• Lessons and doubts from the Chernobyl disaster

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      Patrick Lavery

      Combustion Industry News Editor

One of the many striking passages of Serhii Plokhy’s excellent Chernobyl: History of a Tragedy (Penguin, 2018) comes in the preface:

“Altogether, 50 million curies of radiation were released by the Chernobyl explosion, the equivalent of 500 Hiroshima bombs. All that was required for such catastrophic fallout was the escape of less than 5 percent of the reactor’s nuclear fuel. Originally it had contained more than 250 pounds of enriched uranium – enough to pollute and devastate most of Europe. And if the other three reactors of the Chernobyl power plant had been damaged by the explosion of the first, then hardly any living and breathing organisms would have remained on the planet. For weeks after the accident, scientists and engineers did not know whether the explosion of the radioactive Chernobyl volcano would be followed by even deadlier ones. It was not, but the damage done by the first explosion will last for centuries.”

This is perhaps only a slight stretch – damage to the other three reactors would have had to have been of a certain nature to have led to a widespread release of radiation. But this passage is good at capturing the disturbing fact that although the Chernobyl nuclear accident as it was could be counted the worst man-made disaster in history, it could have been a great deal worse. And it is only one of the fascinating aspects of the history that Plokhy, a professor of Ukrainian history at Harvard University, tells in rich detail and with a fine sense of delivery. Many of those aspects are directly relevant to today, in particular nuclear safety in the context of nuclear power’s role in mitigating climate change, the interplay of public opinion, environmental and economic concerns, and the geopolitical tensions between Russia and Ukraine.

As one would expect from a professional historian, the book is broad in its coverage of the disaster. While there is a painstaking reconstruction of the timeline of the actions that led to the accident, and the immediate response to it, there is also in-depth description of the domestic and international political ramifications over the subsequent months and years, as well as accounts of the fates of individuals and their families that were caught up in tragedy. Explanation of the peculiarities of the Soviet political system also help to explain the causes and consequences of the disaster.

The Soviet Union was the first nation to open a nuclear power station, in 1954. Three years later, an explosion of a waste tank at the Mayak plutonium production site (for nuclear weapons) in the Chelyabinsk region of the east Urals, which released 20 million curies of radiation, forced the evacuation of numerous villages in the area, and also led to the distribution of potassium iodide pills to protect people from thyroid cancer, though the accident itself was kept secret. It was this disaster that gave the blueprint for the response strategy for the Chernobyl accident 32 years later. Also directly related to Chernobyl, but for design reasons, was an accident at Leningrad Nuclear Power Plant in 1975. This plant, completed in 1973, used the same RMBK (high powered channel reactor) design as the four units at Chernobyl, and experienced a drastic rise in radioactivity which destabilised the reactor, due to a ‘positive void effect’ during a shift in operating modes. Control rods inserted into the reactor to slow the chain of nuclear reactions had, at a certain shallow depth of insertion, the effect of speeding them up, and this melted one of the fuel channels in the reactor. An emergency shutdown of the reactor successfully prevented a more serious accident, but subsequent ‘cleaning’ of the reactor with nitrogen was followed by release of the cleaning fluid into the environment through an exhaust pipe – some 1.5 million curies of radionuclides with it. (One curie can contaminate around 9.5 billion litres of milk, making it unfit for human consumption.)

Importantly, like the Mayak disaster, the Leningrad accident was kept under wraps. Details of the accident were not communicated to operators of other nuclear plants, due to a culture of secrecy and a failure to admit mistakes (which would bring disrepute to the Soviet system). What was communicated were instructions for the improvement of control rods for RMBK reactors, but the Chernobyl operators, unaware of the details of the Leningrad accident, considered it a minor issue.

