TO CALL it a hot ticket might, in the circumstances, seem a tad tasteless. But no session at this year's International Conference on Advances in Nuclear Power Plants, held in the south of France, was as well attended as the late-running special plenary hastily arranged to provide an update on the nuclear disaster at the Fukushima Dai-ichi plant in Japan and its attendant radioactive leaks. Akira Omoto of the University of Tokyo, an industry veteran who is a member of Japan's Atomic Energy Commission, Hideki Masui, who works on seismic hazards at TEPCO, the plant's operator (where Mr Omoto has also been employed), and Kenichi Sato of GE Hitachi, which makes the type of reactor involved, provided a range of updates and insights (each of them was keen to stress that he was speaking in a personal capacity).

Much of what they had to say was already familiar to many in the audience; the subject has been closely followed in the nuclear industry. But the sheer power of the story, some fresh details, and the continuing mysteries about some of its particular twists, kept the audience captivated. And hearing the story from people as in the know as anyone is likely to clarify some things—including which questions need further clarification.

Mr Omoto stressed that it was not the earthquake, nor the tsunami, which doomed the plant, but the combination of the two. The earthquake in and of itself did not do too much damage; it shook the reactor buildings slightly more than they were designed to be shaken, but they were built well and seem not to have suffered much harm. The three reactors running at the time shut down as they were meant to. But the earthquake did one crucial other thing: it knocked out the connections which brought electricity from the grid to the power plant. After the earthquake, the plant was on its own. Its sister plant, Fukushima Dai-ni, stayed on the grid through the earthquake and the tsunami; if it hadn't then things there, too, might taken a drastic turn for the worse.

The earthquake's effects had been only a bit worse than Fukushima's designers had expected; the tsunami which arrived just under an hour later was much worse. Fukushima Dai-ichi, which sits ten metres above sea level, was originally designed to withstand a 3.1 metre wave. A “Severe Accident Management” review in 2002 increased estimates of the tsunami threat at all Japan's power plants, raising the average wave height they needed to be designed to cope with to about double the height of the biggest waves in the historical record. The maximum wave to be planned for at Fukushima was pushed up to 5.7 metres. The March 11th wave was about 15 metres. It smashed the plant's sea water intake systems, flooded electrical switching facilities and diesel generators, and carried off diesel fuel tanks; because of the layout of the site the water did considerable further damage as it flowed back out.

All this meant that the plant no longer had the facilities to cool its reactors which, though shut down, were still generating plenty of heat that had to be dealt with. Cooling systems that didn't require alternating current—which is what the grid or the diesel generators would have provided—worked for a while, but eventually failed. The reactors began to overheat and damage themselves. One implication of this is that designers should think about external challenges to their reactors coming in pairs—and not necessarily pairs which share a common cause, like earth tremors and tsunamis.

Triple whammy

Another insight the presentations provided, though, was that Fukushima was in some ways not merely a double blow but a triple one. As well as being rocked by the earthquake and swamped by the tsunami, the plant was later hit by a sequence of explosions due to hydrogen leaking from the damaged reactors. This should not have happened. According to Mr Sato, the "Severe Accident Review" which included the reassessment of the tsunami risk was a decision to put in hardened-vent systems, which had been required in similar American reactors for a decade. These were designed to make sure that the containment vessels surrounding the reactors could have gas bled off from them safely if they became over-pressurised, whatever else was going on. Any hydrogen that was released into the airtight containment vessels after the cladding of the fuel rods in the reactors themselves began to oxidise in the now-hotter-than-designed-for steam should in principle either have stayed in the containment vessels or have been vented.

One way or another things did not work out like that; either the hydrogen got out of the containment vessels through some part of the venting systems but then failed to make it all the way to the open air, as it should have done, or it simply leaked through the seals on top of the containment vessels (the latter explanation seems to make most sense to many, but Mr Omoto insisted that the precise source of the explosive gas is as yet not clear to him and his colleagues). Either way hydrogen ended up in the space inside the reactor buildings but outside the containment vessels, where it mixed with fresh air. The subsequent explosions blew the tops off reactor buildings 1 and 3.

The blast that decapitated unit 3 did more damage, perhaps because there was more hydrogen involved or perhaps because the building was built differently, using concrete, not steel. The two are not entirely independent, as different designs and materials could have affected the way hydrogen accumulated; indeed listening to the talks provided a strong sense of how in trying to understand such disasters nothing which goes on can be taken as independent of everything else.

The blast at unit 3 was not only particularly destructive of that building. It also seems, Mr Omoto said, to have opened up a “blowout panel” in the side of building 2. That may explain why building 2 kept its roof on—the open panel could have let hydrogen out from the upper levels or have lessened the shock of an explosion in the enclosed space within. That said, if the opened panel is in any way related to the fact that the eventual explosion in building 2 took place lower down in the structure there is no reason to cheer. The lower explosion seems to have been responsible for releasing a lot of radioactive water from unit 2, probably from the containment vessel, into the area below the adjacent turbine hall. That radioactive water, some of which leaked out into the sea, now represents a huge radiological challenge.

