Chernobyl Reactor 4 started up before the end of 1983 in order to meet a Soviet Union deadline for energy production targets. In the rush, an antiquated design was used and some of the commissioning tests were bypassed.
A worrying problem emerged: how to keep the reactor cool if power was lost.
Active cooling is required in nuclear reactors, running or idle, to remove the heat generated by radioactive decay. In the event of a reactor shutdown, back-up power must keep the water pumps running.
The standby diesel generators took over a minute to run up to speed, too long for the reactor core to be without cooling. A solution – a way of keeping the cooling pumps running for a short time – was proposed by electrical engineers and an experiment scheduled to prove that the problem had been solved.
Normal safety controls were over-ridden to allow the experiment to proceed.
An expert investigator later described the safety test as akin to ordering airplane pilots to switch off their engines mid-flight to check that they could be restarted.
And what if they couldn’t?
The operators who started the test on 25th April 1986 were unaware of the multiple design flaws in the RBMK-1000 nuclear reactors. The design had been hurriedly adapted from military to civilian use. The reactors were unstable at low power, a loss of water not only stopped cooling, but caused a positive increase reactor power, as did the initial entry of the boron control rods (designed to stop a runaway reaction) as their tips were made of a different material.
Instability had been observed in Reactor 2 in Chernobyl and in Lithuania, but the consequences were far less serious because the safety systems worked as intended. With several RMBK-1000 reactors in operation in the Soviet Union, a serious accident was just a matter of time.
There were many conspiracy theories – A US attack on the Soviet Union, A Russian attempt to cover up the overspend on Duga 1, the early warning system that never worked.
Occam’s razor. The simplest explanation is the most probable: a well-intentioned safety test which failed to take account of multiple design faults and went horribly, disastrously wrong.
But it could have been even worse.
As the scale of the April 1986 accident became clear, the team sent to manage the disaster struggled to come to terms with the fact that a further, far worse accident, was still possible.
The damaged reactor core was still active.
The first danger was that hot core would descend into the pool of fire-fighting water which had collected in the basement under the reactor. The water would flash into steam and explode into the atmosphere, flinging out further radioactive debris.
Divers were sent on a suicide mission to open valves in the dark and release the water. They were successful, but the danger was far from over.
Having capped the core, the core temperature continued to rise. If it reached the critical temperature, then a much more serious nuclear explosion would hurl radioactive particles into the atmosphere and winds would blow them in all directions.
Or the burning core would continue to descend, melting down into the water table – so called China Syndrome.
Radiation affected the circuitry of machines, so miners were brought in to hand dig tunnels under the complex, working in shifts at temperature above 40 degrees, digging and hauling the earth away. The initial plan was to inject liquid nitrogen beneath the reactor core to cool it from below, but the hopelessly ambitious project was never followed through.
Instead the tunnels were filled with concrete to provide a foundation to contain the smouldering core.
The heroic actions of the “liquidators”, those sent to contain and clean up, saved countless lives.
Often at the cost of their own.
Chernobyl memorial – To those who saved the world