Fukushima Daiichi Accident

(Updated May 2020)

Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011. All three cores largely melted in the first three days.

The accident was rated 7 on the INES scale, due to high radioactive releases over days 4 to 6, eventually a total of some 940 PBq (I-131 eq).

Four reactors were written off due to damage in the accident – 2719 MWe net.

After two weeks, the three reactors (units 1-3) were stable with water addition and by July they were being cooled with recycled water from the new treatment plant. Official 'cold shutdown condition' was announced in mid-December.

Apart from cooling, the basic ongoing task was to prevent release of radioactive materials, particularly in contaminated water leaked from the three units. This task became newsworthy in August 2013.

There have been no deaths or cases of radiation sickness from the nuclear accident, but over 100,000 people were evacuated from their homes to ensure this. Government nervousness delays the return of many.

Official figures show that there have been 2259 disaster-related deaths (e.g. from maintaining the evacuation), in contrast to little risk from radiation if early return had been allowed.

The Great East Japan Earthquake of magnitude 9.0 at 2.46 pm on Friday 11 March 2011 did considerable damage in the region, and the large tsunami it created caused very much more. The earthquake was centred 130 km offshore the city of Sendai in Miyagi prefecture on the eastern coast of Honshu Island (the main part of Japan), and was a rare and complex double quake giving a severe duration of about 3 minutes. An area of the seafloor extending 650 km north-south moved typically 10-20 metres horizontally. Japan moved a few metres east and the local coastline subsided half a metre. The tsunami inundated about 560 sq km and resulted in a human death toll of about 19,000 and much damage to coastal ports and towns, with over a million buildings destroyed or partly collapsed.

Eleven reactors at four nuclear power plants in the region were operating at the time and all shut down automatically when the earthquake hit. Subsequent inspection showed no significant damage to any from the earthquake. The operating units which shut down were Tokyo Electric Power Company's (Tepco) Fukushima Daiichi 1, 2, 3, and Fukushima Daini 1, 2, 3, 4, Tohoku's Onagawa 1, 2, 3, and Japco's Tokai, total 9377 MWe net. Fukushima Daiichi units 4, 5&6 were not operating at the time, but were affected. The main problem initially centred on Fukushima Daiichi units 1-3. Unit 4 became a problem on day five.

The reactors proved robust seismically, but vulnerable to the tsunami. Power, from grid or backup generators, was available to run the residual heat removal (RHR) system cooling pumps at eight of the eleven units, and despite some problems they achieved 'cold shutdown' within about four days. The other three, at Fukushima Daiichi, lost power at 3.42 pm, almost an hour after the earthquake, when the entire site was flooded by the 15-metre tsunami. This disabled 12 of 13 back-up generators onsite and also the heat exchangers for dumping reactor waste heat and decay heat to the sea. The three units lost the ability to maintain proper reactor cooling and water circulation functions. Electrical switchgear was also disabled. Thereafter, many weeks of focused work centred on restoring heat removal from the reactors and coping with overheated spent fuel ponds. This was undertaken by hundreds of Tepco employees as well as some contractors, supported by firefighting and military personnel. Some of the Tepco staff had lost homes, and even families, in the tsunami, and were initially living in temporary accommodation under great difficulties and privation, with some personal risk. A hardened emergency response centre on site was unable to be used in grappling with the situation, due to radioactive contamination.

Three Tepco employees at the Daiichi and Daini plants were killed directly by the earthquake and tsunami, but there have been no fatalities from the nuclear accident.

Among hundreds of aftershocks, an earthquake with magnitude 7.1, closer to Fukushima than the 11 March one, was experienced on 7 April, but without further damage to the plant. On 11 April a magnitude 7.1 earthquake and on 12 April a magnitude 6.3 earthquake, both with epicenter at Fukushima-Hamadori, caused no further problems.

The two Fukushima plants and their siting

The Daiichi (first) and Daini (second) Fukushima plants are sited about 11 km apart on the coast, Daini to the south.

The recorded seismic data for both plants – some 180 km from the epicentre – shows that 550 Gal (0.56 g) was the maximum ground acceleration for Daiichi, and 254 Gal was maximum for Daini. Daiichi units 2, 3 and 5 exceeded their maximum response acceleration design basis in E-W direction by about 20%. The recording was over 130-150 seconds. (All nuclear plants in Japan are built on rock – ground acceleration was around 2000 Gal a few kilometres north, on sediments).

The original design basis tsunami height was 3.1 m for Daiichi based on assessment of the 1960 Chile tsunami and so the plant had been built about 10 metres above sea level with the seawater pumps 4 m above sea level. The Daini plant was built 13 metres above sea level. In 2002 the design basis was revised to 5.7 metres above, and the seawater pumps were sealed. In the event, tsunami heights coming ashore were about 15 metres, and the Daiichi turbine halls were under some 5 metres of seawater until levels subsided. Daini was less affected. The maximum amplitude of this tsunami was 23 metres at point of origin, about 180 km from Fukushima.

In the last century there have been eight tsunamis in the region with maximum amplitudes at origin above 10 metres (some much more), these having arisen from earthquakes of magnitude 7.7 to 8.4, on average one every 12 years. Those in 1983 and in 1993 were the most recent affecting Japan, with maximum heights at origin of 14.5 metres and 31 metres respectively, both induced by magnitude 7.7 earthquakes. The June 1896 earthquake of estimated magnitude 8.3 produced a tsunami with run-up height of 38 metres in Tohoku region, killing more than 27,000 people.

The tsunami countermeasures taken when Fukushima Daiichi was designed and sited in the 1960s were considered acceptable in relation to the scientific knowledge then, with low recorded run-up heights for that particular coastline. But some 18 years before the 2011 disaster, new scientific knowledge had emerged about the likelihood of a large earthquake and resulting major tsunami of some 15.7 metres at the Daiichi site. However, this had not yet led to any major action by either the plant operator, Tepco, or government regulators, notably the Nuclear & Industrial Safety Agency (NISA). Discussion was ongoing, but action minimal. The tsunami countermeasures could also have been reviewed in accordance with IAEA guidelines which required taking into account high tsunami levels, but NISA continued to allow the Fukushima plant to operate without sufficient countermeasures such as moving the backup generators up the hill, sealing the lower part of the buildings, and having some back-up for seawater pumps, despite clear warnings.

A report from the Japanese government's Earthquake Research Committee on earthquakes and tsunamis off the Pacific coastline of northeastern Japan in February 2011 was due for release in April, and might finally have brought about changes. The document includes analysis of a magnitude 8.3 earthquake that is known to have struck the region more than 1140 years ago, triggering enormous tsunamis that flooded vast areas of Miyagi and Fukushima prefectures. The report concludes that the region should be alerted of the risk of a similar disaster striking again. The 11 March earthquake measured magnitude 9.0 and involved substantial shifting of multiple sections of seabed over a source area of 200 x 400 km. Tsunami waves devastated wide areas of Miyagi, Iwate and Fukushima prefectures.

(See also background on Earthquakes and Seismic Protection for Nuclear Power Plants in Japan)

Events at Fukushima Daiichi 1-3 & 4

It appears that no serious damage was done to the reactors by the earthquake, and the operating units 1-3 were automatically shut down in response to it, as designed. At the same time all six external power supply sources were lost due to earthquake damage, so the emergency diesel generators located in the basements of the turbine buildings started up. Initially cooling would have been maintained through the main steam circuit bypassing the turbine and going through the condensers.

Then 41 minutes later, at 3:42 pm, the first tsunami wave hit, followed by a second 8 minutes later. These submerged and damaged the seawater pumps for both the main condenser circuits and the auxiliary cooling circuits, notably the Residual Heat Removal (RHR) cooling system. They also drowned the diesel generators and inundated the electrical switchgear and batteries, all located in the basements of the turbine buildings (the one surviving air-cooled generator was serving units 5 & 6). So there was a station blackout, and the reactors were isolated from their ultimate heat sink. The tsunamis also damaged and obstructed roads, making outside access difficult.

All this put those reactors 1-3 in a dire situation and led the authorities to order, and subsequently extend, an evacuation while engineers worked to restore power and cooling. The 125-volt DC back-up batteries for units 1 & 2 were flooded and failed, leaving them without instrumentation, control or lighting. Unit 3 had battery power for about 30 hours.

At 7.03 pm Friday 11 March a Nuclear Emergency was declared, and at 8.50pm the Fukushima Prefecture issued an evacuation order for people within 2 km of the plant. At 9.23 pm the Prime Minister extended this to 3 km, and at 5.44 am on 12th he extended it to 10 km. He visited the plant soon after. On Saturday 12th he extended the evacuation zone to 20 km.

Inside the Fukushima Daiichi reactors

The Fukushima Daiichi reactors are GE boiling water reactors (BWR) of an early (1960s) design supplied by GE, Toshiba and Hitachi, with what is known as a Mark I containment. Reactors 1-3 came into commercial operation 1971-75. Reactor capacity is 460 MWe for unit 1, 784 MWe for units 2-5, and 1100 MWe for unit 6.

When the power failed at 3.42 pm, about one hour after shutdown of the fission reactions, the reactor cores would still be producing about 1.5% of their nominal thermal power, from fission product decay – about 22 MW in unit 1 and 33 MW in units 2&3. Without heat removal by circulation to an outside heat exchanger, this produced a lot of steam in the reactor pressure vessels housing the cores, and this was released into the dry primary containment (PCV) through safety valves. Later this was accompanied by hydrogen, produced by the interaction of the fuel's very hot zirconium cladding with steam after the water level dropped.

As pressure started to rise here, the steam was directed into the suppression chamber/ wetwell under the reactor, within the containment, but the internal temperature and pressure nevertheless rose quite rapidly. Water injection commenced, using the various systems provide for this and finally the Emergency Core Cooling System (ECCS). These systems progressively failed over three days, so from early Saturday water injection to the reactor pressure vessel (RPV) was with fire pumps, but this required the internal pressures to be relieved initially by venting into the suppression chamber/ wetwell. Seawater injection into unit 1 began at 7pm on Saturday 12th, into unit 3 on 13th and unit 2 on 14th. Tepco management ignored an instruction from the prime minister to cease the seawater injection into unit 1, and this instruction was withdrawn shortly afterwards.

