Chernobyl Reactor Blew Up Like a Bomb : Can't Happen Here? It Happened There

Mikhail S. Gorbachev is reported this week to be visibly frustrated by the uncanny series of natural, man-made and man-aggravated disasters that have plagued his country during his regime.

Sadly, even as news arrives of the latest catastrophe--a natural gas explosion in which nearly 500 are dead or missing--troubling evidence from previous disasters continues to accumulate, posing more challenges to Gorbachev's perestroika program. The latest findings about the Chernobyl nuclear disaster strongly suggest that about 15 Chernobyl-type reactors in the Soviet Union should be shut down.

Last year I started to investigate the Chernobyl disaster, inspired by some of the procedures Caltech physicist Richard Feynman devised as a member of a presidential commission looking into the Challenger accident. Shortly before his death early last year, Feynman described how he had discovered, during a visit to a NASA facility in Huntsville, Ala., that NASA engineers considered a space shuttle catastrophe a near statistical certainty before the disaster happened.

Editing a version of Feynman's account for a physics magazine, it occurred to me that it might be interesting to ask the leading experts on Chernobyl in the United States how likely they considered another Chernobyl-like accident. During the next year I interviewed top people at the Nuclear Regulatory Commission, the Department of Energy, their counterparts in Canada and specialists in the academic world, industry and government laboratories. The results were disconcerting.

The standard explanation for Chernobyl has been that a steam explosion destroyed the reactor, suggesting that it was like a pot boiling over on your stove or, at worst, a pressure-cooker mishap that put a mess of radioactive pea soup on your ceiling. In fact, the reactor blew up like an atomic bomb. It of course did not blow up with the full efficiency or force of an atomic bomb, but the physical processes that blew up the reactor were the same forces that cause a bomb to explode. Moreover, the corrective measures taken by the Soviets do not preclude an equally severe accident and some of the measures may make an accident more rather than less likely.

I first got an inkling of what happened at Chernobyl during an interview with Richard Wilson, a reactor expert at Harvard who has visited Chernobyl twice. When I asked Wilson to comment on a description of the accident as a "slow nuclear explosion," expecting him to pooh-pooh the claim, he said: "To ever say it was not a nuclear explosion is just plain wrong."

The Chernobyl reactors have the unfortunate defect of speeding up, under certain conditions, rather than slowing down when water is lost. Compounding this defect, the top of the reactor is poorly sealed, so that surprisingly little excess pressure will rupture the lid. As a result, a loss of water could cause nuclear reactions to go out of control, generating enough pressure to blow up the reactor vessel and release large quantities of radioactive material into the atmosphere.

Immediately before the accident on the night of April 25-26, 1986, the plant's operators were preparing to do an unusual test, after which they planned to shut the reactor down for annual maintenance and refueling. They took a series of reckless measures that put the reactor into its most unstable state, unleashing an uncontrolled and uncontrollable nuclear reaction. "Once power began to rise rapidly," says Edward Purvis, head of the Department of Energy's study team, "nothing could have been done to stop it."

In just a second or two, this first explosion sent the reactor's power soaring from close to zero to between 50 and 100 times the plant's maximum normal capacity. This was an enormously more violent event than had ever been described or imagined in the voluminous U.S. literature on reactor safety.

Eyewitness accounts and Soviet computer analyses strongly suggest that there was a second explosion, perhaps 10 times more violent than the first. While interpretations vary, the most plausible is that the severely damaged core collapsed on itself, formed a critical mass or several critical masses and blew up like a bomb. This explosion "completed the destruction of Chernobyl Unit 4," Purvis says.

Harold Denton, the former Nuclear Regulatory Commission safety chief who managed emergency operations at Three Mile Island in 1979, said that he and his colleagues worried at that time that a nuclear explosion could take place. But they brought in consultants who concluded that TMI's molten core could not form a critical mass and explode.

Asked to assess the probability of another catastrophic accident at a Chernobyl-type reactor, Denton said: "We would not license such a reactor here."

For decades, spokesmen for the nuclear industry have insisted that reactors cannot blow up like bombs. But the Chernobyl reactor did blow up like a bomb, and a similar accident is remotely conceivable in Canada's reactors. Perhaps industry spokesmen should have said that most reactors, or all reactors in the United States, cannot blow up like bombs.

Peter Gale, the UCLA bone marrow specialist who flew to the Soviet Union to help Chernobyl victims, has said he believes that Chernobyl was the catalytic event that prompted Gorbachev to launch his bold arms-control initiatives. If Chernobyl had been a mere steam explosion brought on mainly by operator error, Gale's view of things would make little sense. But it makes good sense if Gorbachev understood the accident to have been a nuclear explosion that created, on a small scale, the devastated landscape that we would have on a grand scale in the wake of an all-out nuclear war.

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