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Chernobyl Nuclear Accident – What Happened?

26 huhtikuun, 2021
The Dark Horse – Nuclear Power and Climate Change

This is a sample chapter from our recent book The Dark Horse – Nuclear Power and Climate Change (link to Amazon store). The book was published in 2020, and this article came out on the 35th anniversary of the Chernobyl accident. It is our effort to counter the misinformation out there regarding the most serious nuclear accident of our history and what its impacts are.


By far the most serious nuclear power plant accident happened on 26th of April, 1986, in Chernobyl nuclear power station, in what is currently Ukraine. The hydrogen/steam explosion in the core and the fire that followed spread large amounts of radioactive matter to the immediate surroundings as well as farther away. Some emergency workers received radiation doses so high that they died due to acute radiation poisoning and its later complications during the next few years. The raging fire in the core lifted the lighter radioactive particles high into the air and during the following days, winds blew them across Europe, where they fell with rainfall.

The fallout was so severe in the nearby areas that people had to be evacuated. Officials should have distributed iodine pills to the public and advised them not to eat vegetables or drink milk produced in the area, since they were contaminated with iodine-131. But Soviet officials dropped the ball. Without their paranoid secrecy on all things nuclear, the health effects would have been much lower.

As it is with all things, nuclear reactors can be designed well, or less well. They can also be designed horribly. This category is reserved for the RBMK, which was the type of reactor in Chernobyl. It was designed in the 1950s, and all the Soviet scientists had to work with were the small “graphite piles” that were used to produce plutonium for the weapons program. The main goals for the RBMK were low costs and flexible operation. Low cost was achieved by making the reactor very large and allowing it to use uranium fuel that did not have to be enriched to similar levels as the fuel for western reactors. Flexible operation meant that the reactor could be refuelled without shutting it down. This had two goals: the primary goal was to minimize interruptions to power production. The secondary goal was to make it easier to use the reactor for weapons grade plutonium production, should the need arise.

These parameters led to using graphite as the moderator for the nuclear reaction instead of heavy water, which was four times as expensive. The reactor ended up being very large, so it would have been very expensive to build a containment building around it. Some have estimated it would have doubled the cost of the reactor. So containment was left out. Costs were also cut by leaving additional safety systems to a minimum. The prevailing Soviet doctrine concluded that containment buildings and safety systems were unnecessary, since in principle the RBMK was completely safe, provided it was operated exactly according to the instruction manual.

However, even that manual omitted some important weaknesses of the RBMK, which were never told to the operators. The design choices combined with the use of low-enriched uranium fuel meant that the reactor could become unstable in certain conditions. Most reactors are designed in a way that if the coolant (often water) is removed, the reactor shuts down. With RBMK, the opposite happens. If the cooling water turns to steam and the cooling system dries up even partially, the chain reaction in the core accelerates. If reaction controls do not operate fast enough or if they are turned off, this will lead to even more water turning to steam, accelerating the chain reaction further, which will eventually lead to a meltdown. This was especially likely when the reactor was operating at lower power, during which the coolant pressure and boiling point were lower.

Finally, the control rods of the reactor were badly designed. The tip of the rod was made of graphite, which accelerates the reaction. When inserting a completely removed rod into the reactor, the fission reaction accelerates for about five seconds before starting to slow down. This was not a big problem when a single rod was inserted. But inserting all 211 control rods at the same time to the reactor operating at full power had catastrophic effects. To top it all off, the mechanism of inserting the control rods was slow even by 1980s standards. Western designs of similar age inserted control rods in three seconds, while the same operation took RBMK some 20 seconds.

The people at the Kurchatov Institute who designed the reactor were not stupid, and they were well aware of the inherent problems in their design. The Soviet officials did not want to hear of any problems, however. Everything nuclear was by definition top secret in the Soviet Union, and the operating principles of the RBMK were no exception. Reactor operators were forbidden to discuss any possible problems with each other. So when the operators at the Ignalina nuclear power plant discovered the problem with the control rods in 1983, they told no one about it. Nobody imagined that someone would manually turn off all the safety features and then run the reactor at dangerously low power.

Chernobyl – What happened?

A safety experiment was scheduled for reactor 4 in the Chernobyl nuclear power station in April 1986. The goal of the experiment was to ensure that the reactor could be cooled if there was a power loss during a shutdown. Previous experiments had failed, and the politically appointed management, who had only superficial knowledge of nuclear power, were hard-pressed to make certain that this time the experiment would succeed. Partly due to the embarrassing former failures, and partly due to the low perceived risk, Soviet nuclear safety officials were not notified of the experiment.