Inside unit 1, it is understood that the water level dropped to the top of the fuel about three hours after the scram (about 6 pm) and the bottom of the fuel 1.5 hours later (7.30 pm). The temperature of the exposed fuel rose to some 2800°C so that the central part started to melt after a few hours and by 16 hours after the scram (7 am Saturday) most of it had fallen into the water at the bottom of the RPV. After that, RPV temperatures decreased steadily.

As pressure rose, attempts were made to vent the containment, and when external power and compressed air sources were harnessed this was successful, by about 2.30 pm Saturday, though some manual venting was apparently achieved at about 10.17 am. The venting was designed to be through an external stack, but in the absence of power much of it apparently backflowed to the service floor at the top of the reactor building, representing a serious failure of this system (though another possibility is leakage from the drywell). The vented steam, noble gases and aerosols were accompanied by hydrogen. At 3.36 pm on Saturday 12th, there was a hydrogen explosion on the service floor of the building above unit 1 reactor containment, blowing off the roof and cladding on the top part of the building, after the hydrogen mixed with air and ignited. (Oxidation of the zirconium cladding at high temperatures in the presence of steam produces hydrogen exothermically, with this exacerbating the fuel decay heat problem.)

In unit 1 most of the core – as corium comprised of melted fuel and control rods – was assumed to be in the bottom of the RPV, but later it appeared that it had mostly gone through the bottom of the RPV and eroded about 65 cm into the drywell concrete below (which is 2.6 m thick). This reduced the intensity of the heat and enabled the mass to solidify.

Much of the fuel in units 2&3 also apparently melted to some degree, but to a lesser extent than in unit 1, and a day or two later. In mid-May 2011 the unit 1 core would still be producing 1.8 MW of heat, and units 2&3 would be producing about 3.0 MW each.

In mid-2013 the Nuclear Regulation Authority (NRA) confirmed that the earthquake itself had caused no damage to unit 1.

In unit 2, water injection using the steam-driven back-up water injection system failed on Monday 14th, and it was about six hours before a fire pump started injecting seawater into the RPV. Before the fire pump could be used RPV pressure had to be relieved via the wetwell, which required power and nitrogen, hence the delay. Meanwhile the reactor water level dropped rapidly after back-up cooling was lost, so that core damage started about 8 pm, and it is now provisionally understood that much of the fuel then melted and probably fell into the water at the bottom of the RPV about 100 hours after the scram. Pressure was vented on 13th and again on 15th, and meanwhile the blowout panel near the top of the building was opened to avoid a repetition of unit 1 hydrogen explosion. Early on Tuesday 15th, the pressure suppression chamber under the actual reactor seemed to rupture, possibly due to a hydrogen explosion there, and the drywell containment pressure inside dropped. However, subsequent inspection of the suppression chamber did not support the rupture interpretation. Later analysis suggested that a leak of the primary containment developed on Tuesday 15th. Most of the radioactive releases from the site appeared to come from unit 2.

In Unit 3, the main back-up water injection system failed at about 11 am on Saturday 12th and early on Sunday 13th, water injection using the high pressure system failed also and water levels dropped dramatically. RPV pressure was reduced by venting steam into the wetwell, allowing injection of seawater using a fire pump from just before noon. Early on Sunday venting the suppression chamber and containment was successfully undertaken. It is now understood that core damage started about 5:30 am and much or all of the fuel melted on the morning of Sunday 13th and fell into the bottom of the RPV, with some probably going through the bottom of the reactor pressure vessel and onto the concrete below.

Early on Monday 14th PCV venting was repeated, and this evidently backflowed to the service floor of the building, so that at 11 am a very large hydrogen explosion here above unit 3 reactor containment blew off much of the roof and walls and demolished the top part of the building. This explosion created a lot of debris, and some of that on the ground near unit 3 was very radioactive.

In defuelled unit 4, at about 6 am on Tuesday 15 March, there was an explosion which destroyed the top of the building and damaged unit 3's superstructure further. This was apparently from hydrogen arising in unit 3 and reaching unit 4 by backflow in shared ducts when vented from unit 3.

Units 1-3: Water has been injected into each of the three reactor units more or less continuously, and in the absence of normal heat removal via external heat exchanger this water was boiling off for some months. In the government report to IAEA in June it was estimated that to the end of May about 40% of the injected water boiled off, and 60% leaked out the bottom. In June 2011 this was adding to the contaminated water on site by about 500 m3 per day. In January 2013 4.5 to 5.5 m3/hr was being added to each RPV via core spray and feed water systems, hence 370 m3 per day, and temperatures at the bottom of RPVs were 19°C in unit 1 and 32°C in units 2&3, at little above atmospheric pressure.

There was a peak of radioactive release on 15th, apparently mostly from unit 2, but the precise source remains uncertain. Due to volatile and easily-airborne fission products being carried with the hydrogen and steam, the venting and hydrogen explosions discharged a lot of radioactive material into the atmosphere, notably iodine and caesium. NISA said in June that it estimated that 800-1000 kg of hydrogen had been produced in each of the units.

Nitrogen is being injected into the containment vessels (PCVs) of all three reactors to remove concerns about further hydrogen explosions, and in December this was started also for the pressure vessels. Gas control systems which extract and clean the gas from the PCV to avoid leakage of caesium have been commissioned for all three units.

Throughout 2011 injection into the RPVs of water circulated through the new water treatment plant achieved relatively effective cooling, and temperatures at the bottom of the RPVs were stable in the range 60-76°C at the end of October, and 27-54°C in mid-January 2012. RPV pressures ranged from atmospheric to slightly above (102-109 kPa) in January, due to water and nitrogen injection. However, since they are leaking, the normal definition of "cold shutdown" does not apply, and Tepco waited to bring radioactive releases under control before declaring "cold shutdown condition" in mid-December, with NISA's approval. This, with the prime minister's announcement of it, formally brought to a close the 'accident' phase of events.

The AC electricity supply from external source was connected to all units by 22 March. Power was restored to instrumentation in all units except unit 3 by 25 March. However, radiation levels inside the plant were so high that normal access was impossible until June.

Event sequence following earthquake (timing from it: 14:46, 11 March)

Unit 1 Unit 2 Unit 3 Loss of AC power + 51 min + 54 min + 52 min Loss of cooling + 1 hour + 70 hours + 36 hours Water level down to top of fuel* + 3 hours + 74 hours + 42 hours Core damage starts* + 4 hours + 77 hours + 44 hours Reactor pressure vessel damage* +11 hours uncertain uncertain Fire pumps with fresh water + 15 hours + 43 hours Hydrogen explosion (not confirmed for unit 2) + 25 hours

service floor + 87 hours

suppression chamber + 68 hours

service floor Fire pumps with seawater + 28 hours + 77 hours + 46 hours Off-site electrical supply + 11-15 days Fresh water cooling + 14-15 days

Tepco has written off the four reactors damaged by the accident, and is decommissioning them.

By March 2016 total decay heat in units 1-3 had dropped to 1 MW for all three, about 1% of the original level, meaning that cooling water injection – then 100 m3/d – could be interrupted for up to two days.

Results of muon measurements in unit 2 in 2016 indicate that most of the fuel debris in unit 2 is in the bottom of the reactor vessel.

Summary: Major fuel melting occurred early on in all three units, though the fuel remains essentially contained except for some volatile fission products vented early on, or released from unit 2 in mid-March, and some soluble ones which were leaking with the water, especially from unit 2, where the containment is evidently breached. Cooling is provided from external sources, using treated recycled water, with a stable heat removal path from the actual reactors to external heat sinks. Temperatures at the bottom of the reactor pressure vessels have decreased to well below boiling point and are stable. Access has been gained to all three reactor buildings, but dose rates remain high inside. Nitrogen is being injected into all three containment vessels and pressure vessels. Tepco declared "cold shutdown condition" in mid-December 2011 when radioactive releases had reduced to minimal levels.

(See also background on nuclear reactors at Fukushima Daiichi.)

Fuel ponds: developing problems

Used fuel needs to be cooled and shielded. This is initially by water, in ponds. After about three years under water, used fuel can be transferred to dry storage, with air ventilation simply by convection. Used fuel generates heat, so the water is circulated by electric pumps through external heat exchangers, so that the heat is dumped and a low temperature maintained. There are fuel ponds near the top of all six reactor buildings at the Daiichi plant, adjacent to the top of each reactor so that the fuel can be unloaded under water when the top is off the reactor pressure vessel and it is flooded. The ponds hold some fresh fuel and some used fuel, the latter pending its transfer to the central used/spent fuel storage on site. (There is some dry storage on site to extend the plant's capacity.)

At the time of the accident, in addition to a large number of used fuel assemblies, unit 4's pond also held a full core load of 548 fuel assemblies while the reactor was undergoing maintenance, these having been removed at the end of November, and were to be repplaced in the core.

A separate set of problems arose as the fuel ponds, holding fresh and used fuel in the upper part of the reactor structures, were found to be depleted in water. The primary cause of the low water levels was loss of cooling circulation to external heat exchangers, leading to elevated temperatures and probably boiling, especially in heavily-loaded unit 4. Here the fuel would have been uncovered in about 7 days due to water boiling off. However, the fact that unit 4 was unloaded meant that there was a large inventory of water at the top of the structure, and enough of this replenished the fuel pond to prevent the fuel becoming uncovered – the minimum level reached was about 1.2 m above the fuel on about 22 April.

After the hydrogen explosion in unit 4 early on Tuesday 15 March, Tepco was told to implement injection of water to unit 4 pond which had a particularly high heat load (3 MW) from 1331 used fuel assemblies in it, so it was the main focus of concern. It needed the addition of about 100 m3/day to replenish it after circulation ceased.

From Tuesday 15 March attention was given to replenishing the water in the ponds of units 1, 2, 3 as well. Initially this was attempted with fire pumps but from 22 March a concrete pump with 58-metre boom enabled more precise targeting of water through the damaged walls of the service floors. There was some use of built-in plumbing for unit 2. Analysis of radionuclides in water from the used fuel ponds suggested that some of the fuel assemblies might be damaged, but the majority were intact.

There was concern about structural strength of unit 4 building, so support for the pond was reinforced by the end of July.