This secrecy was profoundly consequential, but not the only reason for the Chernobyl disaster. From my reading of Plokhy’s work, there were a number of reasons for the accident to have occurred. This is my ranking of them:

  1. The RMBK reactor type was chosen over the VVER (water-water energy reactors) type because it produced double the power at a cheaper price. VVERs, like their American pressurized water reactor counterparts, submerged fuel rods into pressurized water, also using water as the coolant, a safer design than the RMBK reactor, which used graphite to moderate the fission chain reaction and water as the coolant. If a VVER experienced a loss of coolant, the higher heat in the core would lead to less pressurized water, which in turn would slow the chain reaction. For the RMBK reactor, there was not this self-limiting process. RMBKs were cheaper because they only needed 2-3% enriched uranium, and could be built without high-precision equipment, while VVERs, which grew out of military use of uranium, required entirely enriched uranium. Most importantly, the design of the RMBK reactor and its control rods was flawed as the Leningrad accident had demonstrated.
  2. Details of the Leningrad accident were not communicated to other plant operators.
  3. At Chernobyl, a new safety test for using waste heat during shutdown was scheduled to take place during a scheduled shutdown at the plant. This test necessitated the overriding of the emergency shutdown mechanisms that saved the Leningrad plant.
  4. The scheduled shutdown was delayed, and spread over numerous operator shifts, because a different power plant in the grid was having problems, making the grid operators demand prolonged output from Chernobyl Unit 4. Not all operators had been briefed about the new safety test and what it would involve.
  5. Having the assurance that RMBK reactors were “as safe as a samovar” and that explosions were impossible from the designers of the reactor, the operators had a very high confidence in its safety during any mode of operation, such that warning signals that something was going wrong were ignored.
  6. Some construction defects due to the rushed nature of work as a result of unrealistic timeframes set by those in power may also have contributed.

In fact, it took hours for the operators to begin to contemplate, and then accept that the reactor had failed. (Initially, they had refused to accept the idea, even while vomiting from radiation sickness, ascribing their nausea instead to shock.) This was part of the reason why the response to the accident was not as good as it could have been – people did not fully comprehend what the situation was. At the same time, another culture of shifting responsibility up the chain of authority, and for downplaying the seriousness of situations, delayed and muted the response. This was not helped by the fact that the dosimeters used to gauge levels of radiation only went up to 1,000 microroentgens per second (3.6 roentgen per hour), a fraction of the true amount, allowing early reports to understate the severity of the situation. (One dosimeter brought to the site was capable of measuring 200 roentgens per hour, and also went off the scale; when its user reported the reading to his superiors, he was told to “go away”. The actual levels emitted by debris around the reactor, estimated later, were around 10,000 roentgens per hour; 67 million times higher than the background level, 150 microroentgens per hour, and way above the ‘emergency’ acceptable level for operators that could be endured for a few hours, 25 roentgen.) A further factor that made the consequences of the accident worse was that the Chernobyl reactor, like other Soviet nuclear reactors, had no outer concrete shell for radiation containment in the event of an accident.

Denialism kicked in, with no Soviet media report on the accident for almost three days, and then only a brief bulletin on April 28, which read “An accident has taken place at the Chernobyl atomic electricity station. One of the atomic reactors has been damaged. Measures are being taken to eliminate the consequences of the accident. Assistance is being given to the victims. A government commission has been struck to investigate what happened.” Three days later, the May Day parade in Kiev nevertheless was ordered by high officials in Moscow to proceed, despite radiation levels of 2.5 roentgens, far above anything considered safe. Many officials aware of the radiation levels in the city of Kiev did not attend.

While the Soviet system has much to blame for the accident and the response to it, there was a heroic aspect to Soviet culture that aided the response. A well-known motto at the time was “Who if not we?”, and when ‘liquidation’ (clean-up and capping) of the site began in earnest, after the immediate threat of the accident seemed to have passed, tens of thousands volunteered to help, aware of the risk to themselves. (Many others were ordered to help.) During the immediate response to the accident there was also a form of this self-sacrifice, as firefighters and other first-responders began to experience radiation sickness after only hours of exposure, and, suspecting their fate was sealed, chose to undergo further exposure to help prevent exposure to others. The request by fireman Lieutenant Volodymyr Pravyk, as he was being taken to hospital after helping tame the fire at the rector, to others to tell his wife to close the window of the apartment where they lived with their young daughter, is a touching illustration of the naïve realisation of the risk involved.

