Dr Michael Allen describes how a Fukushima worst case scenario could develop. He is an ex-Sandia Engineer. Other experts disagree about worst case from this point. Also, these scenarios are avoided with fresh water in the reactors and on the spent fuel ponds.
Dr. Michael Allen, vice provost for research and dean of graduate studies at Middle Tennessee State University, spent much of his early career at Sandia National Labs studying nuclear reactor accidents of the worst kind and performing simulations to better understand how bad things happen — including core meltdowns.
Allen wouldn’t say if he thinks the crisis in Japan will become the worst nuclear accident in history or whether he believes a full-scale meltdown is inevitable, although he described how it could happen. The situation, he said, is far from over.
“I’m concerned about Japan because I think this is a really bad accident and concerned about their people, their infrastructure,” Allen said today in a telephone interview. “I don’t think this is an accident that is going to go away anytime soon.”
Allen has been at MTSU since 2007, but earlier in his career he worked more than 14 years at Sandia National Laboratories in New Mexico, where — among other assignments — he headed the federal lab’s work on “severe accident phenomenology.” That work included using a research reactor to actually melt the core of another reactor to better assess how the core relocates in an accident, as well as the release of fission products. He also conducted hydrogen and steam explosions at desert test sites outside Albuquerque, using reactor fuel simulants to assess the results.
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 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.
“I’ve done many of these experiments,” he said. “When you drop a molten core into water, there’s a big explosion.”
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.