More details on Chernobyl.

More details on Chernobyl

As cleanup crews at the Chernobyl nuclear power station continue efforts to encase its devastated reactor in concrete, Soviet leaders have begun offering the most detailed account thus far of what they think caused the April 26 accident. In a televised statement last week that was later translated and distributed by the Soviet press agency Tass, Communist Party Chief Mikhail Gorbachev said the reactor's "capacity suddenly increased' during a scheduled shutdown of the #4 reactor.

The Soviet reactor design incorporates what is called a "positive void co-efficient,' explains Frank Graham, vice-president of the Atomic Industrial Forum, Inc., in Bethesda, Md., a nuclear-industry association. That means any loss of water or overheating of water in the pressure tubes through which fuel-cooling water passes could prompt "a surge in the fission action,' causing a rapid increase in reactor power, he says. This is in contrast to most commercial U.S. reactors, Graham says, which begin losing power when their fuel's coolant overheats. Theoretically, Graham told SCIENCE NEWS, the power surge that Gorbachev seemed to be referring to could, if unchecked, have caused "the fuel to come apart"--and, in a reactor of the Chernobyl type, started a graphite fire.

Conceding this scenario is only an "educated guess,' Graham says it might explain the origin of the graphite fire and hydrogen explosion that the Soviets now believe blew the roof off the plant. As the heat generated in a graphite fire melted the pressure tubes--liberating water, steam and oxygen--the zirconium that cladded the fuel would have begun oxidizing, producing copious amounts of heat and hydrogen.

Meanwhile, bone marrow transplant specialist Robert Gale, of the University of California at Los Angeles, has returned from Moscow after assisting in a reported 19 transplants. As many as 100,000 Soviets may eventually suffer radiation-induced health problems, he estimates.