New cooling circuits with heat exchangers adjacent to the reactor buildings for all four ponds were commissioned after a few months, and each reduced the pool temperature from 70°C to normal in a few days. Each has a primary circuit within the reactor and waste treatment buildings and a secondary circuit dumping heat through a small dry cooling tower outside the building.

The next task was to remove the salt from those ponds which had seawater added, to reduce the potential for corrosion.

In July 2012 two of the 204 fresh fuel assemblies were removed from the unit 4 pool and transferred to the central spent fuel pool for detailed inspection to check damage, particularly corrosion. They were found to have no deformation or corrosion. Unloading the 1331 spent fuel assemblies in pond 4 and transferring them to the central spent fuel storage commenced in mid-November 2013 and was completed 13 months later. These comprised 783 spent fuel plus the full fuel load of 548.

The next focus of attention was the unit 3 pool. In 2015 the damaged fuel handling equipment and other wreckage was removed from the destroyed upper level of the reactor building. Toshiba has built a 74-tonne fuel handling machine for transferring the 566 fuel assemblies into casks and to remove debris in the pool, and a crane for lifting the fuel transfer casks. Installation of a cover over the fuel handling machine was completed in February 2018.

The onsite central spent fuel pool in 2011 held about 60% of the Daiichi used fuel, and is immediately west (inland) of unit 4. It lost circulation with the power outage, and temperature increased to 73°C by the time mains power and cooling were restored after two weeks. In late 2013 this pond, with capacity for 6840*, held 6375 fuel assemblies, the same as at the time of the accident. The older ones will be transferred to 65 casks in dry storage, with total capacity of at least 2930 assemblies – each dry cask holds 50 fuel assemblies. Eventually these will be shipped to JNFL’s Rokkasho reprocessing plant or to Recyclable Fuel Storage Company’s new Mutsu facility. The dry storage area held 408 fuel assemblies at the time of the accident, and 1004 have been transferred there since (to mid-2014).

Summary: The spent fuel storage pools survived the earthquake, tsunami and hydrogen explosions without significant damage to the fuel or significant radiological release, or threat to public safety. The new cooling circuits with external heat exchangers for the four ponds are working well. Temperatures are normal. Analysis of water has confirmed that most fuel rods are intact. All fuel assemblies have been removed from unit 4 pool. Those at unit 3 will be removed next.

(See also background on Fukushima Fuel Ponds and Decommissioning section below.)

Radioactive releases to air

Regarding releases to air and also water leakage from Fukushima Daiichi, the main radionuclide from among the many kinds of fission products in the fuel was volatile iodine-131, which has a half-life of 8 days. The other main radionuclide is caesium-137, which has a 30-year half-life, is easily carried in a plume, and when it lands it may contaminate land for some time. It is a strong gamma-emitter in its decay. Cs-134 is also produced and dispersed, it has a two-year half-life. Caesium is soluble and can be taken into the body, but does not concentrate in any particular organs, and has a biological half-life of about 70 days. In assessing the significance of atmospheric releases, the Cs-137 figure is multiplied by 40 and added to the I-131 number to give an "iodine-131 equivalent" figure.

As cooling failed on the first day, evacuations were progressively ordered, due to uncertainty about what was happening inside the reactors and the possible effects. By the evening of Saturday 12 March the evacuation zone had been extended to 20 km from the plant. From 20 to 30 km from the plant, the criterion of 20 mSv/yr dose rate was applied to determine evacuation, and is now the criterion for return being allowed. 20 mSv/yr was also the general limit set for children's dose rate related to outdoor activities, but there were calls to reduce this.In areas with 20-50 mSv/yr from April 2012 residency is restricted, with remediation action taken. See later section on Public health and return of evacuees.

A significant problem in tracking radioactive release was that 23 out of the 24 radiation monitoring stations on the plant site were disabled by the tsunami.

There is some uncertainty about the amount and exact sources of radioactive releases to air. (See also background on Radiation Exposure.)

Japan’s regulator, the Nuclear & Industrial Safety Agency (NISA), estimated in June 2011 that 770 PBq (iodine-131 equivalent) of radioactivity had been released, but the Nuclear Safety Commission (NSC, a policy body) in August lowered this estimate to 570 PBq. The 770 PBq figure is about 15% of the Chernobyl release of 5200 PBq iodine-131 equivalent. Most of the release was by the end of March 2011.

Tepco sprayed a dust-suppressing polymer resin around the plant to ensure that fallout from mid-March was not mobilized by wind or rain. In addition it removed a lot of rubble with remote control front-end loaders, and this further reduced ambient radiation levels, halving them near unit 1. The highest radiation levels on site came from debris left on the ground after the explosions at units 3&4.

Reactor covers

In mid-May 2011 work started towards constructing a cover over unit 1 to reduce airborne radioactive releases from the site, to keep out the rain, and to enable measurement of radioactive releases within the structure through its ventilation system. The frame was assembled over the reactor, enclosing an area 42 x 47 m, and 54 m high. The sections of the steel frame fitted together remotely without the use of screws and bolts. All the wall panels had a flameproof coating, and the structure had a filtered ventilation system capable of handling 40,000 cubic metres of air per hour through six lines, including two backup lines. The cover structure was fitted with internal monitoring cameras, radiation and hydrogen detectors, thermometers and a pipe for water injection. The cover was completed with ventilation systems working by the end of October 2011. It was expected to be needed for two years. In May 2013 Tepco announced its more permanent replacement, to be built over four years. It started demolishing the 2011 cover in 2014 and finished in 2016. It then plans to remove concrete and other rubble on the top floor of the building. A crane and other equipment for fuel removal would then be installed in a new cover over the building, similar to that over unit 4.

More substantial covers were designed to fit around units 3&4 reactor buildings after the top floors were cleared up in 2012.

A cantilevered structure was built over unit 4 from April 2012 to July 2013 to enable recovery of the contents of the spent fuel pond. This is 69 x 31 m cover (53 m high) and it was fully equipped by the end of 2013 to enable unloading of used fuel from the storage pond into casks, each holding 22 fuel assemblies, and removal of the casks. This operation was accomplished under water, using the new fuel handling machine (replacing the one destroyed by the hydrogen explosion) so that the used fuel could be transferred to the central storage on site. Transfer was completed in December 2014. Animation video of the process here.

A different design of cover is being built over unit 3, and foundation work had begun in 2012. Large rubble removal took place from 2013 to 2015, including the damaged fuel handling machine. An arched cover has been prefabricated, 57 m long and 19 m wide, to be supported by the turbine building on one side and the ground on the other. A crane to remove the 566 fuel assemblies from the pool will be fitted, with its operation removing some remaining rubble. Toshiba’s 74-tonne fuel handling machine for transferring the fuel assemblies into casks is expected be installed in 2017, and the fuel is to be removed from mid-2018. Fuel debris retrieval from within the reactor is scheduled from about 2021, after that from units 1&2 is started.

Spent fuel removal from units 1&2 pools is scheduled in 2018, and fuel debris retrieval from within the reactors from 2020.

Tests on radioactivity in rice have been made and caesium was found in a few of them. The highest levels were about one quarter of the allowable limit of 500 Bq/kg, so shipments to market are permitted.



Maps from MEXT aerial surveys carried out approximately one year apart show the reduction in contamination from late 2011 to late 2012. Areas with colour changes in 2012 showed approximately half the contamination as surveyed in 2011, the difference coming from decay of caesium-134 (two year half-life) and natural processes like wind and rain. In blue areas, ambient radiation is very similar to global background levels at <0.5uSv/h which is equal to <4.38 mSv/y.

Summary: Major releases of radionuclides, including long-lived caesium, occurred to air, mainly in mid-March. The population within a 20km radius had been evacuated three days earlier. Considerable work was done to reduce the amount of radioactive debris on site and to stabilise dust. The main source of radioactive releases was the apparent hydrogen explosion in the suppression chamber of unit 2 on 15 March. A cover building for unit 1 reactor was built and is now being dismantled, a more substantial one for unit 4 was built to enable fuel removal during 2014. Radioactive releases in mid-August 2011 had reduced to 5 GBq/hr, and dose rate from these at the plant boundary was 1.7 mSv/yr, less than natural background.

Sequence of evacuation orders based on the report by The National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission: 11 March

14:46 JST The earthquake occurred.

15:42 TEPCO made the first emergency report to the government.

19:03 The government announced nuclear emergency.

20:50 The Fukushima Prefecture Office ordered 2km radius evacuation.

21:23 The government ordered 3km evacuation and to keep staying inside buildings in the area of 3-10km radius. 12 March

05:44 The government ordered 10km radius evacuation.

18:25 The government ordered 20km evacuation. 15 March

11:01 The government ordered to keep staying inside buildings in the area of 20-30km from the plant. 25 March The government requested voluntary evacuation in the area of 20-30km. 21 April The government set the 20km radius no-go area.

Radiation exposure on the plant site

By the end of 2011, Tepco had checked the radiation exposure of 19,594 people who had worked on the site since 11 March. For many of these both external dose and internal doses (measured with whole-body counters) were considered. It reported that 167 workers had received doses over 100 mSv. Of these 135 had received 100 to 150 mSv, 23 150-200 mSv, three more 200-250 mSv, and six had received over 250 mSv (309 to 678 mSv) apparently due to inhaling iodine-131 fume early on. The latter included the two unit 3-4 control room operators in the first two days who had not been wearing breathing apparatus. There were up to 200 workers on site each day. Recovery workers are wearing personal monitors, with breathing apparatus and protective clothing which protect against alpha and beta radiation. So far over 3500 of some 3700 workers at the damaged Daiichi plant have received internal check-ups for radiation exposure, giving whole body count estimates. The level of 250 mSv was the allowable maximum short-term dose for Fukushima Daiichi accident clean-up workers through to December 2011, 500 mSv is the international allowable short-term dose "for emergency workers taking life-saving actions". Since January 2012 the allowable maximum has reverted to 50 mSv/yr.

Tepco figures submitted to NRA for the period to end January 2014 showed 173 workers had received more than 100 mSv (six more than two years earlier) and 1578 had received 50 to 100 mSv. This was among a total of 32,024, 64% more than had worked there two years earlier. Since April 2013 none of the 13,154 who had worked on site had received more than 50 mSv, and 96% of these had less than 20 mSv dose. Early in 2014 there were about 4000 on site each weekday.