The explosions could be conceived of as a third independent blow because of the ways in which they made the bad situation worse. They splattered radioactive and other debris around, making it even harder to move through the plant and get things done. This splattering also muddied the picture of what needed fixing; radioactive debris scattered hither and yon made it harder to work out what the most important sources of radiation were. This seems to have been part of what led to the confusion over the spent-fuel pool in reactor building 4, which, like the precise processes that led to the hydrogen explosions, remains something of a mystery.

Like units 1 and 3, unit 4 lost its roof. Unlike those two, though, building 4 had no fuel in its reactor, and thus no obvious source of hydrogen. What it did have was a lot of spent fuel being kept in a pool near the roof of the building. Spent fuel, like the core of a shut-down reactor, is still hot and needs cooling, though not as much as of it as the reactor core. Unit 4's pool contained a lot of relatively hot spent fuel. One possible cause of its de-roofing seemed to be that the spent fuel started to boil off its protective water because the cooling pumps for its pool had stopped working. The overheated fuel then produced hydrogen and subsequently exploded. This scenario and some radiation measurements led to the belief, promulgated by Gregory Jaczko, chairman of America's Nuclear Regulatory Commission, that the pool in unit 4 had lost most or all of its water. That would have been very serious; damaged fuel exposed directly to air can give off all sorts of nastiness, including gamma rays. TEPCO said at the time that this had not happened, but a loss of cooling in the spent fuel pools was obviously a concern. Powerful fire hoses and water drops from helicopters were hastily brought into play before a remarkably long-armed concrete pouring vehicle—the Putzmeister—proved a reliable topper-up of last resort.

The Japanese now say that the spent-fuel pool in building 4 never boiled dry, as was feared. The fuel in it looks intact in aerial photographs, and samples retrieved from the pool are said to rule out disastrous overheating of the fuel, though no data to confirm this were presented in Nice. Radiation from the debris spread around by the earlier explosions—“Fukushima background”, as Mr Sato put it—may have confused people into thinking that there was radiation coming from that spent fuel. Which leaves the question of what it was, exactly, that took the roof off building 4: a knock-on effect from the particularly dramatic explosion at building 3 might be one possibility. One conclusion that seems very likely from the Fukushima post mortems is that building a row of reactors domino-close to each other is a poor idea.

Steady as she goes

Almost two months on, the situation is much more stable. Systems have been set up for cleaning some of the contaminated water on the site, which among other things provide the 500 tonnes a day that are being used to cool the reactors down. Proper cooling systems that bleed heat off to the air are being installed, as are permanent cooling systems for the spent-fuel pools. Work is being undertaken to reinforce the rickety structure in the buildings. TEPCO hopes to bring the reactors to cold shutdown—which means getting them below the boiling point of water—within six months, though it is not committed to a strict schedule. That will still leave years, even a decade, of hard and expensive work decommissioning the site. The amount of contaminated water that will have to be dealt with is remarkable.

Cold shutdown does not necessarily mean an early answer to one of the other great question remaining: how badly did things go in the reactor cores? Various clues can be gleaned from radiation measurements, but it remains unclear just how much fuel melted in the various reactors and what amount if any escaped the reactor vessels and the containment (while staying confined on the site). Indirect evidence should help get the answer in the months to come. Direct evidence will take a lot longer. In Nice Marie Pierre Comets, who runs France's nuclear regulator, pointed out that it was not until six years after the Three Mile Island accident that a robot finally got into the reactor pressure vessel and revealed what it looked like.

A thorough account of the damage done and the lessons to be learned from Fukushima should be available a lot sooner than that; not everything has to be seen to be appreciated. But the Japanese do not seem to be rushing. The presidential commission on America's Deepwater Horizon disaster was up and running less than a month after the rig exploded—the governmental inquiry into Fukushima seems not to be up to speed almost two months on. That is in part because a lot of Japan's nuclear expertise is still being used to manage the ongoing response, but it is clear that many would now like to see it a priority.

Delegates from other countries, too, though grateful to their Japanese colleagues for the enlightenment they offered in Nice, would welcome access to the sort of information that an inquiry would amass and analyse sooner rather than later. That investigation should show whether the hydrogen explosions could have been avoided if the operators had acted differently, which is clearly a question on many minds. It will also detail flaws in design, and quite possibly in risk assessment and maintenance, too. But it may also show the workings of a currently under-appreciated success.

The fact that Japan organised a prompt evacuation, provided iodine pills and kept radioactive material out of the food chain means that experts expect Fukushima to have a negligible public-health effect, at least in terms of radiation (stress, fear and being removed from one's home are forms of harm less easily measured). Less widely acknowledged is how well safety procedures for the staff in the plant appear to have worked. According to Mr Masui, not a single worker at the Fukushima Dai-ichi plant received a radiation dose of more than 250 milliSieverts, the raised limit for emergency responders set by government dispensation. That is five times the maximum annual dose for a nuclear worker, but it is well short of what is seen as a serious health risk. Given the circumstances—the lack of instrumentation, the explosions, the power outage, the psychological pressure, the possibility of bereavement and so on—the disciplined behaviour needed to avoid really bad exposures has been impressive. There is undoubtedly a lot wrong with the culture of Japan's nuclear establishment, and various plants have had well-chronicled safety lapses. In this particular case, though, at least one part of the safety culture of the sorely tried workers seems to have held up remarkably.