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"bubbling pool" and the pressure in the cell is relieved by bubbling should a pump, a separator or a pipe burst.
In the afternoon Legasov continued, now concentrating in greater detail upon the reasons for the accident and its progress. Although he followed quite closely the written report which participants had been given he added several important points. In particular he said that the operators felt that they were under extreme pressure to complete the planned experiment that night since they knew that it would be a full year before they would have another chance. It was "a tremendous psychological mistake" on the part of the designers of the RBMK reactor that they did not foresee that additional protective systems would be needed in the core in order to trip the reactor and keep it cool even if (as occurred in the Chernobyl accident):
(a) the operators deliberately switched off the standard protection systems and in addition
(b) completely disobeyed the safety rules concerned with the minimum number of control rods which must be inserted.
This, he said, was the case against the RBMK designers: "Now, with hindsight we can see that it could have been prevented in a very easy way using technical means" (by which he meant engineered safety features, not written rules). He illustrated what had happened by means of an analogy. It was, he said, as if the pilot of a passenger plane suddenly started testing the 'plane in the flight: opening and closing the doors and switching off safety systems. He suggested that the Soviets had realised, somewhat later than other countries, the
to protect against this kind of human fallibility.
As for the detailed progress of the accident: this is involved. In essence what Legasov says happened was as follows:
The operators tried to power the coolant pumps using electricity from a "free-wheeling" turbo-alternator. As the alternator slowed down, so of course did the pumps which it was driving and so the amount of steam being produced increased. It was this that triggered the accident. The operators tried to insert the control rods but the rods were mostly so far out of the core (only 6 were inserted instead of the minimum of 30 required by the rules) that long before the rods could have shut the reactor down it had run away, the power rocketing up. The steam, now produced in vast quantities, burst the pressure tubes. Next the uranium dioxide pellets disintegrated with a further explosive generation of steam which blew the top cover (pile-cap) off the reactor and exposed the hot fuel to the air. Hydrogen and carbon monoxide were produced by the oxidation (in steam and air) of graphite and zirconium. These gases burned
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