No radiation casualties (acute radiation syndrome) occurred, and few other injuries, though higher than normal doses were being accumulated by several hundred workers on site. High radiation levels in the three reactor buildings has hindered access there.

Monitoring of seawater, soil and atmosphere is at 25 locations on the plant site, 12 locations on the boundary, and others further afield. Government and IAEA monitoring of air and seawater is ongoing. Some high but not health-threatening levels of iodine-131 were found in March, but with an eight-day half-life, most I-131 had gone by the end of April 2011.

A radiation survey map of the site made in March 2013 revealed substantial progress: the highest dose rate anywhere on the site was 0.15 mSv/h near units 3 and 4. (Soon after the accident a similar survey put the highest dose rate at 300 mSv/h near rubble lying alongside unit 3.) The majority of the power plant area was at less than 0.01 mSv/h. These reduced levels are reflected in worker doses: during January 2013, the 5702 workers at the site received an average of 0.86 mSv, with 75% of workers recorded as receiving less than 1 mSv. In total, only about 2% of workers received over 5 mSv and the highest dose in January was 12.65 mSv for one worker.

Media reports have referred to "nuclear gypsies" – casual workers employed by subcontractors on a short-term basis, and allegedly prone to receiving higher and unsupervised radiation doses. This transient workforce has been part of the nuclear scene for at least four decades, and at Fukushima their doses are very rigorously monitored. If they reach certain levels, e.g. 30 mSv but varying according to circumstance, they are reassigned to lower-exposure areas.

Summary: Six workers received radiation doses apparently over the 250 mSv level set by NISA, but at levels below those which would cause radiation sickness.

Radiation exposure and fallout beyond the plant site

On 4 April 2011, radiation levels of 0.06 mSv/day were recorded in Fukushima city, 65 km northwest of the plant, about 60 times higher than normal but posing no health risk according to authorities. Monitoring beyond the 20 km evacuation radius to 13 April showed one location – around Iitate – with up to 0.266 mSv/day dose rate, but elsewhere no more than one-tenth of this. At the end of July the highest level measured within 30km radius was 0.84 mSv/day in Namie town, 24 km away. The safety limit set by the central government in mid-April for public recreation areas was 3.8 microsieverts per hour (0.09 mSv/day).

In June 2013, analysis from Japan's Nuclear Regulation Authority (NRA) showed that the most contaminated areas in the Fukushima evacuation zone had reduced in size by three-quarters over the previous two years. The area subject to high dose rates (over 166 mSv/yr) diminished from 27% of the 1117 km2 zone to 6% over 15 months to March 2013, and in the ‘no residence’ portion (originally 83-166 mSv/yr) no areas remained at this level and 70% was below 33 mSv/yr. The least-contaminated area is now entirely below 33 mSv/yr.

In August 2011 The Act on Special Measures Concerning the Handling of Radioactive Pollution was enacted and it took full effect from January 2012 as the main legal instrument to deal with all remediation activities in the affected areas, as well as the management of materials removed as a result of those activities. It specified two categories of land:

Special Decontamination Areas consisting of the “restricted areas” located within a 20 km radius from the Fukushima Daiichi plant, and “deliberate evacuation areas” where the annual cumulative dose for individuals was anticipated to exceed 20 mSv. The national government promotes decontamination in these areas. These areas are subdivided into three: dose 1- 20 mSv/yr (green) dose 20-50 mSv/yr (yellow) and dose over 50 mSv/yr and over 20 mSv/yr average over 5 years (red).

Intensive Contamination Survey Areas including the so-called Decontamination Implementation Areas, where an additional annual cumulative dose between 1mSv and 20mSv was estimated for individuals. Municipalities implement decontamination activities in these areas.

In May 2013, the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) reported, following a detailed study by 80 international experts. It concluded that "Radiation exposure following the nuclear accident at Fukushima Daiichi did not cause any immediate health effects. It is unlikely to be able to attribute any health effects in the future among the general public and the vast majority of workers." The only exception are the 146 emergency workers that received radiation doses of over 100 mSv during the crisis. They will be monitored closely for "potential late radiation-related health effects at an individual level." UNSCEAR’s follow-up white paper in October 2015 said that none of the new information appraised after the 2013 report “materially affected the main findings in, or challenged the major assumptions of, the 2013 Fukushima report."

By contrast, the public was exposed to 10-50 times less radiation. Most Japanese people were exposed to additional radiation amounting to less than the typical natural background level of 2.1 mSv per year.

People living in Fukushima prefecture are expected to be exposed to around 10 mSv over their entire lifetimes, while for those living further away the dose would be 0.2 mSv per year. The UNSCEAR conclusion reinforces the findings of several international reports to date, including one from the World Health Organisation (WHO) that considered the health risk to the most exposed people possible: a postulated girl under one year of age living in Iitate or Namie that did not evacuate and continued life as normal for four months after the accident. Such a child's theoretical risk of developing any cancer would be increased only marginally, according to WHO's analysis.

Eleven municipalities in the former restricted zone or planned evacuation area, within 20 km of the plant or where annual cumulative radiation dose is greater than 20 mSv, are designated Special Decontamination Areas, where decontamination work is being implemented by the government. A further 100 municipalities in eight prefectures, where dose rates are equivalent to over 1 mSv per year are classed as Intensive Decontamination Survey Areas, where decontamination is being implemented by each municipality with funding and technical support from the national government. In the Special Decontamination Areas, decontamination is proceeding and was complete to target levels in one municipality by June 2013.

In October 2013 a 16-member IAEA mission reported on remediation and decontamination in the Special Decontamination Areas. Its preliminary report said that decontamination efforts were commendable but driven by unrealistic targets. If annual radiation dose was below 20 mSv, such as generally in Intensive Decontamination Survey Areas, this level was “acceptable and in line with the international standards and with the recommendations from the relevant international organisations, e.g. ICRP, IAEA, UNSCEAR and WHO.” The clear implication is that people in such areas should be allowed to return home. Furthermore the government should increase efforts to communicate this to the public, and should explain that its long-term goal of achieving an additional individual dose of 1 mSv/yr is unrealistic and unnecessary in the short term. Also, there is potential to produce more food safely in contaminated areas.

Radioactivity, primarily from caesium-137, in the evacuation zone and other areas beyond it has been reported in terms of kBq/kg (compared with kBq/m2 around Chernobyl. A total of 3,000 km 2 was contaminated above 180 kBq/m2, compared with 29,400 km 2 from Chernobyl). However the main measure has been presumed doses in mSv/yr. The government has adopted 20 mSv/yr as its goal for the evacuation zone and more contaminated areas outside it, and supports municipal government work to reduce levels below that. The total area under consideration for attention is 13,000 km2. In 2016 the Ministry of Environment announced that material with less than 8 kBq/kg caesium would no longer be specified as waste, and subject to restrictions on disposal. It allowed use of contaminated soil for embankments, where the activity was less then 8 kBq/kg, and unrestricted use if less than 100 Bq/kg. Most of the stored wastes have decayed to below the 8 kBq/kg level.

Summary: There have been no harmful effects from radiation on local people, nor any doses approaching harmful levels. However, some 160,000 people were evacuated from their homes and only from 2012 were allowed limited return. In October 2013, 81,000 evacuees remained displaced due to government concern about radiological effects from the accident.

Public health and return of evacuees

Permanent return remains a high priority, and the evacuation zone is being decontaminated where required and possible, so that evacuees (81,000 from this accident according to METI) can return without undue delay. There are many cases of evacuation stress including transfer trauma among evacuees, and once the situation had stabilised at the plant these outweighed the radiological hazards of returning, with 2259 deaths reported (see below). There were also 267,000 tsunami survivor refugees remaining displaced in February 2014.

In December 2011 the government said that where annual radiation dose would be below 20 mSv/yr, the government would help residents return home as soon as possible and assist local municipalities with decontamination and repair of infrastructure. In areas where radiation levels are over 20 mSv/yr evacuees will be asked to continue living elsewhere for “a few years” until the government completes decontamination and recovery work. The government said it would consider purchasing land and houses from residents of these areas if the evacuees wish to sell them.

In November 2013 the NRA decided to change the way radiation exposure was estimated. Instead of airborne surveys being the basis, personal dosimeters would be used, giving very much more accurate figures, often much less than airborne estimates. The same criteria would be used, as above, with 20 mSv/yr being the threshold of concern to authorities.

In February 2014 the results of a study were published showing that 458 residents of two study areas 20 to 30 km from the plant and a third one 50 km northwest received radiation doses from the contaminated ground similar to the country’s natural background levels. Measurement was by personal dosimeters over August-September 2012.

By December 2018, 2259 disaster-related deaths, that were not due to radiation-induced damage or to the earthquake or to the tsunami, had been identified by the Japanese authorities. About 90% of deaths were for persons above 66 years of age. Of these, about 70% occurred within the first three months of the evacuations. (A similar number of deaths occurred among evacuees from tsunami- and earthquake-affected prefectures. These figures are additional to the 19,000 that died in the actual tsunami.)

