4.8 Chernobyl 1986
The Chernobyl nuclear power station consisted of four RBMK reactors with a further two under construction. In the early hours of the morning of 26 April 1986 the operators of Unit 4 carried out an experiment on the reactor which ironically was designed to improve safety.
In the event of the station becoming isolated from the grid, diesel generators would automatically start up to provide power for the main coolant pumps. It does however take a few seconds for the diesel generators to start producing power. The operators wished to carry out a test to see if electricity could be generated for a few seconds, following isolation from the grid, by using the kinetic energy stored in the rotating turbogenerators and thus provide power for the main coolant pumps for the short period between the reactor being isolated from the grid and diesel start up. In order to carry out the experiment a number of normal operating conditions were breached and safety interlocks overridden in violation of safety procedures.
In the event, the electricity generated by the running down turbogenerators was insufficient to provide adequate power to the coolant pumps. The reactor began to close down by motoring the control rods into the core. Due to the way the experiment had been carried out most of the rods were entirely out of the core at the time of the test, in clear contravention of written operating instructions. Because of this and the practice of motoring, as opposed to dropping the rods as would occur in most other reactor designs, the reactor could not be shut down quickly enough. The heat in the core boiled the coolant water and the positive void coefficient caused the power of the reactor to increase dramatically. This sudden power increase caused water in the core to flash to steam, wrecking the reactor core and the structures above the core, and setting fire to the reactor's graphite moderator.
The reactor, unlike a PWR, had no effective containment and a large fraction of the fission products in the core was released into the atmosphere.
Two men in the hall above the reactor were killed. A number of the other operators who went to investigate what had happened, together with a number of fire fighters, were also killed--a total of 31 deaths. No off-site member of the public was killed.
The total amount of radioactivity released was very large, (of the order of 1018Bq). In terms of iodine-131 it was equivalent to about 350 Windscale accidents, or about half a million Three Mile Island accidents.
There are a number of features of the design and operation of the Russian RBMK reactor which either helped to cause, or exacerbated the effects of, the accident. Perhaps the most important are:
(i)
(ii)
(iii)
(iv)
(v)
a design which allows a positive void coefficient to exist at low power (the condition at which the experiment was being carried out);
slow shutdown mechanisms;
reliance on operational procedures rather than engineered safety systems;
deliberate shutting off of emergency systems;
lack of an effective containment.
Improvement in any one of these feature would have prevented, or greatly mitigated, the effects of the Chernobyl accident.
A Chernobyl-type accident is impossible in a PWR, which has a negative instead of a positive void coefficient and which would automatically shut down in the event of the sort of operator malpractice that occurred at Chernobyl. It is also impossible to have a moderator fire in a PWR since it uses water, not graphite, as a moderator.
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