The plan was to run the reactor at a low power level of 700 MW (thermal), and then cut the flow of steam to the generator. The generator, running at full tilt, would keep on spinning, and its behaviour would be monitored and measured. This would help ensure that the power it generated would be enough to keep the coolant pumps running long enough for the diesel-powered backup generators to start up.

It was planned that the reactor would be powered down slowly to the desired level during the early morning hours of 25th April. From there, the dayshift, who had been familiarized with the experiment, would be able to take over and start the experiment. As the day shift came in, another power station got dropped from the grid, and the grid operator in the area asked Chernobyl operators to postpone the experiment. This was agreed on, but the preparations were kept underway, and even some of the safety features were turned offline in advance. The grid operator gave permission only late that night, at 23:04, to continue with the powering down. The day shift was long gone, and even the evening shift was starting to leave. The night shift that had just arrived got orders to go ahead with the experiment as soon as possible. The reactor power level was promptly lowered. Here one of the operators made a mistake: he pushed the control rods in too deep, and the reactor was practically shut down.

The experiment could not be carried out with a reactor that was shut down, and the operator managing the experiment demanded swift action. The automatic system that moved the control rods in the core was turned off (an essential safety feature in the RBMK). Most of the rods were then pulled out from the core manually. The chain reaction kicked in, and in a few minutes the power level increased to 160-200 megawatts thermal. This was not enough for the experiment, and the automatic warnings were bypassed, more rods were pulled out and preparations for the experiment were continued. Numerous warnings came on, but the operators did not realise that the momentary shut down and other preparations had caused the reactor to go unstable and unsafe for the experiment, which could only be remedied by letting the reactor run for a while on a higher power level.

The experiment was then started by cutting the steam flow to the generators. The generator slowed down and the power to the pumps responsible for the pressure and circulation of coolant water went down. As the pressure went down, the boiling point of the circulating water lowered. As the water started to boil and turn to steam, the chain reaction in the RBMK accelerated – which is the opposite of what happens in most other reactors. Apparently, nobody had told the operators of this critical “feature” of the RBMK.

As the chain reaction accelerated, the automatic safety system started lowering control rods into the reactor to keep the power more stable. But only 12 rods had been left to be controlled by the automatic system, while the other 199 rods were on manual control and drawn completely out of the reactor. 36 seconds after the experiment was started, the 12 rods were fully inserted, but the power level kept increasing. At this point, someone decided to initiate emergency shutdown.

Emergency shutdown started pushing all the remaining rods into the reactor at once. Due to the bad design of the control rods we mentioned earlier, this accelerated the nuclear reaction for a few seconds before it would slow down, especially with many rods being pushed in at the same time. Although this “feature” had been noted three years earlier in the Ignalina power plant, nobody had shared the information with Chernobyl operators. The reaction accelerated so fast that at least part of the rods overheated and got stuck before the part of the rod that would slow down the reaction got inserted into the reactor.

During the next few seconds, the chain reaction went hypercritical and totally out of control. The power level indicators showed 33,000 megawatts, while the reactor’s designed operational power was 3,200 megawatts. It is impossible to reconstruct what happened next, but the best guess is that the tremendous heat energy produced caused a massive steam explosion, which popped the 2,000-ton top of the reactor vessel through the roof of the reactor building. This explosion wrecked the rest of the cooling systems in place, and the RBMK pulled one last trick from its sleeve: With all cooling gone, nothing held back the chain reaction. Another, even more massive explosion destroyed the reactor core and sent the highly radioactive fuel and graphite pieces around the compound. As air was drawn into the torn reactor core, the red-hot graphite started burning. Despite heroic and quite literally self-sacrificing actions of the emergency workers, the graphite burned for two weeks, sending most of the remaining radioactive materials high into the atmosphere and surroundings.

The following morning, Soviet leaders woke up, knowing nothing of what had happened and at what scale. It was not unusual in the Soviet Union that environmental destruction or accidents went unreported, and the nuclear program was especially sensitive. The scale of what occurred started unwinding when Finnish border control officials noticed abnormally high radioactivity levels. This information was forwarded to the prime minister Kalevi Sorsa, who decided not to publicize the matter, as it would probably only antagonize Soviet Union.

The following morning, 27th of April, the radiation alarms in the Swedish Forsmark nuclear power station went off. The source was found to be the dirt from one employee’s boots, which carried radioactive fallout from Chernobyl brought to ground by rain. In cooperation with Finland, the Swedes located the likely source to be a nuclear power plant accident in western Soviet Union. The fallout spread all over Europe, and the cesium-137 can still be measured in many places. As denial finally became impossible, Soviet officials had to admit to the world that the accident had happened.

