Frank Munger at the Knoxville News Sentinel is a reporter whose entire beat is devoted to the Department of Energy's Oak Ridge National Laboratory. That means Munger spends a lot of his time reporting on nuclear weapons, waste, and energy, and he knows the right experts to go to in a nuclear crisis situation. For instance, when it comes time to talk worst-case scenarios at Fukushima Daiichi, it makes sense to talk to somebody who spent more than 14 years studying nuclear worst-case scenarios.
Munger's source is Dr. Michael Allen, who used to work at the Sandia National Laboratory, where he, essentially, watched things go horribly wrong for a living.
Nuclear scientists call this "severe accident phenomenology". The kind of accident currently happening at Fukushima Daiichi is something that researchers, like Michael Allen, have tested out under controlled conditions. To a certain extent, we know what happens when a reactor core melts, and we know what happens when nuclear fuel gets exposed from beneath a water bath. We know what happens in those situations because we've intentionally made them occur, and studied the results.
Thanks to those experiments, people like Michael Allen are in a good position to speculate about a worst-case scenario at Fukushima. Allen can't tell you what will happen. There's not really an inevitable direction this disaster will take. But he can tell you that the Fukushima crisis won't be over anytime soon. And he can tell you about the very worst thing that could happen.
The worst of the worst could come if Japan can't come up with a way to sufficiently cool down the reactor fuel cores. That has reportedly become increasingly difficult with workers evacuating the sites — at least temporarily — because of high radiation fields.
"These things play out over a long period of time, longer than people would think," Allen said. "You have an earthquake that lasts maybe a minute, a tsunami that lasts maybe 15 minutes. But these things could go on for months. You could lose all six of the reactors."
If workers are unable to get additional cooling water into the reactor vessel, the molten fuel core will collapse into the water in bottom of the vessel. Eventually the heat from the decaying fuel would boil away the water that's left, leaving the core sitting on the vessel's lower head made of steel.
Should that happen, "It'll melt through it like butter," Allen said.
That, in turn, would cause a "high-pressure melt injection" into the water-filled concrete cavity below the reactor. Because the concrete would likely be unheated, the reaction created by the sudden injection of the reactor's ultra-hot content would be immense, he said.
"It'll be like somebody dropped a bomb, and there'll be a big cloud of very, very radioactive material above the ground," Allen said, noting that it would contain uranium and plutonium, as well as the fission products.
Should these events happen, the best outcome would be if the winds are blowing 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.
The worst case, Allen said, would be if winds pushed a radioactive cloud south toward Tokyo and Japan's highly populated cities. If that were to happen, he said, the consequences would likely be greater than the 1986 accident at Chernobyl, where an entire area of Ukraine had to be evacuated because of the radioactive conditions that increased the risk of developing cancer.
There's a lot more good information about nuclear accidents, and accident research, in the article this came from. I highly recommend reading the whole thing. This is scary stuff. And I hate to add to the fear. But I know that worst-case scenarios are being debated, and I think this one has more weight behind it than most.