TNAG-1503-FCO40-2061-Guangdong-nuclear-power-station-project-at-Daya-Bay-safety-c-1986 — Page 61

SUMMARY

This meeting of 400 or so experts from Member States had been convened in order to permit the Russians to present their account of the Chernobyl nuclear accident. Opportunity was afforded for Member States to put technical questions so as to improve their understanding of the Russian account.

From my notes of the meeting, the notes of UKAEA colleagues and the documents tabled by the Russians, I have prepared this factual report.

THE ACCIDENT ITSELF

It

There has emerged a satisfactory working knowledge and understanding of the accident. In brief, the Russians wished to measure the ability of a turbo-alternator to power certain of the cooling pumps whilst free-wheeling to a standstill. would be advantageous briefly to extend the operation of the cooling pumps in this manner if, following accidental disconnection of the station from the grid, steam had to be diverted from the turbine and the reactor shut down.

As the alternator slowed down, so did the pumps and a point was reached where they were no longer pumping enough water through the reactor to keep it cool. At this point the automatic safety system should have tripped the reactor but it did not, because the operator had turned off the safety system although forbidden to do so. Accordingly the reactor began to generate additional steam and this formed extra bubbles and voids in the cooling water: The amount of heat being generated rose to a result of the voids, because the reactor has a positive void coefficient. A run-away situation had developed, the additional heat generating still more steam.

The operators tried to extricate themselves by releasing the control rods so as to trip the reactor, but they were too late and part of the reactor went prompt-critical. The temperature then rose steeply, fuel disintegrated, steam pressure burst pressure tubes and lifted the pile-cap. Several tons of fuel escaped. Ten to twenty percent of the radioactive iodine and caesium escaped and were carried some hundreds of miles by the wind, causing contamination.

The Russians stated that their reactor design has deficiencies and that these, coupled with operator errors and a deliberate flouting of safety instructions, led to the accident.

PREVENTIVE ACTION BY THE RUSSIANS

To prevent such an accident happening again in another of their RBMK reactors, the Russians have decided to increase the amount and speed of insertion of control rods. The fuel enrichment will be raised to maintain the discharge burnup. They will, in addition, supply the operator with better facilities with which to control the reactor, and give him better training. Half of the reactors have been shut down pending these improvements.

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