Analysis Japan's nuclear powerplants have performed magnificently in the face of a disaster hugely greater than they were designed to withstand, remaining entirely safe throughout and sustaining only minor damage. The unfolding Fukushima story has enormously strengthened the case for advanced nations – including Japan – to build more nuclear powerplants, in the knowledge that no imaginable disaster can result in serious problems.
Let's recap on what's happened so far. The earthquake which hit on Friday was terrifically powerful, shaking the entire planet on its axis and jolting the whole of Japan several feet sideways. At 8.9 on the Richter scale, it was some five times stronger than the older Fukushima plants had been designed to cope with.
If nuclear powerplants were merely as safe as they are advertised to be, there should have been a major failure right then. As the hot cores ceased to be cooled by the water which is used to extract power from them, control rods would have remained withdrawn and a runaway chain reaction could have ensued – probably resulting in the worst thing that can happen to a properly designed nuclear reactor: a core meltdown in which the superhot fuel rods actually melt and slag down the whole core into a blob of molten metal. In this case the only thing to do is seal up the containment and wait: no radiation disaster will take place1, but the reactor is a total writeoff and cooling the core off will be difficult and take a long time. Eventual cleanup will be protracted and expensive.
In fact, though the quake was far beyond design limits, all the reactors went into automatic shutdown perfectly: triumph number one. Control rods slammed into the cores, absorbing the neutrons spitting from the fuel rods and pinching off the uranium-fission chain reactions powering the plant.
However, the cores were still producing heat and radiation at this point: intermediate radioactive isotopes of caesium and iodine are created during normal running. They have short half-lives and decay to insignificant levels within days of a shutdown, but for that time the reactor will still produce a few per cent of the heat it puts out in normal running – and this is still a lot of heat. If it is not dealt with, it can eventually melt down parts of the core, though the resulting mess will not be nearly as bad as a runaway meltdown.
Thus, even with the control rods in, the core still needs to be cooled for some days until the "residual" heating dies away and so power and water need to be supplied for this purpose. Backup cooling driven by diesels came on at all the plants without trouble, despite the way-beyond-spec hit from the quake: triumph number two.
For a few hours all was well. Then the tsunami – again, bigger than the plant had been built to cope with – struck, knocking out the diesel backups and the backup diesel backups.
Needless to say, this being a nuclear powerplant, there was another backup and this one worked despite having been through a beyond-spec quake and the tsunami. Battery power cut in and the cores continued to be cooled, giving the plant operators some hours of leeway to bring in mobile generators: triumph number three.
Unfortunately it appears that the devastation from the quake and tsunami was sufficient that mobile power wasn't online at all the sites before the temperatures inside the cores began to climb seriously. At this stage the cores are sitting immersed in cooling water inside their terrifically thick and strong airtight containment vessels. As the water is not being circulated and cooled any more, it is getting hotter, turning to steam, and pressure is building inside the vessel. Left alone the vessel interior will presently become hot enough to start melting the tough alloy casings of the fuel rods, at which stage the interior will fill with long-half-life radioactive materials – and will thus have to be buttoned up tightly and abandoned for a long time, creating a mess.