THE precise details of what has gone wrong at the nuclear power plants in north-eastern Japan following the magnitude 9.0 earthquake that struck the area on March 11th remain hazy. But a picture is beginning to emerge as events unfold and information is made available by the plants' operators and the Japanese authorities.

Start with the basics. Nuclear energy is produced by atomic fission. A large atom (uranium or plutonium) breaks into two smaller ones, releasing energy and neutrons. These neutrons may then trigger the break-up of further atoms, creating a chain reaction. The faster the neutron, the fewer break-ups it provokes. This is because an incoming neutron has to be captured to provoke fission, and fast neutrons are harder to capture. As a result, the chain reaction will peter out unless the neutrons can be slowed down sufficiently.

There also need to be enough fissionable atoms about for the neutrons to bump into—in other words, a critical mass. That is why uranium fuel has to be enriched, for only one of the two naturally occurring isotopes of the metal is fissile, and it is much the rarer of the two. In water-cooled reactors like the ones at Fukushima, the right combination of slow neutrons and enriched fuel leads to a self-sustaining process which produces energy that can be used to boil water, make steam and drive a turbine to generate electricity. Besides cooling the fuel (and thus producing the steam) the water also acts as a so-called moderator, slowing down the neutrons and keeping the reaction going.

So what happens when things cease to run smoothly, as when an earthquake interferes with the plant's systems? When designing reactors, engineers attempt to achieve what they call “defence in depth”. The idea is that if any specific defence fails, another will make good the shortfall. This is a principle that Fukushima Dai-ichi, the worst hit of the nuclear plants, has been testing to destruction. The defences have failed badly at all three of the reactors which were running at the time the earthquake hit.

Some defences are simply barriers. The pellets of nuclear fuel are encased in hard alloys based on zirconium (which lets neutrons pass freely through), to make fuel rods. The reactor core which includes these rods, and the water it sits in, are contained within a thick steel pressure vessel. That, in turn, sits within a larger steel structure, the primary containment vessel. Around all this sits the steel and concrete of the secondary containment structure.

Other defences are actions, rather than things, some of them automatic and some of them not. The first action to be taken in the case of an earthquake is an emergency shutdown, which is achieved by thrusting control rods that sit below the reactor up into the reactor's core. These rods, made of neutron-absorbing materials such as boron, mop up excess neutrons and quench the chain reaction.

However, there are other nuclear reactions in a core that do not depend on neutrons. Some byproducts of the nuclear fission are themselves radioactive. These decay, producing heat. Though that heat is but a fraction of a reactor's normal power—about 3%, in the case of the Fukushima machines—it is still similar to what comes out of a commercial jet engine operating at full throttle. That can warm things up pretty quickly in the absence of a cooling system.

So, the next action needed is to get a set of pumps running to keep cool water flowing into the reactor vessel and the consequent steam coming out. This seems to have happened according to plan, thanks to back-up diesel generators. At Fukushima Dai-ni, Fukushima Dai-ichi's sister plant about 11 kilometres away, there were problems with some of the cooling systems. These, though, have been put to rest and the plant is now fully shut down. At Fukushima Dai-ichi, meanwhile, the generators and the power system they drove did not survive the tsunami, which hit about an hour after the earthquake and was much larger than the designers had been told to prepare for. Among the problems was the fact that crucial electrical switching equipment was in a basement, and therefore got flooded.

Attempts to get the cooling system working with batteries and generators brought in from elsewhere were insufficient, and without a flow of coolant, the cores of the three reactors at Fukushima Dai-ichi began to heat up. More heat means more steam and less water in the reactor vessel. If the water level drops far enough, the fuel rods of the reactor proper, which are meant to stay submerged, will be exposed to the steam and other gases. That means they heat up even more quickly, possibly beyond the temperature they can cope with. The fuel rods' zirconium casings then begin to react with the steam. This produces, among other things, hydrogen gas. Also, the contents of the rods and then the rods themselves begin to melt.

This appears to have happened in the Fukushima reactors. On Saturday Japan's Nuclear and Industrial Safety Agency reported that the temperature inside one reactor reached a staggering 2,700°C and the pressure shot up above 8 atmospheres (compared with the 4 atmospheres experienced during normal operation). To prevent the containment vessel from cracking or blowing open altogether, releasing a cloud of radioactive smoke into the air, the plant's operators vented some of the steam and hydrogen, bringing the pressure down to a more comfortable 5.5 atmospheres.

Steam, contaminated with some radioactive elements, flowed out of the pressure vessel and into the larger containment vessel that surrounds it. This primary containment vessel looks like an old-fashioned light bulb turned upside down and balanced on top of a doughnut. The reactor's pressure vessel is a cylinder suspended in the light bulb's neck. The doughnut contains a large reservoir of water, and steam released into the containment vessel is meant to end up condensed in that reservoir. This, though, seems not to have happened as it was meant to.

Pressure built up in the primary containment, too, and it seems that the operators then released steam laced with hydrogen into the secondary containment structures—in other words, the building housing the reactor—first in unit one, then in unit three. In both cases, that release was followed by an explosion when something sparked the hydrogen. (It is also possible that the hydrogen did not come from the reactors at all; there are various uses for hydrogen in such plants and it may have been some of this which exploded.) The explosions were spectacular and unnerving, but were also, like the venting, better than the alternative. The secondary containment is meant to lose its roof if there is a blast within it, because otherwise such a blast will do more damage to the primary containment.