The premature deaths reported in 2012 were mainly related to the following: (1) somatic effects and spiritual fatigue brought on by having to reside in shelters; (2) transfer trauma – the mental or physical burden of the forced move from their homes for fragile individuals; and (3) delays in obtaining needed medical support because of the enormous destruction caused by the earthquake and tsunami. However, the radiation levels in most of the evacuated areas were not greater than the natural radiation levels in high background areas elsewhere in the world where no adverse health effect is evident, so maintaining the evacuation beyond a precautionary few days was evidently the main disaster in relation to human fatalities.

http://www.reconstruction.go.jp/topics/20121102_sinsaikanrensi.pdf

http://www.reconstruction.go.jp/topics/240821_higashinihondaishinsainiokerushinsaikanrenshinikansuruhoukoku.pdf

In its December 2018 report, the Fukushima prefectural government said that the number of ‘indirect’ deaths in the prefecture was greater than the number (1829) killed in the earthquake and tsunami. It put the figure at 2259 as determined by municipal panels that examine links between the disaster’s aftermath and deaths. The figure is greater than for Iwate and Miyagi prefectures, with 434 and 879 respectively, though they had much higher loss of life in the earthquake and tsunami – about 14,200. The disparity is attributed to the older age group involved among Fukushima’s evacuated quake/tsunami survivors, about 90% of indirect deaths being of people over 66. Causes of indirect deaths include physical and mental stress stemming from long stays at shelters, a lack of initial care as a result of hospitals being disabled by the disaster, and suicides. The high rate of these deaths continues three years later as the evacuation is maintained for about 135,000 people – apparently some 75,000 from the nuclear accident and 60,000 from the natural disaster itself. Evaluation of ‘indirect deaths’ is according to a model developed by Niigata prefecture after the 2004 earthquake there. (Japan Times 20/2/14)

Evacuees receive JPY 100,000 (about $1000) per month in psychological suffering compensation. The money is tax-exempt and paid unconditionally. In October 2013, about 84,000 evacuees received the payments. Statistics indicate that an average family of four has received about JPY 90 million ($900,000) in compensation from Tepco. The average compensation for real estate was JPY 49.1 million ($490,000), JPY 10.9 million ($110,000) for lost wages, and JPY 30 million ($300,000) as “consolation money” for pain and suffering. (Asahi Shimbun 26/10/13)

The Fukushima prefecture has 17,000 government-financed temporary housing units for some 29,500 evacuees from the accident. The prefectural government said residents could continue to use these until March 2015. The number compares with very few built in Miyagi, Iwate and Aomori prefectures for the 222,700 tsunami survivor refugees there. (Japan Times 17/11/13) Another reported contrast from the Reconstruction Agency is that some $30 billion had been paid to 84,000 nuclear accident refugees but only some $20 billion to 300,000 tsunami survivors in the Tohoku region.

Evacuation orders have been progressively lifted, apart from some 300 km 2 designated areas with annual dose levels above 20 mSv with continuous occupation.

In April 2019, the first residents of Okuma, the closest town to the plant, were allowed to return home.

According to a survey released by the prefectural government in April 2017, the majority of people who voluntarily evacuated their homes after the accident and who are now living outside of Fukushima prefecture do not intend to return. A Mainichi report said that 78.2% of respondents to the survey preferred to continue living in the area to which they had moved, while only 18.3% intended to move back to the prefecture. Of the voluntary evacuees still living in Fukushima prefecture, 23.6% planned to stay where they were, while 66.6% hoped to return to their original homes.

An August 2012 Reconstruction Agency report also considered workers at Fukushima power plant. Of almost 1500 surveyed, many were stressed, due to evacuating their homes (70%), believing they had come close to death (53%), the loss of homes in the tsunami (32%), deaths of colleagues (20%) and of family members (6%) mostly in the tsunami. The death toll directly due to the nuclear accident or radiation exposure remained zero, but stress and disruption due to the continuing evacuation remains high.

Tokyo’s Board of Audit reported in October 2013 that 23% of recovery funding – about JPY 1.45 trillion ($14.5 billion) – had been misappropriated. Some 326 out of about 1400 projects funded had no direct relevance to the natural disaster or Fukushima Daiichi accident. (Mainichi 1/11/13)

Summary: Many evacuated people remain unable to fully return home due to government-mandated restrictions based on conservative radiation exposure criteria. However, 2259 premature disaster-related deaths were caused by maintaining the evacuation beyond a prudent week or so. Decontamination work is proceeding while radiation levels decline naturally. The October 2013 IAEA report makes it clear that many evacuees should be allowed to return home.

Managing contaminated water

Removing contaminated water from the reactor and turbine buildings had become the main challenge in week 3, along with contaminated water in trenches carrying cabling and pipework. This was both from the tsunami inundation and leakage from reactors. Run-off from the site into the sea was also carrying radionuclides well in excess of allowable levels. By the end of March all storages around the four units – basically the main condenser units and condensate tanks – were largely full of contaminated water pumped from the buildings. Some 1000 storage tanks were set up progressively, including initially 350 steel tanks with rubber seams, each holding 1200 m3. A few of these developed leaks in 2013.

Accordingly, with government approval, Tepco over 4-10 April released to the sea about 10,400 cubic metres of slightly contaminated water (0.15 TBq total) in order to free up storage for more highly-contaminated water from unit 2 reactor and turbine buildings which needed to be removed to make safe working conditions. Unit 2 is the main source of contaminated water, though some of it comes from drainage pits. NISA confirmed that there was no significant change in radioactivity levels in the sea as a result of the 0.15 TBq discharge.

Tepco built a new wastewater treatment facility to treat contaminated water. The company used both US proprietary adsorbtion and French conventional technologies in the new 1200 m3/day treatment plant. A supplementary and simpler SARRY plant to remove caesium using Japanese technology and made by Toshiba and Shaw Group was installed and commissioned in August 2011. These plants reduce caesium from about 55 MBq/L to 5.5 kBq/L – about ten times better than designed. Desalination is necessary on account of the seawater earlier used for cooling, and the 1200 m3/day desalination plant produces 480 m3 of clean water while 720 m3 goes to storage. A steady increase in volume of the stored water (about 400 m3/d net) is due to groundwater finding its way into parts of the plant and needing removal and treatment.

Early in 2013 Tepco started to test and commission this Advanced Liquid Processing System (ALPS), developed by EnergySolutions and Toshiba. Each of six trains is capable of processing 250 m3/day to remove 62 remaining radioisotopes. By the end of 2014, an Advanced ALPS of 500 m3/d had been added, making total capacity 2000 m3/d. NRA approved the extra capacity in August 2014.

The ALPS is a chemical system which will remove radionuclides to below legal limits for release. However, because tritium is contained in water molecules, ALPS cannot remove it, which gives rise to questions about the discharge of treated water to the sea. Tritium is a weak beta-emitter which does not bio-accumulate (half-life 12 years), and its concentration has levelled off at about 1 MBq/L in the stored water, with dilution from groundwater balancing further release from the fuel debris.

The Japanese government plans to start preparations in 2020 for the disposal of the contaminated water. ALPS-treated water is currently stored in tanks onsite which will reach full capacity by the summer of 2022. As of March 2020, more than 1 million tonnes was in storage in more than 900 tanks at the plant site.

Disposal will be either into the atmosphere or the sea. In November 2019 the trade and industry ministry stated that annual radiation levels from the release of the tritium-tainted water are estimated at between 0.052 and 0.62 microsieverts if it were disposed of at sea and 1.3 microsieverts if it were released into the atmosphere, compared with the 2100 microsieverts (2.1 mSv) that humans are naturally exposed to annually.

The clean tritiated water was the focus of attention in 2014. A September 2013 report from the Atomic Energy Society of Japan recommended diluting the ALPS-treated water with seawater and releasing it to the sea at the legal discharge concentration of 0.06 MBq/L, with monitoring to ensure that normal background tritium levels of 10 Bq/L are not exceeded. (WHO drinking water guideline is 0.01 MBq/L tritium) The IAEA is reported to support release of tritiated water to the ocean, as does Dr Dale Klein, chairman of Tepco’s nuclear reform monitoring committee (NRMC) and former chairman of US Nuclear Regulatory Commission. The government had an expert Task Force considering the options. In March 2020, TEPCO released a conceptual study in relation to two disposal methods of the ALPS-treated water – discharge into the sea and vapour discharge – with TEPCO going to undertake further studies into the dilution rates of tritium through both options. The Japanese government has the final authority to decide which disposal policy to implement.

In 2016 Kurion (now owned by Veolia) completed a demonstration project for tritium removal at low concentrations, with its new Modular Detritiation System (MDS),* in response to a JPY 1 billion commission from METI.

* In this, an electrolyser produces hydrogen and oxygen, with the tritium reporting in the hydrogen. This is fed through a catalytic exchange column with a little water which preferentially takes up the tritium. The concentrated tritiated water is fed through a ‘getter bed’ of dry metal hydrate, where the tritium replaces hydrogen, and the material is stored, being stable up to 500°C. It can be incorporated into concrete and disposed as low-level waste. The tritium is concentrated 1000 to 20,000 times. The MDS is the first system to be able economically to treat large volumes of water with low tritium concentrations, and builds on existing heavy water tritium removal systems. Each module treats up to 7200 litres per day.

In 2014 a new Kurion (now owned by Veolia) strontium removal system was commissioned. This is mobile and can be moved around the tank groups to further clean up water which has been treated by ALPS.

In June 2015, 108 m3/day of clean water was being circulated through each reactor (1-3). Collected water from them, with high radioactivity levels, was being treated for caesium removal and re-used. Apart from this recirculating loop, the cumulative treated volume was then 1.232 million cubic metres. In storage onsite was 459,000 m3 of fully treated water (by ALPS), and 190,000 m3 of partially-treated water (strontium removed), which was being added to at 400 m3/day due to groundwater inflow. Almost 600 m3 of sludge from the water treatment was stored in shielded containers.

In October 2018 Tepco announced that some 80% of its 900,000 m3 of stored water from reactor buildings remained above legal limits for discharge to the sea. Treatment using ALPS is ongoing at about 210 m3/day.

By the end of June 2011, Tepco had installed 109 concrete panels to seal the water intakes of units 1-4, preventing contaminated water leaking to the harbour. From mid-June some treatment with zeolite of seawater at 30 m3/hr was being undertaken near the water intakes for units 2&3, inside submerged barriers installed in April. From October, a steel water shield wall was built on the sea frontage of units 1-4. It extends about one kilometre, and down to an impermeable layer beneath two permeable strata which potentially leak contaminated groundwater to the sea. The inner harbour area which has some contamination is about 30 ha in area. The government in September 2013 said that “At present, statistically-significant increase of radioactive concentration in the sea outside the port of the TEPCO’s Fukushima Daiichi NPS has not been detected.” And also that “The results of monitoring of sea water in Japan are constantly below the standard of 10 Bq/L” (the WHO standard for Cs-137 in drinking water). In 2012 the Japanese standard for caesium in food supply was dropped from 500 to 100 Bq/kg. In July-August 2014 only 0.6% of fish caught offshore from the plant exceeded this lower level, compared with 53% in the months immediately following the accident.

Apart from the above-ground water treatment activity, there is now a groundwater bypass to reduce the groundwater level above the reactors by about 1.5 metres, pumping from 12 wells and from May 2014, discharging the uncontaminated water into the sea. This prevents some of it flowing into the reactor basements and becoming contaminated. In addition, an impermeable wall is being constructed on the sea-side of the reactors, and inside this a frozen soil wall will further block water flow into the reactor buildings.

