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256 pages, Kindle Edition
First published April 26, 2016
“Those whose bodies were recovered are buried in welded zinc coffins, to prevent their radioactive remains from contaminating the soil.”
The principal concern of a nuclear reactor - particularly an RBMK reactor, because of its graphite moderator - is that cooling water continuously flows into the core. Without it there could be an explosion or meltdown. Pumps driving the flow of water rely on electricity generated by the plant’s own turbines, but in the event of a blackout the electrical supply can be switched to the national grid. If that fails, diesel generators on site will automatically start up to power the water pumps, but these take about 50 seconds to gather enough energy to operate the massive pumps. There are six emergency tanks containing a combined 250 tons of pressurised water which can be injected into the core within 3.5 seconds, but an RBMK reactor needs around 37,000 tons of water per hour - 10 tons-per-second - so 250 tons does not cover the 50 second gap.
Thus: 100 the test of a ‘run-down unit’. If a power failure occurred, the fission reaction would still be producing heat, while the remaining water in the pipes would continue its momentum for a short time and therefore steam would still be produced. In turn, the turbines would still rotate and generate electricity, albeit at an exponentially falling capacity. This residual electricity could be used to drive the water pumps for a few vital moments, giving the diesel generators sufficient time to get up to speed and take over, and it’s the hardware behind this that was being tested.
…In order to open the plant ahead of schedule, Chernobyl’s Plant Manager Viktor Bryukhanov, along with members of various Ministries involved with the construction and testing of a new plant, signed off on safety tests that were never conducted, with the unwritten promise of completing them later. As reckless as it sounds, this was fairly routine practise in the USSR, as completing work ahead of schedule entitled everyone involved to significant bonuses and awards.
The afternoon staff had been briefed on the test and knew exactly what to do, but their shift ended and they went home. Evening staff took over, but then they too left, leaving the relatively inexperienced night crew - who had never conducted a test before - the responsibility of starting a test they were not prepared for and had not anticipated doing.
To make matters worse, Unit 4 was at the end of a fuel cycle. One of the features of the RBMK design is ‘online refuelling’, which is the ability to swap out spent fuel while the reactor is at power. Because fuel burn-up is not even throughout the core, it was not uncommon for the reactor to contain both new and old fuel, which was usually replaced every two years. On April 26 th, around 75% of the fuel was nearing the end of its cycle. 102 This old fuel had, by now, been given time to accumulate hot and highly radioactive fission products, meaning any interruption in the flow of cooling water could quickly damage the older fuel channels and generate heat faster than the reactor was designed to cope with.
Unit 4 was scheduled for a lengthy shutdown and annual maintenance period upon conclusion of the test, during which all of the old fuel would be replaced. It would have been far more sensible to conduct the test with fresh fuel, but management decided to push ahead anyway.
“Imagine personnel of a plane which is flying very high. Whilst flying they begin testing the plane, opening the doors of the plane, shutting off various systems... The facts show that even such a situation should have been foreseen by the designers.” - Valeri Legasov, USSR Delegation Leader, 25-29 August 1986, Vienna.