Jaakko Leppänen – June 10, 2019 (updated June 19, 2019)
The purpose of this blog is to provide insight and expert view on various topics related to nuclear energy in a clear and comprehensible manner, yet without dumbing things down or cutting any corners. All previous texts were written in Finnish, but since we have some international readers as well, I thought it would be interesting to translate a recent post on HBO’s mini series ”Chernobyl” into English. The original text makes several references to previous posts, also in Finnish, so I have tried to fill in the missing pieces with additional notes.
The motivation for writing about this TV-show was not to point out errors or debunk events that did not occur, but rather to provide a commentary on how the story line is linked to actual events, and the science and technology behind them. I will correct some factual errors along the way, but that is not the main purpose of this writing. My professional background is in reactor physics, so there is a strong emphasis on the events that lead to the reactor explosion.
The main references used for this writing are the INSAG-7 report by the International Nuclear Safety Advisory Group (INSAG) of the International Atomic Energy Agency (IAEA), and the reports from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).
The HBO mini series is divided into five episodes:
- Episode 1 – 1:23:45
- Episode 2 – Please Remain Calm
- Episode 3 – Open Wide, O Earth
- Episode 4 – The Happiness of All Mankind
- Episode 5 – Vichnaya Pamyat
I will cover the events and the related technical/scientific background in the same order as the story line proceeds, trying to avoid spoilers for future episodes as much as possible. A few concluding remarks are included at the end.
Episode 1 – 1:23:45
The accident occurred on Saturday, April 26 1986, at unit 4 of the Chernobyl nuclear power plant in Ukraine, former Soviet Union. The reactor core was destroyed by a massive explosion following a routine turbine test, which was carried out as the reactor was being shut down for annual maintenance. The reactor was of the RBMK-type, which essentially means a channel-type, water-cooled, graphite-moderated boiling water reactor. The specific characteristics of the RBMK will be discussed in later episodes.
The actual events begin from the moment immediately following the explosion, which took place at 01:23:45 am. The operators of unit 4 night shift are trying to figure out what had just happened. The panels are flashing with alarms, and the reactor is no longer responding to any controls. Neutron-absorbing control rods have stopped moving, and it is uncertain whether the main circulation pumps are still supplying coolant water into the reactor core.
The explosion is at first suspected to have been caused by hydrogen in the turbine hall. Hydrogen gas is commonly used in power plants for cooling the generator, so an explosion in one of the hydrogen storage tanks seems like a plausible explanation. The opening scene is probably a pretty realistic description of actual events. There are several very common misconceptions related to operator actions and the nature of the turbine test, but in reality the explosion came as a complete surprise to everyone. What had just happened was against everything they knew about nuclear reactors, which is why they are looking so desperately for an alternative explanation.
The first reaction by deputy chief-engineer Anatoly Dyatlov is to ensure that water is flowing into the reactor core. Even though the reactor was shut down just before the explosion, there is concern that the fuel will still overheat and melt. This is related to so-called decay heat. When a nuclear reactor is operated at power, radioactive fission products and transuranic actinides (neptunium, plutonium, etc.) are accumulated in the fuel. The radioactive decay of short-lived isotopes releases a lot of heat.
Unlike fission power, this decay heat does not fall to zero at the moment the chain reaction is terminated, and failure to maintain cooling may result in severe core damage. Since the control rod indicators show no movement either, there is also concern that the fission power may not be completely shut down. The pumps and control rods can no longer be operated remotely from the control room, so the operators rush to perform these tasks manually.
Elevated radiation levels are measured inside the reactor building, but the numbers are not alarmingly high. The exposure reading is 3.6 Roentgen, which in SI units of radiation dose is about 34 mSv – the equivalent of about 3 full-body CT-scans.i The detected radiation does not necessarily contradict with the assumption that the explosion was caused by hydrogen in the turbine hall, since the explosion could have ruptured a feed-water line carrying water back into the reactor core. The RBMK is a boiling water type reactor, so the steam turning the turbines is taken directly from the primary circuit. Certain short-lived isotopes (e.g N16) are produced in the water by neutron activation as the coolant flows through the reactor core.
