Major Blast, Meltdown Seen by U.S.

The Soviet power plant accident involved a non-nuclear explosion that was probably caused by a loss of reactor coolant and was followed by a reactor meltdown, according to Reagan Administration sources and some experts in the United States who are familiar with the Chernobyl facility.

They said that much of the intense radiation escaped within the first few hours of the accident, which is believed to have occurred on Saturday.

“There was quite an explosion,” said one official, adding that the site of the blast is “possibly” continuing to emit radioactive gases, although nuclear experts said that such continuing radiation is likely to be at low levels.

Melting of Nuclear Fuel

“A chemical explosion which led to a meltdown--that’s the way I understand it to be,” a senior Administration official said. A meltdown involves the partial or total melting of a nuclear reactor’s fuel.

Intelligence data show a roof blown away, walls at least partly crumbled and evidence of continuing fire three days after the explosion, according to Administration sources.

A U.S. nuclear expert who has visited the Chernobyl complex said the four reactors were housed in sheet metal buildings, none of them inside a pressure vessel that might have succeeded in containing radioactivity in the event of an accident.

Although there were reports in Washington that the situation has stabilized, experts said Tuesday that the fire could continue for days. And because of intense radiation lingering at the plant, it would be impossible to get into the immediate area. Therefore, they said, the Soviets may have no choice but to allow the fire to burn itself out.

CIA Briefs Senators

“There is so much radioactivity in the vicinity of the fire that to get humans anywhere near the fire is impossible,” said Sen. Malcolm Wallop (R-Wyo.), after a closed CIA briefing for senators.

The Soviet Union has said that two people were killed as a result of the accident. But Kenneth L. Adelman, director of the U.S. Arms Control and Disarmament Agency, told a Senate committee that this account is “frankly preposterous in terms of an accident of this magnitude.”

About 1,000 people were believed to work at each of the four reactors during each shift, although officials were uncertain how many people were in the plant at the time of the blast or whether any warnings were given before the explosion occurred.

The fire is in blocks of graphite, a substance that is a crystalline form of carbon and which encases the uranium fuel in the reactor’s core. The graphite slows the speed of neutrons emitted by the fuel, thus making them more susceptible to the fission process.

Although it is a highly effective moderator, graphite is also extremely volatile, and U.S. reactor experts speculated that a water leak, allowing water and super-heated graphite to form an explosive mixture, may have caused a chemical blast. It is also thought possible that refueling operations may have caused the accident.

“We suspect they had a loss-of-coolant accident,” said a top official with G A Technologies, a San Diego company that built the only commercial reactor in this country that is somewhat similar to the Chernobyl plant.

After the loss of coolant, temperatures in the reactor would soar, and the first line of defense would be to pump cooling water into the reactor core. But experts suspect that the Soviet reactor was so hot that the emergency cooling water probably turned to steam as soon as it hit the core, creating pressures too great for the reactor vessel to contain.

“That explosion probably opened up the graphite to air, and at high temperatures in the presence of air, it will burn,” said the G A Technologies official, who asked that he not be identified by name.

He said graphite is “just like coal, but it is very dense” and that under those circumstances, it would burn at very high temperatures.

The heat would seriously damage the uranium fuel pellets, thus venting radioactive byproducts of the fission process into the air.

Early Damage the Worst

This scenario holds that the greatest damage would have been done within hours of the accident and that workers in the immediate area would have probably been killed by the explosion; others undoubtedly would have been exposed to heavy dosages of radiation--possibly to fatal levels.

“It doesn’t take much heat in one area” to warp the channels through which cooling water is supposed to flow, said Edward Siegel, who was with the International Atomic Energy Agency in Vienna when he visited the Chernobyl plant in 1977, a year before it was finished.

He said loss of coolant is “the only way a local area could get hot enough” to damage the reactor. If that happened, it might be impossible to insert the rods into the reactor that would shut it down, and cooling water might be blocked from reaching other areas, thus allowing the problem to expand, Siegel said.

The accident’s catalyst was “obviously a failure of the cooling system,” said Sen. Alan K. Simpson (R-Wyo.), who with Wallop had taken part in the Senate investigation of the nuclear incident at Pennsylvania’s Three Mile Island plant in 1979.

Differing View Presented

However, Julius Goodman, a former Soviet physicist who now works at California State University, Long Beach, said he does not consider a coolant loss a likely cause of the accident because, he noted, the Chernobyl reactor was cooled by hundreds of individual cooling loops. In contrast, U.S. reactors use one primary cooling loop and various backup systems.

“I think it was some kind of combination of human error and unexpected technical malfunction which led to damage (of the reactor’s core),” Goodman said.

He noted that Soviet reactors are designed so that workers can refuel part of the reactor while it is still running, simply by replacing part of the graphite core that houses the uranium. U.S. reactors must be shut down during refueling operations.

The graphite must also be “cleaned” of radioactive contamination from time to time, and that is done by allowing the reactor to heat up beyond its normal operating temperatures.

D. Allan Bromley, professor of physics at Yale University, described that operation as a “very ticklish, precarious business” because if the reactor gets too hot, the stainless steel container around the graphite bricks could melt, exposing the hot graphite to air. If that happened, it would burst into flames, Bromley said in Washington at a meeting of the American Physical Society.

Water Becomes Steam

Basically, most power plants are similar, regardless of the type of fuel they burn.

The heart of any such system is the “fire box,” the part of the plant that produces heat. Heat is used to convert water to steam, which in turn drives the giant turbines that generate electricity.

Non-nuclear plants burn coal, oil, gas or even wood to produce heat. In nuclear plants, the “fire box” is a nuclear reactor. In nuclear plants, radioactive byproducts must be contained.

To sustain the nuclear reaction, a “moderator” must be used to slow neutrons emitted during the fissioning process. In most reactors in the United States, water is used for that purpose. The Soviet reactor used graphite.

To keep temperatures within operating limits, all reactors must be cooled constantly. Like most reactors in the United States, the Chernobyl plant used water for that purpose.

Lee Dye reported from Los Angeles and James Gerstenzang from Washington. Times staff writers Rudy Abramson, Doyle McManus and Karen Tumulty in Washington also contributed to this story.