In the absence of a clear picture from the Japanese government of what is happening at the Fukushima Daiichi nuclear plant, a number of experts have stepped in to offer their take on what is currently unfolding and the worst-case scenarios that might occur. Middle Tennessee State University's Dr. Michael Allen spent his early career at Sandia National Labs performing simulations of the world's worst nuclear-reactor accidents, including what happens when nuclear fuel is no longer submerged in water. (Yesterday, U.S. authorities in Japan said the pools of water in Unit 4 containing spent fuel rods, which are different from the reactor cores, had boiled dry.) Allen told the Knoxville News Sentinel that he's unsure whether a full-scale meltdown is inevitable, but he explained how it might occur.
Allen said the situation was likely to play out for a long period of time: "These things could go on for months. You could lose all six of the reactors." If workers are unable to get cooling water into the reactor vessel, as they're trying to do with helicopters and water cannons, heat from the decaying fuel would boil away any water. If that happens, Allen says, "It'll melt through it like butter."
The effect of that would be a "high-pressure melt injection" into the water-filled concrete cavity below the reactor. The sudden injection of the ultrahot contents of the reactor into cold concrete would be "like somebody dropped a bomb, and there'll be a big cloud of very, very radioactive material above the ground," Allen said.
Should these events happen, the best outcome would be for the winds to blow east and push the radioactive plume over the Pacific Ocean, he said. "It (the radioactivity) will fall out in the ocean and everything will be fine," he said. If, however, the radioactive cloud moves toward Tokyo and other cities, the result would be worse than Chernobyl.
The U.K.'s chief scientific officer, professor John Beddington, also spoke to the British Embassy in Tokyo and had a different, slightly more optimistic take on what would happen if that injection occurs, based mostly on how far the radioactive cloud would spread.
You get some radioactive material going up to about 500 metres up into the air. Now, that’s really serious, but it’s serious again for the local area. ... If you then couple that with the worst possible weather situation i.e. prevailing weather taking radioactive material in the direction of Greater Tokyo and you had maybe rainfall which would bring the radioactive material down do we have a problem? The answer is unequivocally no. Absolutely no issue. The problems are within 30 km of the reactor. And to give you a flavour for that, when Chernobyl had a massive fire at the graphite core, material was going up not just 500 metres but to 30,000 feet.
While both Beddington and Allen's analyses are focused on the reactors themselves, the New York Times says that the most danger actually lies with those spent fuel rods rather than the reactor cores, according to figures provided by Tokyo Electric Power. The electric utility said that a total of 11,195 spent fuel-rod assemblies were stored at the site. Spent fuel rods generate less heat than newer ones, but there are strong indications that they have begun to melt and release extremely high levels of radiation. The result could be a similar scenario that Allen and Beddington sketched out, but with far more nuclear material.
Ex-Sandia engineer talks about some of the worst things that could happen in Japan [Knoxville News Sentinel]
Situation at Fukushima nuclear plant [British Embassy - Tokyo]
Danger of Spent Fuel Outweighs Reactor Threat [NYT]