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Soviets unveil lessons from Chernobyl.

Soviets unveil lessons from Chernobyl

A detailed picture of events that led to the crippling of the Chernobyl nuclear plant last April emerges in a 382-page report formally released by the Soviet government on Monday. The report, unveiled in Vienna, Austria, at the opening of an International Atomic Energy Agency five-day meeting on the accident, not only documents extensive operator error but also identifies what the Soviets are now conceding as a number of serious design flaws in the reactor--flaws that they acknowledge contributed substantially to making the Chernobyl accident so catastrophic.

According to an English translation of the report that was prepared by the U.S. government, the accident occurred when operators began a test of one of the No. 4 reactor's two turbogenerators, essentially to see if it could be used to power the reactor's emergency cooling system. To simulate the conditions under which the emergency cooling system might need the generator's backup power, the test required disengaging the system from the reactor and disconnecting the generator from external sources of electric power, according to the report. Moreover, on the off chance that the test might prove unsuccessful, the operators decided to prepare the second turbogenerator to undergo an identical test. As these tests apparently also required removing certain other automatic safety systems from the generators, both generators were vulnerable and hard to control during the accident. Leaving both generators vulnerable at the same time violated strict, written procedures.

As the reactor power was decreased in preparation for the tests, the reactor became unstable--with uneven heating occurring throughout its more than 1,600 pressure tubes, each the equivalent of a small reactor core (SN:8/16/86, p.101). In their attempt to stabilize the reactor, the operators hooked up additional coolingwater pumps. But the measure overcompensated, rapidly increasing the flow rate of cooling water through the reactor. In fact, says the report, it would have caused an automatic shutdown of the reactor if operators hadn't blocked additional emergency control systems.

To compensate again, operators began sharply reducing the water's flow rate, thereby increasing coolant temperature --a factor that can increase reactor power. When the test was finally begun, a "continuing decrease in the water flow rate through the channels of the reactor under conditions of an increase in power led to intense steam formation and then to a crisis' that included the overheating of the fuel and its disintegration. Ensuing chemical reactions helped initiate a massive steam explosion that ripped the plant apart, started at least 30 individual fires and ultimately released about 3.5 percent of the radioactive material in the reactor. The fallout may eventually cause 24,000 cancer deaths, according to calculations in the Soviet report.

If the reactor had been enclosed in a containment building, much of that radioactivity might never have been unleashed. However, the report notes, the Soviets found it difficult to manufacture these large containment structures and so instead engineered a complex series of other safety measures to compensate. At the meeting Monday, Valery A. Legasov, head of the Soviet delegation, conceded that the accident proved these measures unreliable. Moreover, the report highlights a host of additional design liabilities plaguing Chernobyl-style reactors. In preparation for a move to begin rectifying many of these, Legasov said, half of the USSR's reactors have already been shut down.

The Soviets intend to continue their aggressive development of nuclear power. Citing concern over the environmental damage now caused globally by the emission of combustion pollutants, the new report states that all future Soviet base-load electric-generating stations will be nuclear powered--not fossil-fuel fired.
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Author:Raloff, Janet
Publication:Science News
Date:Aug 30, 1986
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