It is soon noticed that all dosimeters are displaying the same reading, which is also the maximum count for the detector model. This is also based on actual events. The personnel carried film badges, that were all over-exposed. The operators were working on pumps and valves inside the reactor building, and had no means to evaluate the true radiation levels. Firefighters, who soon arrived at the site, did not carry any radiation detectors at all.
The seriousness of the situation was not recognized by anyone, which was a major contributing factor to the high radiation doses received by the staff and the emergency rescue crews. It is estimated that in some spots the radiation levels exceeded natural background by more than a factor of one billion (1,000,000,000), leading to a lethal dose in a matter of minutes.
While walking past a window, Dyatlov spots black debris on the ground in front of the reactor building. This looks like graphite, which is used inside the the RBMK reactor core as a neutron moderator (this topic will be revisited in the next episode). When the first fire rescue unit arrives at the site, a firefighter picks up one of the black objects, and soon suffers from radiation burns on his hand. Despite the clear evidence that the upper part of the reactor building has been completely destroyed, and the reactor shaft is exposed to open air, the staff continues to debate, whether or not an explosion in the reactor core is even possible.
Fig. 1: A piece of a graphite moderator block outside the reactor building after the explosion. The flat diameter of the object is 25 cm.
Fig. 2: An aerial photograph of the destroyed reactor building.
In reality, the emergency operation was somewhat more extensive than that portrayed on the show. The explosion started more than 30 fires on the bitumen roofs and inside the buildings. The first paramilitary fire rescue unit arrived on the site within five minutes from the explosion, and by 4 am more than 250 firemen were fighting the flames. All major fires were localized and extinguished by 4:50 am. The operating staff worked their best to flood the reactor using the emergency auxiliary feed-water pumps, but the water could not reach the core through damaged pipework. The attempts had to be abandoned by morning.
A total of 600 emergency workers participated in the operation. By morning, workers exposed to high levels of radiation were starting to develop symptoms of acute radiation syndrome (ARS). Some 150 people were admitted to hospital, more than 100 diagnosed with ARS, and 28 died within the following weeks and months. Five of these were firemen, working on the burning roof of the reactor building, where the dose rate was probably the highest. All people who developed and died of ARS in Chernobyl received their dose inside or within close proximity of the reactor building, during the first night of the accident.
On Saturday morning, the situation at Chernobyl NPP unit 4 was as follows. A major part of the reactor core was destroyed by a massive explosion. The thousand-tonne reactor cover plate (upper biological shield) was lifted up, and turned sideways on top of the reactor vessel. The upper section of the reactor building was completely destroyed. Almost every physical barrier preventing the release of radioactive emissions was lost. Radioactive noble gases had already escaped the damaged fuel, and microscopic fuel fragments (so-called ”hot particles”) and larger pieces of graphite, fuel rods and other core internals were dispersed within 150 meters of the reactor building.
The nuclear chain reaction had been stopped by the disruption of critical geometry, but the fuel remaining in the reactor shaft was still producing a considerable amount of decay heat. The next concern was that the fuel would melt, releasing more radioactive emissions into the atmosphere. It was soon realized, that this could no longer be prevented by any practical means.
Sometime during Saturday, a new threat emerged, as a fire broke out in the reactor shaft.ii The exact time is not known, since a thick cloud of white steam was constantly rising from the open core, but by Saturday evening the flames soared 50 meters above the reactor building (in the TV-show this fire started immediately after the explosion). The reactor fire formed a major new source of emissions, as gaseous fission products, aerosols and hot particles were lifted high up in the atmosphere, and dispersed across the Europe and Soviet Union.iii
i) For simplicity, I will be using sievert as the unit for all radiation doses, even though it technically measures the equivalent/effective dose associating exposure to long-term stochastic risk. The appropriate quantity in this context would be the absorbed dose, measured in gray, which is associated with acute effects. Mixing sievert and gray is a common practice, as in most cases the decimal values are the same.
