In its final report, TEPCO concluded that Fukushima Daiichi plant manager Masao Yoshida and other utility executives, including those at headquarters, carried out the seawater injection following appropriate procedures and didn’t delay the exercise.

In video footage recently released by the utility, conversations are documented where TEPCO staff in Tokyo request staff to delay the use of seawater in the reactor.

“We think using seawater in a hasty way would be wasteful because materials will be corroded,” an unidentified company official at Tepco headquarters in Tokyo is heard telling then plant manager Masao Yoshida two days after the quake.

“We don’t have the option to use fresh water. That will cause further delays,” Yoshida replies, emphasizing there was no time to find enough fresh water to do the job.

In September, 2011, officials and experts from the Japan Atomic Energy Agency said their research had indicated that the meltdown of reactor number 2 could have been avoided if water was injected to cool the reactor 4 hours earlier. Water injection was started at 8 PM on March 14 after the cooling system failed at 1 PM that day. The meltdown could have been avoided if injection had started by 4 PM.

Could the meltdown have been avoided if injection had started without delay? How much damage could have been mitigated?

March 14th 2011 – the Unit 2 RCIC system shut down and the fuel rods were fully exposed, and despite worker attempts to refill the reactor with water they were unable to prevent the rods from being fully exposed again. The explosion at Unit 3 had damaged the temporary cooling systems at Unit 2, which was also having problems with its venting system

In the early hours of March 15th, an explosion was heard supposedly damaging the pressure-suppression system, and the radiation levels on site rose over the legal limits, prompting TEPCO to evacuate most of its workers, leaving under a hundred men on-site. By 20:00, the majority of the fuel had dropped to the bottom of the reactor pressure vessel.

March 16th 2011 – A TEPCO press release states that workers had been withdrawn at 06:00 JST because of abnormal noises coming from one of the reactor pressure suppression chambers.

March 21st 2011 – TEPCO is still unable to restore power and the cooling pumps were damaged beyond repair, the utility placed an emergency order for new pumps for Reactor 2.

March 22nd 2011 – Steam still being released and rising from Reactor 2

March 27th 2011 – TEPCO reported measurements of very high radiation levels, over 1000 mSv/h, in the basement of the Unit 2 turbine building (outside the containment structure). Japan’s Nuclear and Industrial Safety Agency said that “The level of radiation is greater than 1,000 millisieverts. It is certain that it comes from atomic fission … But we are not sure how it came from the reactor.” An aerial video recorded by a Ground Self-Defense Force helicopter showed steam emanating from Reactor 2, and revealed that the roof had been badly damaged.

March 28th 2011 – The Japanese Nuclear Safety Commission states that it “assumed” melted fuel rods in Unit 2 have released radioactive substances into the coolant water, which subsequently leaked out through an unknown route to the Reactor 2 turbine building basement. To reduce the amount of leaking water, TEPCO reduced the amount of water pumped into Unit 2, from 16 tons per hour to 7 tons per hour, which lead to higher reactor temperatures.

March 29th 2011 – Richard Lahey, former head of safety research for boiling-water reactors at General Electric, speculated that the reactor core may have melted through the reactor containment vessel onto a concrete floor, raising concerns of a major release of radioactive material

March 30th 2011 – NISA released some concerns about a possible Unit 2 breach at either the suppression pool, or the reactor vessel, saying that “air may be leaking”, very probably through “weakened valves, pipes and openings under the reactors where the control rods are inserted”, but that “there is no indication of large cracks or holes in the reactor vessels”.

April 2nd 2011 – TEPCO admits for the first time that contaminated water from the unit 2 is flowing into the sea. Workers had discovered a crack about 20 cm (8 inches) wide in the maintenance pit, which lies between Reactor 2 and the sea, and holds cables used to power seawater pumps. Workers poured concrete into the crack to stop the water, which was emitting radiation at 1 Sv/h.

April 3rd 2011 – The attempt to plug the leak near unit 2 fails when the concrete fails to set. TEPCO then reattempts to plug up the trench that leads to the damaged storage pit with a combination of superabsorbent polymer, sawdust and shredded newspaper, which also fails. Radioactive water continues to leak into the sea. Radiation levels around the plant are estimated at 1 Sv/h.