Those trying to tame Unit 4 had grave concerns after the initial explosion and a number of subsequent explosions. Having little understanding of what had actually happened, nor of the structural state of the chambers around the reactor, nor of the state of the nuclear fuel and control rods, nor of the material that had been dumped by helicopter onto the broken roof of the reactor (a combination of sand, boron and lead), for weeks after the accident it was feared that there could be further explosions, even worse than the initial one. There was a separate concern, too, that the heat of the reactor would lead it to melt into the ground and into the groundwater. The contaminated groundwater would then find its way into the nearby river system, then into the Black Sea, and ultimately the world’s oceans, poisoning them all. At one point, a 500 km exclusion zone was being discussed, much greater than the 30 km already established.

These concerns speak to the scale of risk presented by the Chernobyl plant, and it is impossible to read the book without thinking of the long-term safety of the nuclear industry. If one plant could imperil much of life on Earth, why ever build it? If the answer was ‘it was considered entirely safe’, and such a disaster happened, why are governments today considering nuclear power plants as a component in mitigating the worst of climate change? The answer the industry would give is that of all available power generation, nuclear power is statistically the safest, that is, there is greater risk to human life from all other dispatchable types of power generation (as well as wind power). Design of reactors has improved since the Chernobyl era, construction practices are safer, operating practices have improved, and software may also contribute to safer operation.

Calculation of such risks has been central to the European Commission’s recent controversial recommendation to consider nuclear power “sustainable”. A report examining various power generation technologies includes Figure 3.5-2 below, which summarises frequency and fatalities for a range of technologies. As one can observe, the Chernobyl disaster in its latent fatalities (those resulting long-term from the accident) is probably the worst ever power-related disaster, comparable to the very worst of dam failures. However, second (PWR, similar to the VVER) and third generation reactors are modelled as 100 to over 1000 times less risky. The report notes, however, that “for nuclear energy, due to the very low number of historical severe nuclear accidents and their significance for risk assessment, an approach based on the use of a simplified, site-specific, Level 3 probabilistic safety assessment is used to quantify the risks associated with hypothetical severe accidents.”

diagram showing the comparison of frequency-consequence curves for full energy chains in OECD and non-OECD countries for the period 1970-2008

As this suggests, it is partly the very nature of the rarity of such accidents that their frequency and consequences are not well understood. With so few data points, it is difficult to determine statistically what was a consequence of Chernobyl, to separate coincidence from causation. Estimates of the consequences of the Chernobyl accident vary enormously. The internationally recognised toll is of 31 from the immediate results of the accident, while a UN estimate in 2005 was that only 50 deaths could be directly attributed to the disaster, though in time that could climb to 4,000. By that year, however, Ukraine was already paying 19,000 families special welfare as a result of losing a breadwinner in the family as a result of the Chernobyl accident. (That number is now 36,525.) Hundreds of thousands (as many as 830,000) of ‘liquidators’ from across the Soviet Union were involved in the clean-up of the plant, and Plokhy writes that their average level of radiation exposure was 12 rem, 120 times the yearly dosage considered safe by the International Commission on Radiological Protection. (Plokhy estimates 600,000 liquidators.) In the contaminated areas of Ukraine, the rate of death in 2007 was 26 people in every 1000, compared to 16 per 1000 in the rest of Ukraine. And Ukraine is not even the country most affected by radiation from the Chernobyl accident – that tragic crown is worn by Belarus. It seems reasonable to suggest that that official death toll of 31 is 1000 times too low, perhaps more. (Greenpeace International’s estimate is 90,000 deaths.) The historical data in Figure 3.5-2 is not in fact reassuring as to nuclear safety – and if 10, 20, or 50% of Unit 4’s fuel was spread in the explosion, instead of 5%, as could have plausibly happened, the chart would be worse still.

Another source of uncertainty in regard to the risks posed by the Chernobyl accident is that there are still gaps in the understanding of the physical state and behaviour of the disaster site. Last year, 35 years after the accident, it was reported that the reactor was heating up again for reasons that were “unclear”. The situation was described as having “no chance of a repeat of 1986”, but this assumes stability of other factors, such as the integrity of the containing structures built after the accident.