With the primary containment vessel of unit one still too full of steam, the radical decision was made to flood it with seawater. This greatly increased the amount of water available to soak up the heat, and should have made sure that the reactor was fully covered by water and thus not hot enough to damage itself any further. The seawater was also laced with boric acid, to soak up stray neutrons that the control rods missed. This treatment is, among other things, a death warrant for the reactors. Such flooding soils them to the point of rendering them useless.

The flooding has other disadvantages, too. Radioactive steam will still bubble up to the top of the water in the containment vessel, but will be harder to vent as the systems for doing so in an orderly manner are now underwater. And should the containment vessel spring a leak, then some of the water, containing radioactive debris, may flow out and contaminate other parts of the site. At unit three the plant managers decided to flood only the pressure vessel hanging in the neck of the containment vessel, not the whole of the primary containment.

At unit two, the decision was apparently made to put seawater only into the reactor vessel, but the intervention went awry. At some point, the entire reactor inside the pressure vessel was above the waterline: the kettle had boiled dry. After that, in the early morning of Tuesday 15th, there were reports of possible explosions within the building and damage to the doughnut at the base of the primary containment. It seems likely that this unit, unlike the other two, may have suffered significant damage to its primary containment. If this is the case, there may be widespread contamination at the site.

It could have been worse. If the zirconium melts, the fuel pellets embedded in it can melt, too, sinking to the bottom of the pressure vessel. If enough molten fuel gathers this way, a critical mass may be assembled, reigniting the fission reaction. The fuel could also burn through the vessel and start forcing radioactive steam continuously into the sky and spreading it around. (The fire within the reactor core at Chernobyl, which had only token containment, did this quite effectively.) This set of events is often referred to as a meltdown, though the word is not recognised as a term of art by the nuclear industry or its regulators.

So far, this most dramatic turn of events has not come to pass. The levels of radioactivity recorded around the site are high, but are unlikely to do great harm beyond it. Unlike Chernobyl, there is no obvious mechanism for spreading the damage at Fukushima Dai-ichi, though there could be further explosions if, on melting, the red-hot fuel hits a body of cold water and vaporises it explosively.

But though the reactors themselves cannot burn, other parts of the plant can and have—and that seems to have produced the most severe radiation hazard yet. On March 15th a fire started in the building housing reactor 4, possibly as a result of further hydrogen leaks. The reactor currently contains no fuel (like two more reactors at the plant, it was off line for maintenance when the quake struck) but there is spent fuel in storage tanks in the building. The fire is thought to have got to this fuel, which should have been entirely submerged, but may well not have been. Spent fuel tanks in the other buildings may also have been damaged by the hydrogen explosions. In response to this, when the fire was out the plant's owners, the Tokyo Electric Power Company, evacuated all personnel other than those involved in seawater pumping.

This decision seems to have been prompted by the fact that in the aftermath of the fire, the radiation levels outside the plant became truly dangerous for the first time, and within—where there had already been a number of casualties—things had got even worse. Background radiation delivers the average person a radiation dose of about 3 millisieverts (mSv) a year. In Europe and America, workers at nuclear plants are meant to receive at most 50mSv a year. People outside the plants should receive less than 1mSv a year on top of their background dose. On March 15th the plant reported a dose at the perimeter of 11.9mSv an hour (though this dropped back to 6mSv), meaning a worker's maximum permitted dose for a year would be exceeded in a single shift. Inside the plant higher levels have been noted, with monitors between the buildings reporting cumulative doses of up to 400mSv.

The immediate vicinity of the plant, out to 20 kilometres, has been evacuated. People between 20 and 30 kilometres away are being urged to stay in their homes. Radiation has been detected further afield than this, but at low levels. In general, experiencing radiation levels a few times higher than background for a few hours is not considered harmful. That said, there is every likelihood of further releases—and it is impossible completely to rule out large ones if the situation at unit two worsens.

According to the International Atomic Energy Agency, all three reactors have now been flooded with seawater. The pressure in the pressure vessels and containments of reactors one and two is reported to be stable. A release of pressure from the containment vessel of number 2 reactor appears to be planned. The status of the various spent fuel stashes is not clear, which is disturbing. In the end they may contribute more contamination to the environment, especially that beyond the immediate vicinity, than the reactors themselves.

It is too early to say how Fukushima fared with the calamity, all things considered. Much of the damage seems to have been caused by the tsunami wrecking the diesel generators—a single failure that resulted in a series of others, and was, in turn, compounded by them. Surely, though, planning for such contingencies can reasonably be considered part and parcel of the technology writ large. And this failed on too many fronts. Switching rooms were flooded. Auxiliary power systems failed. And that is before the full extent of the damage suffered by the reactors is known for sure. True, in accordance with safety regulations, these were designed to withstand tremors of magnitude 8.2. That they survived relatively unscathed through a magnitude 9.0 earthquake—ie, one that, given the scale's logarithmic nature, was approximately 15 times more powerful—seems remarkable. But an expensive installation was ruined, lives were lost and hazardous levels of radiation leaked into the atmosphere. If no further harm is done, an amount of damage comparatively small when set against the many thousands of lives lost across all affected areas might be seen as a victory—but hardly one to celebrate.