April 5th 2011 – It is determined that the leak into the cable storage pit at Unit 2 was likely due to a faulty joint where the pit meets a duct. The pit leads to a gravel layer beneath, resulting in highly radioactive water pouring directly into the sea. Levels of radioactive iodine-131 in seawater near the facility are found to be 7.5 million times the legal limit. TEPCO drills a hole into the pit near reactor 2, from which highly radioactive water is leaking, and inject water glass (sodium silicate) into the pit to prevent further leaking.

April 11th 2011 – NHK reported that radioactive water filling a tunnel near Unit 2 had risen 12 cm since a leak in a trench was injected with water glass on April 6th.

April 15th 2011 – Nuclear fuel is reported to have melted and fallen to the lower containment sections of reactors 1, 2 and 3.

April 18th 2011 – Robots entered Reactor 2, but were severely hindered by fogging on the robot’s camera lens from the high humidity inside of the reactor building (over 90%).

April 19th 2011 – TEPCO begins transferring excess radioactive cooling water from the reactor 2’s basement and maintenance tunnels to a waste processing facility.

April 27th 2011 – TEPCO revised its estimate of damaged fuel in Unit 2 from 30% to 35%.

May 15th 2011 – TEPCO stated that the pressure vessel “is likely to be damaged and leaking water at Units 2 and 3”, which means most of the thousands of tons of water pumped into the reactors had leaked.

May 18th 2011 – Four workers in protective suits and SCBA enter unit 2 for the first time since the March 15 explosion, to check on radiation levels and other conditions inside the building. The workers receive a dose of between 3 and 4 mSv each.

May 23rd 2011 – TEPCO admits that Reactor 2 suffered a “meltdown” about 100 hours after the earthquake.

May 29th 2011 – TEPCO announces that cooling has been restored for Unit 2 spent fuel pool.

June 8th 2011 – A Japanese government report states that nuclear fuel has possibly “melted through” the base of the pressure vessels in the first three reactors and says the implications may be more severe than a meltdown scenario.

June 21st 2011 – A radiation reading of 430 millisieverts per hour is recorded in a mezzanine between the first floor and basement of Reactor 2.

September 16th 2011 – Research indicates that the meltdown of reactor number 2 could have been avoided if water was injected to cool the reactor 4 hours earlier. Water injection was started at 8 PM on March 14 after the cooling system failed at 1 PM that day. The meltdown could have been avoided if injection had started by 4 PM.

October 30th 2011 – A French study by the Institute for Radiological Protection and Nuclear Safety revealed that the Fukushima nuclear disaster caused the biggest discharge of radioactive material into the ocean in history.

November 2nd 2011 – Boric acid is injected into reactor number 2 after the discovery of xenon in its containment vessel.

November 30th 2011 – TEPCO reports that a new computer simulation of the meltdown shows nuclear fuel rod material melting through the pressure vessel and deep inside the concrete of the primary containment vessel at Units 1, 2, and 3.

December 18th 2011 – 230 tons of highly radioactive water is discovered in a tunnel below Reactor 2. TEPCO admits that this radioactive water may be mixing with the ground water, yet claims that the tunnel is not connected to the sea. The government orders TEPCO to survey the rest of the underground facilities for more radioactive water collections.

January 19th 2012 – An attempt is made to view the state of the melted fuel in reactor 2 using a fibre-optic endoscope, however clear images of the water level and fuel location could not be obtained. TEPCO finds an additional 500 tons of water containing 16,200 Bq/cm3 of radioactive caesium in a pit near reactor 2 as a result of additional inspections ordered by government.

March 27th 2012 – TEPCO measures atmospheric radiation at several points inside the containment vessel of reactor 2 for the first time, and reports values of 31.1 and 72.9 Sv/h. The utility states that radiation is too high for robots, endoscopes, and other devices to work properly.

Source: Reuters

Source: TEPCO

Source: TEPCO

Source: Wikipedia

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