Pripyat, a town near the Chernobyl nuclear power station, was evacuated in the afternoon of 27th April. Kiev, which was 90 kilometres (55 miles) away from Chernobyl, went on preparing their parade for the 1st of May. Nobody was giving the public iodine tablets, even though it was clear that the destroyed reactor had leaked large amounts of iodine-131. Iodine-131 is a radioactive isotope that is often the most dangerous radiological consequence of nuclear accidents, but also easily prevented from causing harm by giving the population iodine tablets. It also disappears in a few weeks. A short notification about the accident was finally read in the evening news of 28th April, but there still was no mention about any dietary restrictions.

It is common to see the Chernobyl accident referred to as a typical example of how dangerous nuclear reactors can be. This has little to do with facts and a lot to do with perceptions. This type of reactor, with the unique properties that led to the accident, has not been built in decades, and was only built in the Soviet Union. While there are still a few such reactors operating, they have all been retrofitted with improved safety measures. One rather surprising and even scary fact remains less discussed: Chernobyl reactors 1, 2 and 3 remained in operation even after the accident. Reactor 3 was the last to shut down, in December 2000. It would be wise to replace the remaining operating RBMK’s with safer designs as soon as possible.

Recently, many people have seen or heard of the HBO mini-series about the Chernobyl accident. While highly entertaining, it is also fictitious in some important ways. Jaakko Leppänen, a research professor of nuclear technology at VTT Technical Research Centre of Finland Ltd, wrote a simultaneous blog as the original episodes were aired, fact-checking the events as they unfolded in the mini-series. The blog can be read here:

Health impacts of Chernobyl

Chernobyl has claimed a few dozen lives, according to the World Health Organization and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). According to UNSCEAR’s latest, most comprehensive report[i] to date on Chernobyl:

  • 134 plant staff and emergency workers suffered acute radiation syndrome (ARS) from high doses of radiation.
  • In the first few months after the accident 28 of them died.
  • Although another 19 ARS survivors had died by 2006, those deaths had different causes not usually associated with radiation exposure.
  • Further, some 6,000 thyroid cancer cases were reported in the three most affected countries of Belarus, Ukraine and four most affected regions in the Russian Federation. 15 of them had proven fatal, although very likely not all of them were caused by the Chernobyl fallout.

At most, there are 62 confirmed fatalities from the worst nuclear accident in our history. Statistically, there might be some 4,000 extra fatalities all in all due to the low doses of radiation spread around, according to some LNT models. However, these models have what the UNSCEAR calls unacceptable uncertainties at these low doses and has recommended that they are not used for epidemiological purposes[ii],[iii].

The cleaning up afterward was done by roughly 600,000 “liquidators”, of whom only a small number received significant amounts of radiation. Iodine-131 was the largest health hazard for those living in the surrounding area. As we know, iodine-131 has a short half-life so it was essentially gone in two months. The risk on the longer time frame is from fission products such as cesium-137 which has a half-life of 30 years and caesium-134 with a half-life of two years.

Luckily, these health risks are proving to be less serious than many have thought. Around 6,000 thyroid cancers have been diagnosed (up to 2005) in the group most at risk (mainly children living in nearby areas), with 15 fatalities. Even as most thyroid cancers were treated successfully, almost all cancers could have been avoided with prompt actions by the government to distribute iodine pills.

Very small doses were delivered to a larger population of around 5 million; the LNT model suggests 5,000 could die prematurely. Even smaller lifetime doses, in the neighbourhood of 1 mSv or less (a week or so in Pispala), are estimated to have been delivered to hundreds of millions of people living in Europe and the rest of the world. The Union of Concerned Scientists has done their own calculations and point out that (according to LNT model) the final death toll would be somewhat higher (at 27,000 total fatalities) than that mentioned above, due to these very small doses.

These doses, however, are well within the variation of background radiation, and similar or even much smaller than doses routinely received from medical imaging and such. As discussed earlier, there is little evidence that small doses have the linear health consequences that LNT suggests, and the major expert organizations on radiation and its health effects have concluded that LNT model should not be used for epidemiological purposes. For example, conclusive studies have found that no additional cancer cases have been caused by Chernobyl fallout in Finland (2 mSv lifetime dose).

Some groups have not been content with the relatively low mortality estimates of the World Health Organization and other agencies that comprised the Chernobyl Forum.[iv] They have proceeded to make their own, non-peer reviewed studies on the matter, resulting in tens, even hundreds of times more estimated fatalities.