In October 2013 guidelines for rainwater release from the site allowed Tepco to release water to the sea without specific NRA approval as long as it conformed to activity limits. Tepco has been working to 25 Bq/L caesium and 10 Bq/L strontium-90.

Summary: A large amount of contaminated water has accumulated on site and has been treated to remove all but traces of tritium, which limits the potential to release treated water to the sea. Some radioactivity has been released to the sea, but this has mostly been low-level and it has not had any significant impact beyond the immediate plant structures. Concentrations outside these structures have been below regulatory levels since April 2011.

IRID and NDF involvement

The International Research Institute for Nuclear Decommissioning (IRID) was set up in August 2013 Japan by JAEA, Japanese utilities and reactor vendors, with a focus on Fukushima 1-4.

In September 2013 IRID called for submissions on the management of contaminated water at Fukushima. In particular, proposals were sought for dealing with: the accumulation of contaminated water (in storage tanks, etc); the treatment of contaminated water including tritium removal; the removal of radioactive materials from the seawater in the plant's 30 ha harbour; the management of contaminated water inside the buildings; measures to block groundwater from flowing into the site; and, understanding the flow of groundwater. Responses were submitted to the government in November.

In December 2013 IRID called for innovative proposals for removing fuel debris from units 1-3 about 2020.

In August 2014 the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) was set up by government as a planning body with management support for R&D projects, taking over IRID’s planning role. It will work closely with IRID, whose focus now is on developing mid- and long-term decommissioning technologies. NDF will also work closely with Tepco Fukushima Daiichi D&D Engineering Co. which has responsibility for operating the actual decommissioning work there. The NDF will be the main body interacting with government (METI) to implement policy.

Fukushima Daiichi 5&6

Units 5&6, in a separate building, also lost power on 11 March due to the tsunami. They were in 'cold shutdown' at the time, but still requiring pumped cooling. One air-cooled diesel generator at Daiichi 6 was located higher and so survived the tsunami and enabled repairs on Saturday 19th, allowing full restoration of cooling for units 5 and 6. While the power was off their core temperature had risen to over 100°C (128°C in unit 5) under pressure, and they had been cooled with normal water injection. They were restored to cold shutdown by the normal recirculating system on 20th, and mains power was restored on 21-22nd.

In September 2013 Tepco commenced work to remove the fuel from unit 6. Prime Minister Abe then called for Tepco to decommission both units. Tepco announced in December 2013 that it would decommission both units from the end of january 2014. Unit 5 is a 760 MWe BWR the same as units 2-4, and unit 6 is larger – 1067 MWe. They entered commercial operation in 1978 and 1979 respectively. It is proposed that they will be used for training.

Meanwhile Tepco and Mitsubishi plan to build and operate two new 500 MWe coal-burning power plants near Fukushima Daiichi at Hirono Town and Iwaki City. This will partly compensate for the decommissioning of the Fukushima Daiichi units. The investment is expected to be JPY 300 billion ($3 billion). The plants will be state-of-the-art integrated combined gasification cycle with less atmospheric pollution than the coal plants now operating in Japan. The companies planned to apply for a government subsidy to help defray costs. (Mainichi 23/11/13)

Remediation on site and decommissioning units 1-4

Tepco published a six- to nine-month plan in April 2011 for dealing with the disabled Fukushima reactors, and updated this several times subsequently. Remediation over the next couple of years proceeded approximately as planned. in August 2011 Tepco announced its general plan for proceeding with removing fuel from the four units, initially from the spent fuel ponds and then from the actual reactors. At the end of 2013 Tepco announced the establishment of an internal entity to focus on measures for decommissioning units 1-6 and dealing with contaminated water. The name of the new company is Fukushima Daiichi Decontamination & Decommissioning Engineering Company, and it commenced operations in April 2014.

In June 2015 the government revised the decommissioning plan or roadmap for the second time, though without major change. It clarified milestones to accomplish preventive and multilayered measures, involving the three principles of removing the source of the contamination, isolating groundwater from the contamination source, and preventing leakage of the contaminated water. It included a new goal of cutting the amount of groundwater flowing into the buildings to less than 100 m3 per day by April 2016. The schedule for fuel removal from the pond at unit 1 was postponed from late FY17 to FY20, while that for unit 2 was delayed from early FY20 to later the same fiscal year, and that at unit 3 from early FY15 to FY17. Fuel debris removal was to begin in 2021, as before. In September 2017 the government updated the June 2015 decommissioning roadmap, with no changes to the framework, and confirming first removal of fuel debris from unit 1 in 2021. Treatment of all contaminated water accumulated in the reactor buildings was to be completed by 2020.

However, Tepco’s latest roadmap shows fuel removal from the pond at unit 1 is now expected FY27-28, and from unit 2 FY24-FY26. For unit 3, fuel removal is expected to be completed in FY20. Fuel debris removal remains scheduled to begin in FY21.

Tepco has a website giving updates on decommissioning work and environmental monitoring.

Storage ponds : First, debris has been removed from the upper parts of the reactor buildings using large cranes and heavy machinery. Covers will be built as required, and overhead cranes and fuel handling machines necessary to remove the spent fuel assemblies will be reinstalled, as already at unit 4. The fuel handling machine for unit 3 was built by Toshiba in January 2016 and delivered to the port at the plant. It is being fitted in 2017, with a cover, for the 566 fuel assemblies to be removed by mid-2018. Casks to transfer the removed fuel to the central spent fuel facility have been designed and manufactured using existing cask technology.

In July 2012 two unused fuel assemblies were removed from unit 4 pond, and were found to be in good shape, with no deformation or corrosion. Tepco started removal of both fresh and used fuel from the pond in November 2013, 22 assemblies at a time in each cask, with 1331 used and 202 new ones to be moved. This was uneventful, and the task continued through 2014. By 22 December 2014, all 1331 used as well as all 202 new fuel assemblies had been moved in 71 cask shuttles without incident, with weekly updates having been published. All of the radioactive used fuel was removed by early November, eliminating a significant radiological hazard on the site. The used fuel went to the central storage pond, from which older assemblies were transferred to dry cask storage – 1004 moved since the accident by mid 2014 to make way for new inputs from unit 4. The fresh fuel assemblies are stored in the pool of the undamaged unit 6.

From 2018 Tepco expects to move 514 used fuel assemblies from unit 3 to the central pool, as well as 52 new ones and then 292 used fuel assemblies and 100 new ones from unit 1. Finally unit 2 will have its 587 used assemblies and 28 fresh ones moved. The NRA has expressed concern about the unit 1 used fuel.

Reactors 1-3 : First it is necessary to identify the locations of leaks from the primary containment vessels (PCVs) and reactor buildings using manual and remotely controlled dosimeters, cameras, etc., and indirectly analyse conditions inside the PCVs from the outside via measurements of gamma rays, echo soundings, etc. Any leakage points will be repaired and both reactor vessels (RPVs) and PCVs filled with water sufficient to achieve shielding. Then the vessel heads will be removed. The location of melted fuel and corium will then be established. In particular, the distribution of damaged fuel believed to have flowed out from the reactor pressure vessels (RPVs) into PCVs will be ascertained, and it will be sampled and analysed. After examination of the inside of the reactors, states of the damaged fuel rods and reactor core internals, sampling will be done and the damaged core material will be removed from the RPVs as well as from the PCVs. The whole process will be complex and slow, since safety remains paramount. In December Tepco estimates that the fuel will be removed from the reactors within 25 years – in line with US experience at Three Mile Island, though other estimates suggest ten years. Updated plans are on the IRID website.

The four reactors will be completely demolished in 30-40 years – much the same timeframe as for any nuclear plant. As noted above, units 5&6 were decommissioned in 2014 and will be used for training.

Earlier, consortia led by both Hitachi-GE and Toshiba submitted proposals to Tepco for decommissioning units 1-4. This would generally involve removing the fuel and then sealing them for a further decade or two while the activation products in the steel of the reactor pressure vessels decay. They can then be demolished. As noted above, removal of the very degraded fuel will be a long process in units 1-3, but will draw on experience at Three Mile Island in the USA. In January 2012 it was reported that an industry consortium (Hitachi GE Nuclear Energy, Mitsubishi Heavy Industries and Toshiba) would determine how to locate fuel debris inside units 1-3 and how to fill the pressure vessels with water.

Tepco has allocated ¥207 billion ($2.53 billion) in its accounts for decommissioning units 1-4. In March 2020, Tepco announced that ¥1.37 trillion ($12.6 billion) would be required for the removal of damaged fuel from units 2&3, with no estimates being made for the fuel in unit 1 which suffered the most damage of the three reactors.

The government has allocated ¥1150 billion ($15 billion) for decontamination in the region, with the promise of more if needed.

The new International Research Institute for Nuclear Decommissioning (IRID) has a focus on Fukushima 1-4. See section above. Its schedule for units 1-4 is:

Removal of fuel from spent fuel pools Fuel debris retrieval Unit 1 FY 2017 FY 2020 to 2022 Unit 2 FY 2017 to FY 2023 FY 2020 to 2024 Unit 3 2015 FY 2021 to 2023 Unit 4 2014 (completed) not applicable

FY 20XX ends the following March

The Agency for Natural Resources and Energy (ANRE) has calculated that as of January 2017 Tepco needs an estimated ¥22,000 billion ($191 billion), double the original estimate, to implement fuel debris removal, cleanup and for compensation to firms and individuals in Fukushima prefecture. Of this amount, Tepco will pay ¥16,000 billion. Other Japanese nuclear operators will pay ¥4,000 billion through the Nuclear Damage Compensation and Decommissioning Facilitation Corp (NDF), and the Japanese government will pay ¥2,000 billion for cleanup in Fukushima prefecture.

A 12-member international expert team assembled by the IAEA at the request of the Japanese government has reported on remediation strategies for contaminated land. The mission focused on the remediation of the affected areas outside of the 20 km restricted area. The team said that it agreed with the prioritization and the general strategy being implemented, but advised the government to focus on actual dose reduction. They should "avoid over-conservatism" which "could not effectively contribute to the reduction of exposure doses" to people. It warned the government against being preoccupied with "contamination concentrations rather than dose levels," since this "does not automatically lead to reduction of doses for the public." The team's report calls on the Japanese authorities to "maintain their focus on remediation activities that bring best results in reducing the doses to the public."

