Michael Allen has spent much of his early career studying nuclear reactor accidents of the worst kind and performing simulations to better understand how bad things happen, including core meltdowns.
Like many others with roots in the nuclear industry, Allen, vice provost for research and dean of graduate studies at Middle Tennessee State University, is watching the events unfold in Japan with keen interest and concern.
Allen said there's no question there's been at least a partial melting of fuel cores at three of the reactors at the Fukushima Daiichi Nuclear Power Plant, likely a breach in one reactor pressure vessel, and other damage incurred by last week's earthquake and tsunami and the sequence of nuclear events since then.
"I'm concerned about Japan because I think this is a really bad accident and concerned about their people, their infrastructure," Allen said in a telephone interview this week. "I don't think this is an accident that is going to go away any time soon."
Earlier in his career Allen worked more than 14 years at Sandia National Laboratories in New Mexico, where he headed the federal lab's work on "severe accident phenomenology."
Much of his research directly addressed accident scenarios in which the nuclear fuel is no longer submerged in water, a situation that Japanese workers have been battling for days at the Daiichi reactor complex.
Allen agrees with reports that explosions caused by a hydrogen buildup likely blew the roof off the outer containment buildings of at least two of the reactor sites, exposing pools that store highly radioactive spent nuclear fuel rods to the environment. But, based on reports he's heard or read, he thinks the explosion that occurred March 14 at the Daiichi Unit 2 reactor was a steam explosion inside the reactor pressure vessel that probably occurred when part of the exposed fuel core melted and allowed some of the liquefied fuel or super-hot fragments to drop into the water below.
"When that happens, you're going to have a massive steam explosion, which creates extremely high pressure in the reactor pressure vessel," Allen said.
As has been noted in various news reports, the pressure dropped inside the reactor and radiation levels outside the unit rose significantly at about that time. That, he said, would seem to support conjecture that the vessel protecting the nuclear core may have been damaged or possibly ruptured and released some of the radioactive constituents.
The worst of the worst could come if Japan can't come up with a way to 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 the 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.
Should these events happen, the best outcome would be if the winds are blowing east and push the radioactive plume over the 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.
(Frank Munger is a reporter for The Knoxville News Sentinel in Tennessee)