April 26 marks the anniversary of Chernobyl, the worst nuclear power disaster of the 20th century. rs21 member Brian Parkin explains how the event demonstrated the lethal combination of technical arrogance, corporate and state deceit and human fallibility that will forever lie at the heart of nuclear power. This is an edited version of an article that first appeared in The Miner, paper of the National Union of Mineworkers.

Chernobyl – inside the abandoned site. Picture from wallpaperflare.com, used under CC licence.

At 1.23 on the morning of 26 April 1986, the number four reactor at the Chernobyl nuclear generating site near Pripyat in Ukraine went on SCRAM alert – the maximum red alert for an emergency shutdown effected by immediately terminating the fission reaction. Within three seconds all of the reactor’s safety systems had failed, and with control rods jamming the reactor core the temperature rose by 10 times its design level. The resulting first explosion destroyed the 2,000 tonne concrete containment plate (weighing as much as an Apollo space rocket) and blasted it clear of the reactor building. And it was in that instant that station operator Valery Khodemachuk, his body impaled on control rods, became the first pharaoh of the nuclear age – to be entombed in a sarcophagus even taller than the great pyramid of Giza.

 Hard rain

The 1986 Leeds May Day demonstration was a muted affair, coming as it did just 12 months after the historic defeat of the miners. For me it was memorable in one respect: it rained. Later on, a science post-graduate friend of mine rang and asked if we had got wet on our outing, because if you did you will have got some caesium-137 thanks to the Ukraine nuclear disaster. While monitoring overnight rainfall at a Leeds University weather station, he had found clear traces of the radioactive isotope. Borne upwards by easterly spring winds, fall-out in the form of radioactive isotopes of caesium, iodine and xenon had blown across much of Europe.

Critical times

The International Atomic Energy Agency has proved – more or less – over some 50 years to be an effective watchdog and regulator of all matters regarding nuclear power. It has also undertaken the role of ensuring a uniformity of secrecy as far as matters of safety are concerned – particularly in the immediate aftermath of a major nuclear ‘incident’. But with Chernobyl in April to May 1986, the ‘incident’ was of such a massive scale that any amount of cover-up was bound to fail

During the 24 hours preceding the accident at Chernobyl, the operating staff had been engaged in authorised part-load running experiments in order to improve reactor efficiencies. The ‘experiment’ involved trying to maintain reactor stability and preventing xenon formation by varying the time span of control rod insertion. This was being done manually when the reactor temperature first fell and then suddenly surged. The consequent massive peak in temperature and the pace of the run-away reaction overwhelmed both the manual and automatic control rod processes.

The emergency SCRAM alert state was automatically tripped as the core temperature soared with only 28 out of 211 control rods in place – and within 30 seconds 18 out of those 28 had fractured due to heat. It was at this stage that the first steam explosion occurred as the steam discharging from the broken fuel channels instantly and massively expanded, shattering the inner containment and blasting the upper containment forged steel plate through the roof.

Graphite fire

The Russian RBMK reactor, like many designs world-wide and including all of the UK’s entire reactor fleet in 1986, was based on a graphite core through which the fuel and control rods were passed in channels. But the problem with graphite is that although it can moderate the speed at which reaction neutrons move, like coal it is around 90% carbon in content. Consequently, when the 10,000o C core was blasted open to the outside atmosphere, the oxygen in the air instantly reacted with the red-hot graphite and started to burn fiercely. (This led to the rather tasteless joke at the time that Chernobyl was the world’s first ever coal fired nuclear disaster).

With an initial steam explosion followed by a hydrogen explosion and a simultaneous graphite core fire – all within seconds – the power station staff were completely overwhelmed. Which is when a collective act of the most tremendous heroism took place. Fully aware they were already fatally irradiated, the 30 reactor workers dosed themselves with potassium iodine tablets, donned respirators and decontamination suits and entered what remained of the upper reactor level in a bid to supress the fire. Later they all died in lead-clad hospital isolation within 24 hours.

As they tried to cope with the effects of the second explosion however, a third ‘runaway criticality’ explosion – the kind associated with a nuclear weapon – occurred. This removed most of reactor four’s upper building and set fire to the reactor three building’s roof.

Collateral damage

The fires at Chernobyl raged for 14 days, during which time teams of local miners were drafted in to mine under the blazing reactor in order to get concrete ballast under the foundations and prevent a core melt-down into the water table. Thousands of volunteer ‘bio-robots’ were drafted into the station complex and some 500,000 such workers were rationed to a maximum 40 seconds of work near the reactor – during which time they are thought to have received over 50 lifetimes of radiation.

Around Pripyat 53,000 people were immediately evacuated – for ever. And although the International Journal of Cancer estimated around 4,000 deaths in Ukraine from the accident, a further 4,000 ‘excess’ cancer deaths were estimated for neighbouring Belarus. Greenpeace estimated over 200,000 excess cancer deaths in Ukraine, Belarus and Russia for the ten-year period following the reactor explosion.


The Chernobyl Four reactor was a tried and tested design common throughout much of the former Soviet Union and Eastern Europe. It was based on a graphite moderator core which was a material choice common among nuclear power countries – and in particular in the UK where all the Magnox and Advanced Gas-cooled Reactor (AGR) stations operating at the time of Chernobyl were graphite moderated – albeit CO2 gas cooled. Many of the problems associated with graphite block constructed cores, particularly radial cracking around the fuel and control rod channel pathways, have manifested themselves on all of the AGR stations in the UK, four of which are still operating.

The second-by-second cooling requirements of a Chernobyl type and size of reactor are formidable. Each reactor core has 1,600 fuel rod channels, each of which requires a constant flow of 28 metric tonnes of water per hour. It was estimated that a 30 second cooling water failure at Chernobyl would result in a fire, but when the cooling water supply began to fail as the pumps were denied power from the reactor, the diesel power took over 75 seconds to come on line – by which time the core was alight.

Sequentially every safety system went down, leaving a desperate reactor crew in their dying moments to bring the core under control by manhandling control rods into already blocked channels.

Circles of hell

Chernobyl at that time was the first ever level seven nuclear event – only to be surpassed by the Fukushima events of 2011. But the two events involving repeated safety system failures in split-second sequences were the stuff that any future nuclear catastrophe will be made of. When human fallibility becomes a technological hubris that in turn is applied to energy processes hotter than the sun, and then with the addition of power production for profit, the worst can and will happen.

Valery Khodemachuk, the nuclear age’s pharaoh, got a new sarcophagus in 2019. The original one constructed between 1987 and 1989 had begun to fall apart. With the assistance of the European Bank of Reconstruction and Development, his new tomb cost a cool €2.15 billion. It is intended to last for 100 years. The pyramids of ancient Egypt have lasted 6,000 years, whilst the half-life of some of Chernobyl’s isotopes is beyond 180,000 years – longer than the human span on Earth. We have been warned.

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