Fukushima Daini plant

Units 1-4 were shut down automatically due to the earthquake. The tsunami – here only 9 m high – affected the generators and there was major interruption to cooling due to damaged heat exchangers, so the reactors were almost completely isolated from their ultimate heat sink. Damage to the diesel generators was limited and also the earthquake left one of the external power lines intact, avoiding a station blackout as at Daiichi 1-4. Staff laid and energised 8.8km of heavy-duty electric cables in 30 hours to supplement power.

In units 1, 2&4 there were cooling problems still evident on Tuesday 15th. Unit 3 was undamaged and continued to 'cold shutdown' status on 12th, but the other units suffered flooding to pump rooms where the equipment transfers heat from the reactor heat removal circuit to the sea. Pump motors were replaced in less than 30 hours. All units achieved 'cold shutdown' by 16 March, meaning core temperature less than 100°C at atmospheric pressure (101 kPa), but still requiring some water circulation. The almost complete loss of ultimate heat sink for a day proved a significant challenge, but the cores were kept fully covered.

Radiation monitoring figures remained at low levels, little above background.

There is no technical reason for the Fukushima Daini plant not to restart. restart. However, Tepco in October 2012 said it planned to transfer the fuel from the four reactors to used fuel ponds, and this was done. In February 2015 the prime minister said that restarting the four units was essentially a matter for Tepco to decide. Local government is in favour of decommissioning. Tepco has not announced any decision about whether to retire or move towards restarting the four units.

International Nuclear Event Scale assessment

Japan's Nuclear & Industrial Safety Agency originally declared the Fukushima Daiichi 1-3 accident as Level 5 on the International Nuclear Events Scale (INES) – an accident with wider consequences, the same level as Three Mile Island in 1979. The sequence of events relating to the fuel pond at unit 4 was rated INES Level 3 – a serious incident.

However, a month after the tsunami the NSC raised the rating to 7 for units 1-3 together, 'a major accident', saying that a re-evaluation of early radioactive releases suggested that some 630 PBq of I-131 equivalent had been discharged, mostly in the first week. This then matched the criterion for level 7. In early June NISA increased its estimate of releases to 770 PBq, from about half that, though in August the NSC lowered this estimate to 570 PBq

For Fukushima Daini, NISA declared INES Level 3 for units 1, 2, 4 – each a serious incident.

Accident liability and compensation

Beyond whatever insurance Tepco might carry for its reactors is the question of third party liability for the accident. Japan was not party to any international liability convention but its law generally conforms to them, notably strict and exclusive liability for the operator. Early in 2015 Japan ratified the Convention on Supplementary Compensation for Nuclear Damage (CSC). Two laws governing liability are revised about every ten years: the Law on Compensation for Nuclear Damage and Law on Contract for Liability Insurance for Nuclear Damage. Plant operator liability is exclusive and absolute (regardless of fault), and power plant operators must provide a financial security amount of JPY 120 billion ($1.46 billion) – it was half that to 2010. The government may relieve the operator of liability if it determines that damage results from “a grave natural disaster of an exceptional character” (which it did not do here), and in any case total liability is unlimited.

In mid-April 2011, the first meeting was held of a panel to address compensation for nuclear-related damage. The panel established guidelines for determining the scope of compensation for damage caused by the accident, and to act as an intermediary. It was established within the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and was led by Law Professor Yoshihisa Nomi of Gakushuin, University in Tokyo.

On 11 May 2011, Tepco accepted terms established by the Japanese government for state support to compensate those affected by the accident at the Fukushima Daiichi plant. The scheme includes a new state-backed institution to expedite payments to those affected by the Fukushima Daiichi accident. The body receives financial contributions from electric power companies with nuclear power plants in Japan, and from the government through special bonds that can be cashed whenever necessary. The government bonds total JPY 5 trillion ($62 billion). Tepco accepted the conditions imposed on the company as part of the package. That included not setting an upper limit on compensation payments to those affected, making maximum efforts to reduce costs, and an agreement to cooperate with an independent panel set up to investigate its management.

This Nuclear Damage Compensation Facilitation Corporation, established by government and nuclear plant operators, includes representatives from other nuclear generators and will also operate as an insurer for the industry, being responsible to have plans in place for any future nuclear accidents. The provision for contributions from other nuclear operators is similar to that in the USA. The government estimates that Tepco will be able to complete its repayments in 10 to 13 years, after which it will revert to a fully private company with no government involvement. Meanwhile it will pay an annual fee for the government support, maintain adequate power supplies and ensure plant safety. Tepco estimated its extra costs for fossil fuels in 2011-12 (April-March) would be about JPY 830 billion ($10.7 billion).

On 14 June 2011, Japan's cabinet passed the Nuclear Disaster Compensation Bill, and a related budget to fund post-tsunami reconstruction was also passed subsequently.

In September 2011 the Nuclear Damage Compensation Facilitation Corporation started by working with Tepco to compile a business plan for the next decade. This was approved by the Ministry of Economy, Trade and Industry (METI) so that some JPY 900 billion ($11.5 billion) could be released to the company through bonds issued to the Nuclear Damage Facilitation Fund to cover compensation payments to March 2012. The plan also involved Tepco reducing its own costs by JPY 2545 billion ($32.6 billion) over the next ten years, including shedding 7400 jobs. This special business plan was superseded by a more comprehensive business plan in March 2012, involving compensation payments of JPY 910 billion ($11.6 billion) annually. Tepco wanted to include an electricity rate increase of 17% in the plan, to cover the additional annual fuel costs for thermal power generation to make up for lost capacity at idled nuclear power plants.

In February 2012 METI approved a further JPY 690 billion ($8.9 billion) in compensation support from the Nuclear Damage Liability Facilitation Fund, subject to Tepco's business plan giving the government voting rights. In June 2012 shareholders voted to sell the Japanese government 50.11% of Tepco's voting shares and an additional 25.73% with no voting rights, for JPY 1 trillion (about $12.5 billion), paid through the Nuclear Damage Liability Facilitation Fund. This was effected at the end of July, so that Tepco then became government-controlled, at least temporarily. Tepco said it appreciated the chance to 'transform to New Tepco'.

The government and 12 utilities are contributing funds into the new institution to pay compensation to individuals and businesses claiming damages caused by the accident. It received JPY 7 billion ($91 million) in public funds as well as a total of JPY 7 billion from 12 nuclear plant operators, the Tepco share of JPY 2379 million ($30 million) being largest. The percentage of utility contributions was fixed in proportion to the power output of their plants, so Kansai Electric Power Co. provided JPY 1229 million, followed by JPY 660 million by Kyushu Electric Power Co. and JPY 622 million by Chubu Electric Power Co. Japan Nuclear Fuel Ltd., which owns a used nuclear fuel reprocessing plant in Aomori Prefecture, provided JPY 117 million to the entity. The utility companies also pay annual contributions to the body. Tepco is required to make extra contributions, with the specific amount to be decided later.

In June 2013 Tepco requested a further JPY 666 billion ($6.7 billion) in government support through the Nuclear Damage Liability Facilitation Fund, bringing the total amount requested by Tepco to JPY 3.79 trillion ($38 billion). The company said that more than half of the latest request – some JPY 370 billion ($3.7 billion) – resulted from the re-evaluation of the evacuation zone around the damaged plant and a re-examination of the estimated amount "regarding compensation for mental damages, loss or depreciation of valuables such as housing lands and buildings." About JPY 43 billion ($431 million) was due to a higher estimate of compensation coming from damages to the agriculture, forestry and fisheries industries, as well as the food processing and distribution industries. This, it said, also resulted from "harmful rumours" about the possible health effects of consuming food products from the region near the damaged power plant. As restrictions on the transport of foodstuffs from the Fukushima area seemed set to continue, an additional JPY 240 billion ($2.4 billion) was included to cover for the further compensation claims resulting from this.

By mid-May 2014, Tepco had paid JPY 3808 billion ($38 billion) in compensation, fairly evenly split between businesses and individuals, based on decisions of the Nuclear Damage Compensation Facilitation Corporation, and covered by loans from the NDLF Fund. Some $16 billion of this was distributed evenly among 85,000 evacuees – $188,200 each person including children, as directed early in 2011.

In July 2015 the government approved Tepco’s recovery plan, including compensation payments of JPY 7075 billion ($57.2 billion), enabling it to receive JPY 950 billion more than the JPY 6125 billion estimated in April, according to METI.

In December 2013 the government raised the upper limit of its financial assistance to Tepco from JPY 5 trillion to JPY 9 trillion ($86 billion). Early in 2014 the government estimated it would take JPY11 trillion and 40 years to clean up the Fukushima site. The 2013 Japan trade deficit was JPY 11.5 trillion.

Inquiries and reports: the accident itself and decommissioning

In October 2014 the NRA published its Analysis of the TEPCO Fukushima Daiichi NPS Accident, Interim Report. A provisional translation in English was published in February 2015. This focuses on a number of questions which remained unexplained in the 2012 National Diet Investigation Commission report.

In May 2011 a team of 18 experts from 12 countries spent a week at the plant on behalf of the International Atomic Energy Agency (IAEA), and that mission's final report was presented to the IAEA Ministerial Conference in Vienna in June. At the IAEA General Conference in 2012 the Director General promised a comprehensive report which would be "an authoritative, factual and balanced assessment, addressing the causes and consequences of the accident as well as the lessons learned." The IAEA published this report in September 2015, accompanied by five technical volumes.

In February 2015 the IAEA completed its third review mission (as follow-up to that of late 2013, and involving some 180 experts from 42 IAEA member states and other organizations over two years) and reported on decommissioning to METI. In May 2015 its final report was delivered to member states, and was published in September. It is broadly positive regarding progress since 2013, but said that some challenging issues remain. It contains advisory points on topics such as long-term radioactive waste management, measures concerning contaminated water, and issues related to the removal of used fuel and fuel debris.