What is thankfully clear is that the frequency of nuclear accidents has been diminishing, and this supports the idea that better design, construction and operations are making the nuclear industry safer. One listing of existential threats that reflects the famous Doomsday Clock (which in its January 2022 update was kept at 100 seconds, the closest to midnight it has ever been) does not even include an explicit civil nuclear risk. Yet tracking historical performance also undermines the understanding of risk. Nicholas Nassim Taleb’s account of ‘black swan’ events – those so rare or so unexpected they are not predicted – is useful in the nuclear context. He gives an analogy of risk prediction solely using historical data: a turkey being fattened for Christmas would, looking at his treatment prior to the feast, consider that everything is going swimmingly until the fatal day. The Fukushima Nuclear Disaster of 2011 was such a black swan event, brought about by a tsunami caused by a massive earthquake. (Notably, too, the official death toll from that disaster is zero, but the Japanese government has already recognised one death, and there is currently a lawsuit being brought by six young Japanese people who claim they have suffered thyroid cancer as a result of the accident.) Who could have predicted the string of circumstances that led to the Chernobyl disaster? Who knows what circumstances will occur in the future? What might happen, for instance, in future cases of natural disaster, war, hacking, terrorism, neglect, or unforeseen design flaws? Probabilistic safety assessments are designed to try to model combinations of such events, but truth is stranger than fiction. Plokhy raises similar concerns, writing:

“Today, the chances of another Chernobyl disaster taking place are increasing as nuclear-energy technology falls into the hands of rulers pursuing ambitious geopolitical goals and eager to accelerate economic development in order to overcome energy and demographic crises while paying lip service to ecological concerns. While the world attention is focused on the non-proliferation of nuclear arms, an equally great danger looms from the mismanagement of “atoms for peace” in the developing world. The story of Chernobyl points to the need to strengthen international control over the construction and exploitation of nuclear power stations as well as to develop new nuclear technologies.”

I would disagree with Plokhy in such an estimation of risk as “equally great”. I think it is more uncertain than that, less statistically knowable than the phrase implies. (It also implies a discrimination that may be difficult to justify – should the developed world be allowed to deploy nuclear power, but not the developing world?) Yet strategic decisions have to be made about energy systems, including nuclear power, under such uncertainty. I hope such decision makers read Chernobyl: History of a Tragedy, and that the modelling shown in Figure 3.5-2 proves accurate.


As noted earlier, two other entwined matters from Plokhy’s history also speak to today. One is how profoundly the accident gave an impetus to the break-up of the Soviet Union and the campaign for Ukrainian independence. Faced with an ecological nightmare – exemplified by the case of a calf being born with no eyes – eco-activism surged. Ukrainians tapped into an underlaying well of regard for the environment, and felt they were the victims of the Russian authorities’ mishandling of the response to the accident. Mistrust that was already present prior to 1986 ballooned into a popular movement, and by 1991 this did in fact lead to independence. Ukraine’s independence is now at the centre of geopolitical rivalries between Russia, Europe and the USA, which once again have at their heart considerations around energy – this time natural gas. Seeing Ukrainian independence in the light of the Chernobyl disaster helps to better understand today’s situation.

An economic crisis also ensued. The cost of the response to the accident was monumental, and crippled the Soviet economy. Upon independence, Ukrainian authorities shut down the Chernobyl plant, and set up a welfare system for victims of the disaster. However, faced with its own economic meltdown, Ukrainian authorities, supported by much of the population, decided to reactivate the remaining three units at the Chernobyl plant. Similar forces played out in Lithuania, where an eco-political movement centred around closing the Ignalina Nuclear Power led to the Baltic country being the first to declare its independence from the Soviet Union. When Lithuania was faced with an economic blockade from the remaining bulk of the Soviet Union, it decided to restart the Ignalina plant to help ensure economic independence. For both Ukraine and Lithuania, a steady source of electricity and economic considerations trumped the environment. Such decision making is already playing out in the context of climate change, and is likely to for decades to come.