Hurled into the night

sky 25 years ago, the

fireworks of Chernobyl

James Wikie

 

A quarter of a century has passed since the Chernobyl nuclear reactor in what was then the Ukrainian Soviet Socialist Republic blew its top, on 26 April 1986. While the more recent Fukushima disaster demonstrated that even the intrinsically safest nuclear plant is not immune to external hazards, Chernobyl proved that human error can be even more dangerous.
     At the time of the Chernobyl disaster I was working for the International Institute for Applied Systems Analysis (I actually wrote IIASA's 1986 annual scientific report), where I had contact with leading Soviet scientists, and I can attest to the absence of any secrecy or cover-up on the part of the Soviet authorities. They were completely open about what happened.
     From 25 to 29 August 1986 the USSR state committee on the utilisation of atomic energy submitted an unusually detailed report more than 50 pages long to an expert meeting at the International Atomic Energy Agency in United Nations HQ Vienna. Its subject was 'The accident at the Chernobyl nuclear power plant and its consequences', and it was backed up by a wealth of graphic, pictorial and film material. A team of 23 prominent Soviet experts, with support from all the state institutions concerned, analysed the causes, the course of events, and the consequences of the disaster. At the tail end of the cold war it was a remarkable experience for everyone involved.
     I still have my personal copy of that report, which I summarised and published in the Austrian government's diplomatic magazine Austria Today. So, what actually happened at Chernobyl?
     The idea was eminently sensible. When a power station breaks down, various emergency services come into operation to ensure that enough electricity is provided to cover the plant’s own needs. The interruption constitutes no real danger, but until the emergency generators, which are mostly diesel-powered, can be brought into action there is usually a loss of valuable minutes.  
     But steam turbines, which drive the generators of most large thermal power stations – including nuclear ones – continue to revolve at high speed after the steam has been cut off, and indeed require several hours to slow down to a standstill. The massive connected generator rotors also have a flywheel effect.
     It was not the first time that experiments had been undertaken at Chernobyl on utilising the energy of the 'freewheeling' turbo-generator units to provide bridging power over the period until the emergency system could be brought into operation – a completely rational idea, it must once more be emphasised. The fault lay in the methodology.
     In April 1986 the engineers at Chernobyl were presented with a good opportunity for further experiments. Their number 4 reactor – a standard Soviet RBMK-1000 unit, water-cooled and graphite-moderated – had been put into operation in 1983 and was due to be shut down for routine maintenance work. The plan being drawn up for the experiment with the 'freewheeling' turbo-generators 7 and 8 foresaw it being repeated if the first results were unsatisfactory. But, once the reactor had been shut down and the first experiment had been carried out, to have restarted it again for a second experiment would have been impracticable in terms of both time and expense, and the next opportunity for a test would have been at least a year away.
     And so a compromise was attempted. Instead of a complete shutdown, reactor 4 would be 'throttled back' to 25% of its output, between 750 and 1,000 mW (thermal), which would allow the two turbo-generators to be brought up to their operational speed very quickly in the event of an initial test failure.

 

     So much for the theory. The technique of running down a fission reactor is roughly this: the 'brakes' consist of control rods of neutron-absorbing materials like boron or cadmium, which are inserted into the reactor core. To cut a long scientific explanation short, these absorption rods are used to keep the reaction process under control by altering their positions in the reactor core. The swift insertion of a sufficient number of control rods can shut the reactor down within seconds. There is also a series of 'scram rods' that are used exclusively for shutdown operations.
     The experiment began in the early morning of 25 April, accompanied by a series of flagrant violations of the operating regulations. The reactor's emergency cooling system was switched off in accordance with the experiment. Shutting off the automatic regulation allowed the reactor output to fall to only 1% instead of the planned 25% output, and resulted in a so-called 'xenon well'. The xenon gas being formed in the reactor had 'poisoned' it to the extent that only a minimal increase in output was possible.  
     And so, in an incredible display of irresponsibility that eventually brought him 10 years in a gulag, the operator lifted all of the neutron-absorbing rods out of the core, with the idea of increasing neutron circulation and raising output once more. The relevant operating regulation, that not even the Supreme Soviet was empowered to alter, stated that a minimum of 30 fully inserted control rods had to be available at all times for a shutdown operation of two seconds, or in very exceptional circumstances 15 rods for a shutdown time of six seconds.
     The staff went ahead with the experiment nevertheless, running all six main and two reserve coolant pumps experimentally from turbo-generator 8, with resulting turbulence in the cooling system from the excessive water flow, while the reactor was still well below its planned output level. Manual attempts to sustain the main parameters of the system proved ineffective. The staff, instead of shutting the reactor down, blocked the emergency protection signals.
     At 01.22.30 am on 26 April the computer printout informed the operator that excess reactivity had reached a level that demanded immediate shutdown. Undeterred, they continued with their experiment. The reactor continued to operate at a power of about 200 mW (th), with its emergency protection blocked in order to increase output and facilitate a repetition of the experiment. Then the reactor output started to rise again – this time with no form of control.
     At 01.23.40 am the unit shift foreman recognised the seriousness of the situation. Without asking his superiors he gave the order 'Press button AZ-5!' This should have had the effect of inserting all the available scram rods into the core. But the absolute minimum capacity of 15 absorber rods equivalent was not available. The rods, illegally removed from their positions hours before, did not reach the necessary depth in the core. The operator hastily cut off the power to the sleeves of the servo drive, so that the rods would fall into the core under their own weight.  
     Under normal circumstances, with 30 control rods, about two seconds would have sufficed to shut down the reactor, which was now going out of control. The available capacity of neutron-absorber rods would have needed six seconds or more to do the job. The reactor did not grant that length of time.
     At 01.23.42 a.m. on 26 April 1986 an enormous explosion blew the massive top plate off the reactor. The excess water from the pumps, and oxygen from the outside air, were now entering the vented and destroyed reactor core, where there were various chemical reactions with steam and the glowing graphite.
     As a result, a mixture of gases was formed containing hydrogen and carbon monoxide, which then led almost immediately to a second, thermal explosion. In a gigantic fireworks display of glowing particles and fragments, some 3.5% of the reactor's radioactivity was hurled into the night sky to drift over Europe – and to pollute the Earth's atmosphere for decades thereafter.

 

James Wilkie worked for the United Nations in Africa and Asia as well as for the Austrian chancellery and foreign ministry