The Foreword to the Director General’s report in 2015 states: "A major factor that contributed to the accident was the widespread assumption in Japan that its nuclear power plants were so safe that an accident of this magnitude was simply unthinkable. This assumption was accepted by nuclear power plant operators and was not challenged by regulators or by the government. As a result, Japan was not sufficiently prepared for a severe nuclear accident in March 2011." The accident exposed "certain weaknesses" in Japan's regulatory framework, with responsibilities divided among a number of bodies. It also said there were certain weaknesses "in plant design, in emergency preparedness and response arrangement and in planning for the management of a severe accident".

The Director General said: "I am confident that the legacy of the Fukushima Daiichi accident will be a sharper focus on nuclear safety everywhere. I have seen improvements in safety measures and procedures in every nuclear power plant that I have visited. There is widespread recognition that everything humanly possible must be done to ensure that no such accident ever happens again." He added: "This is all the more essential as global use of nuclear power is likely to continue to grow in coming decades.” Furthermore, "there can be no grounds for complacency about nuclear safety in any country. Some of the factors that contributed to the Fukushima Daiichi accident were not unique to Japan. Continuous questioning and openness to learning from experience are key to safety culture and are essential for everyone involved in nuclear power."

The Executive Summary includes recommendations, but the following paragraphs indicate some salient points from the actual investigation.

Before the accident, there was a basic assumption in Japan that the design of nuclear power plants and the safety measures that had been put in place were sufficiently robust to withstand external events of low probability and high consequences. Because of the basic assumption that nuclear power plants in Japan were safe, there was a tendency for organizations and their staff not to challenge the level of safety. The reinforced basic assumption among the stakeholders about the robustness of the technical design of nuclear power plants resulted in a situation where safety improvements were not introduced promptly.

Before the accident, the operator had conducted some reassessments of extreme tsunami flood levels, using a consensus based methodology developed in Japan in 2002, which had resulted in values higher than the original design basis estimates. Based on the results, some compensatory measures were taken, but they proved to be insufficient at the time of the accident.

There were no indications that the main safety features of the plant were affected by the vibratory ground motions generated by the earthquake on 11 March 2011. This was due to the conservative approach to earthquake design and construction of nuclear power plants in Japan, resulting in a plant that was provided with sufficient safety margins. However, the original design considerations did not provide comparable safety margins for extreme external flooding events, such as tsunamis.

Despite the efforts of the operators at the Fukushima Daiichi nuclear power plant to maintain control, the reactor cores in units 1-3 overheated, the nuclear fuel melted and the three containment vessels were breached. Hydrogen was released from the reactor pressure vessels, leading to explosions inside the reactor buildings in units 1, 3 and 4 that damaged structures and equipment and injured personnel. Radionuclides were released from the plant to the atmosphere and were deposited on land and on the ocean. There were also direct releases into the sea.

Venting of the containment was necessary to relieve pressure and prevent its failure. The operators were able to vent units 1 and 3 to reduce the pressure in the primary containment vessels. However, this resulted in radioactive releases to the environment. Even though the containment vents for units 1 and 3 were opened, the primary containment vessels for units 1 and 3 eventually failed. Containment venting for unit 2 was not successful, and the containment failed, resulting in radioactive releases.

People within a radius of 20 km of the site and in other designated areas were evacuated, and those within a radius of 20-30 km were instructed to shelter before later being advised to voluntarily evacuate. Restrictions were placed on the distribution and consumption of food and the consumption of drinking water. At the time of writing, many people were still living outside the areas from which they were evacuated.

No early radiation induced health effects were observed among workers or members of the public that could be attributed to the accident.

Earlier reports 2011-2012

Early in June 2011 the independent Investigation Committee on the Accident at the Fukushima Nuclear Power Stations (ICANPS), a panel of ten experts, mostly academics and appointed by the Japanese cabinet, began meeting. It has two technological advisers. An initial report was published in December 2011 and a final report in July 2012. The panel set up four teams to undertake investigations on the causes of the accident and ensuing damage and on measures to prevent the further spread of damage caused by the accident, but not to pursue the question of responsibility for the accident.

The national Diet later set up a legally-constituted Nuclear Accident Independent Investigation Commission (NAIIC, or National Diet Investigation Commission) of ten members which started its work in December 2011. One of the purposes of NAIIC is to provide suggestions including the “re-examination of an optimal administrative organization” for nuclear safety regulation based on its investigation of the accident. NAIIC reported in July 2012, harshly criticizing the government, the plant operator and the country’s national culture. After conducting 900 hours of public hearings and interviews with more than 1,100 people and visiting several nuclear power plants, the commission’s report concluded that the accident was a “manmade disaster,” the result of “collusion between the government, the regulators and Tokyo Electric Power Co.” It said the “root causes were the organizational and regulatory systems that supported faulty rationales for decisions and actions.” The NAIIC criticized the regulator for insufficiently maintaining independence from the industry in developing and enforcing safety regulations, the government for inadequate emergency preparedness and management, and Tepco for its poor governance and lack of safety culture. The report called for fundamental changes across the industry, including the government and regulators, to increase openness, trustworthiness and focus on protecting public health and safety.

The NAIIC Chairman wrote: "What must be admitted – very painfully – is that this was a disaster 'Made in Japan.' Its fundamental causes are to be found in the ingrained conventions of Japanese culture: our reflexive obedience; our reluctance to question authority; our devotion to 'sticking with the program'; our groupism; and our insularity.” The mindset of government and industry led the country to avoid learning the lessons of the previous major nuclear accidents at Three Mile Island and Chernobyl. "The consequences of negligence at Fukushima stand out as catastrophic, but the mindset that supported it can be found across Japan. In recognizing that fact, each of us (every Japanese citizen) should reflect on our responsibility as individuals in a democratic society."

NAIIC reported that Tepco had been aware since 2006 that Fukushuima Daiichi could face a station blackout if flooded, as well as the potential loss of ultimate heat sink in the event of a major tsunami. However, the regulator, NISA, gave no instruction to the company to prepare for severe flooding, and even told all nuclear operators that it was not necessary to plan for station blackout. During the initial response to the tsunami, this lack of readiness for station blackout was compounded by a lack of planning and training for severe accident mitigation. Plans and procedures for venting and manual operation of emergency cooling were incomplete and their implementation in emergency circumstances proved very difficult as a result. NISA was also criticised for its "negligence and failure over the years" to prepare for a nuclear accident in terms of public information and evacuation, with previous governments equally culpable. Then Tepco’s difficulty in mitigation was compounded by government interference which undermined NISA.

On 7 June 2011 the government submitted a 750-page report to IAEA compiled by the nuclear emergency taskforce, acknowledging reactor design inadequacies and systemic shortcomings. It said that "In light of the lessons learned from the accident, Japan has recognized that a fundamental revision of its nuclear safety preparedness and response is inevitable."

On 11 September 2011 a second report was issued by the government and submitted to the IAEA, summarising both on-site work and progress and off-site responses. It contained further analysis of the earthquake and tsunami, the initial responses to manage and cool the reactors, the state of spent fuel ponds and the state of reactor pressure vessels. It also summarised radioactive releases and their effects.

Meanwhile a July 2011 report from MIT's Centre for Advanced Nuclear Energy Systems provided a useful series of observations, questions raised, and suggestions. Its Appendix has some constructive comment on radiation exposure and balancing the costs of dose avoidance in circumstances of environmental contamination.

In November 2011 the US Institute of Nuclear Power Operators (INPO) released its Special Report on the Nuclear Accident at the Fukushima Daiichi Nuclear Power Station, with timeline. This 97-page report gives a valuable and detailed account of events.

Also in November 2011 the Japan Nuclear Technology Institute published a 280-page report on the accident, with proposals to be addressed in the future.

On 2 December 2011 Tepco released its interim investigation report on the accident (in Japanese).

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) is undertaking a 12-month study on the magnitude of radioactive releases to the atmosphere and ocean, and the range of radiation doses received by the public and workers.

An analysis by the Carnegie Endowment in March 2012 said that if best practices from other countries had been adopted by Tepco and NISA at Fukushima, the serious accident would not have happened, underlining the need for greater international regulatory collaboration.

In April 2012 the US Electric Power Research Institute (EPRI) published Fukushima Daiichi Accident – Technical Causal Factor Analysis, which identified the root cause beyond the flooding and its effects as a failure to consider the possibility of the rupture of combinations of geological fault segments in the vicinity of the plant.

Inquiries and reports: radiation effects

A preliminary report from the World Health Organisation (WHO) in May 2012 estimated the radiation doses that residents of Japan outside the evacuated areas received in the year following the accident. The report's headline conclusion is that most people in Fukushima prefecture would have received a radiation dose of between 1 and 10 mSv during the first year after the accident. This compares with levels of about 2.4 mSv they would have received from unavoidable natural sources. In two places the doses were higher – between 10 and 50 mSv, still below any harmful level. Almost all were “below the internationally-agreed reference level for the public exposure due to radon in dwellings” (about 10 mSv/yr).

The UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in May 2012 reported that despite skin contamination of several workers, no clinically-observable effects have been reported and there is no evidence of acute radiation injury in any of the 20,115 workers who participated in Tepco’s efforts to mitigate the accident at the plant. Eighteen UNSCEAR member states provided 72 experts for the assessment. UNSCEAR also surveyed Fukushima prefecture tol compare its data with Japanese measurements of exposures of some 2 million people living there at the time of the accident.

The results of UNSCEAR’s 12-month study on the magnitude of radioactive releases to the atmosphere and ocean, and the range of radiation doses received by the public and workers were announced in May 2013 are reported above in the subsection on Radiation Exposure.

UNSCEAR’s final report of radiation effects was released in April 2014. This concluded that the rates of cancer or hereditary diseases were unlikely to show any discernible rise in affected areas because the radiation doses people received were too low. People were promptly evacuated from the vicinity of the nuclear power plant, and later from a neighbouring area where radionuclides had accumulated. This action reduced their radiation exposure by a factor of ten, to levels that were "low or very low." Overall, people in Fukushima are expected on average to receive less than 10 mSv due to the accident over their whole lifetime, compared with the 170 mSv lifetime dose from natural background radiation that people in Japan typically receive. "The most important health effect is on mental and social well-being, related to the enormous impact of the earthquake, tsunami and nuclear accident, and the fear and stigma related to the perceived risk of exposure to radiation."