European Greens were among those disappointed by the Chernobyl Forum’s results, so they commissioned and published their own TORCH-report[v] (The Other Report on Chernobyl). It again made good use of the LNT model and stuck to it on a purely theoretical level, whereas the Chernobyl Forum had searched for actual empirical evidence of harm to people’s health. TORCH managed to report 30,000 to 60,000 extra cancer deaths in the whole of Europe.

Next was Greenpeace[vi], who calculated that Chernobyl would eventually cause around 93,000 premature extra fatalities. The method was rather curious: any area that had any amount of fallout was included. If there were any increased fatalities in that area after 1986, these were counted as being due to Chernobyl. This dubious method meant that the increased fatalities that were due to liver cirrhosis in the area of the recently dissolved Soviet Union were promptly attributed to Chernobyl. The report did not question whether there could have been other reasons for liver cirrhosis than radiation from Chernobyl. Perhaps alarms should have gone off, as there are no studies showing that radiation causes liver cirrhosis.

Even this estimate, 10 times higher than that of the Chernobyl Forum, was not enough for everybody. Alexei V. Yablokov, one of the authors involved in the Greenpeace report, a former member of Russian Academy of Science and one of the founders of Greenpeace Russia, wrote a book Chernobyl: Consequences of the Catastrophe for People and the Environment (2007). It is based mainly on a wide variety of materials written in the Slavic languages. The book claims that around a million humans have died or will die due to the Chernobyl accident, growing the official estimates a hundred-fold and Greenpeace’s estimates ten-fold. Experts who have reviewed the book (and who know Russian language and can check some of the sources used) have noted that as a scientific study, the book has negative value. [vii] It uses fictional novels as sources, and scientific literature receives scant attention.

In addition to blaming liver cirrhosis and other diseases that have no known connection with radiation, the book resorts to fabrication and lying when scientific reality does not concur with the author’s preconceptions. For example, in the chapter dealing with Finland, a study concerning the area of Tampere concluded that birth defects had decreased after the Chernobyl accident. Yablokov, on the other hand, first generalized the study to cover the whole of Finland, and then promptly changed “decreased” to “increased”.

These kinds of problems have not stopped some anti-nuclear people from using Yablokov’s book as the definitive proof on the destructiveness of nuclear accidents.[viii]

Indeed, the common strategy for many anti-nuclear advocates seems to be ever-more imaginative and ridiculous claims that are poured on the unsuspecting public, as it is a well-known psychological fact that people often think, despite the actual evidence, that the truth is to be found somewhere in between the extreme claims made. This same tactic is used with Fukushima, as we will learn later in this book.

The disproportion between the actual evidence and the amount of public worry is enormous, especially if it is compared with common, everyday risks our society accepts to ensure everyday access to energy services. Coal is probably the most glaring of these. Coal mining accidents alone kill thousands every year, with tens of thousands getting sick and permanently disabled due to coal dust and other hazards. [ix] Air pollution kills roughly seven million each year, according to WHO.[x] Catastrophic climate change, which might end up being the biggest risk associated with coal burning, is not even included in these numbers.[xi] Greenpeace has done a study on the matter a few years back, as have other environmental NGOs more recently. These studies find that around 20,000 people are killed each year in Europe alone as a result of coal burning. In addition, coal burning distributes heavy metals and other toxins which have no half-lives to the environment.

The scale of this gap between reality and perception is so enormous that it makes it difficult for many to believe. Even if we take Greenpeace’s numbers at face value – for which there is no good reason – it could be said that a Chernobyl accident every few years would be an acceptable price for giving up coal burning from a public health perspective. Sadly, we would soon run out of Chernobyl-type reactors and would have to content ourselves with less harmful accidents.


[ii] ICRP. ICRP Publication 103: the 2007 recommendations of the International Commission on Radiological Protection. Ann ICRP. 2007;37(2-4):1–332.

[iii] UNSCEAR. Report of the United Nations Scientific Committee on the Effects of Atomic Radiation Fifty-ninth Session (21-25 May 2012). New York, NY: UNSCEAR; 2012: Report No. A/67/46.

[iv] The report was done and published by IAEA, WHO, UNDP, FAO, UNEP, UN-OCHA, UNSCEAR, World Bank and the governments of Belarus, Ukraine and Russia. The original report can be downloaded from




[viii] Such as Helen Caldicott, who also claims to be strictly for science and evidence.


[x] 7 million premature deaths annually linked to air pollution, World Health Organization (2014).

[xi] European coal pollution causes 22,300 premature deaths a year, study shows, The Guardian (2013).


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