ii) The reactor fire is often referred to as a ”graphite fire”. To be precise, nuclear-grade (ultra-high purity) graphite does not catch on fire. Graphite can be oxidized when exposed to air at high temperature, i.e., it burns, but the process does not produce enough heat to support a self-sustaining flame. The reactor fire was instead fueled by two flammable gases, hydrogen and carbon monoxide, which are produced when hot graphite interacts with steam. Hydrogen was also produced by the oxidation of zirconium-metal cladding and other core structures. In fact, zirconium metal itself undergoes self-sustained burning at high temperature.
iii) Aerosols are formed when hot gaseous compounds cool down and condensate to form small (sub-micrometer scale) particles, that are suspended in air. In reactor accidents these aerosols are composed of volatile fission products, such as iodine and cesium. Hot particles refer to microscopic fragments of fuel, graphite, etc., that may contain also non-volatile radionuclides, such as isotopes of strontium, neptunium and plutonium.
Episode 2 – Please Remain Calm
The second episode covers the events during the first few days after the explosion from Saturday morning on. In the opening scene, nuclear physicist Ulana Khomyuk (a fictional character) discovers radioactivity outside the window at the Belarus Institute of Atomic Energy in Minsk. A gamma spectrometer reveals that the source is a radioactive isotope of iodine, I131. Since the isotope cannot be of military origin, there must have been an accident in a nuclear reactor.
Isotopic analyses do tell a lot about the source of the emission. Some isotopic ratios (e.g. ratio of Cs134 and Cs137) are different for nuclear explosions compared to reactor accidents. The conclusion that the emission could have only come from a nuclear reactor, however, cannot be made based on a single isotope. I131 is a fission product (not decay product), and it is produced in the fission of both U235 and Pu239. In reality, the sample would have also shown traces of other radionuclides as well, including the two isotopes of cesium.
Radioactive I131 can be considered the most severe immediate threat to population in contaminated areas. The isotope is accumulated in the thyroid gland, where it inflicts a high localized radiation dose, leading to an increased risk of thyroid cancer. I131 is a short-lived isotope (half-life 8 days). Its specific activity is high, but the short half-life also means that the concentration falls to negligible levels within the first few months after the release. Iodine pills containing stable iodine are used to saturate the thyroid, and reduce the intake of I131. This is an effective and important countermeasure, especially for small children.
The radioactive plume reached Finland and Scandinavia during Sunday, April 27, less than two days after the explosion. Elevated radiation levels were first observed at the Forsmark nuclear power plant in Sweden. In Finland, the dose rates peaked on Monday, reaching about 5 µSv/h. This corresponds to the level of cosmic radiation on a commercial airliner at cruising altitude (natural level of background radiation in Finland is 0.05-0.30 µSv/h).
Some of my senior colleagues, who worked at VTT’s FiR 1 research reactor during the accident, have told about similar experiences as Khomyuk’s character on the show. When returning from lunch break into the reactor building, the radiation monitors picked up the contamination on their shoes. There are several similar accounts from the nuclear industry.
The scene where Legasov explains Shcherbina how a nuclear reactor works was probably intended as an introduction to the topic for the viewer as well. One of the most important takeaways from this scene for future reference is the role of graphite in the RBMK reactor. Since the explanation is not fully accurate, I will try to provide an alternative description using the technically appropriate terms.
The operation of all nuclear reactors is based on a self-sustaining chain reaction, carried on by neutrons. The fuel is comprised of fissile uranium atoms, which after absorbing a neutron are split, or fissioned, into two intermediate-mass nuclides. Fission releases energy, and 2-7 new neutrons, which continue the reaction chain by splitting more uranium atoms. The self-sustaining state is referred to as criticality. The prerequisite of criticality is that neutron emission rate matches the rate of neutron loss. If the source rate exceeds the rate of neutron loss, the particle population and fission power are increasing. In the opposite case, neutrons are lost faster than they are produced, and the population and fission power are decreasing. These two operating states are referred to as super- and sub-criticality, respectively.i
In order to reach the self-sustaining state, neutrons passing through the reactor core must have relatively high probability of causing new fissions while interacting with the uranium fuel. Natural uranium consists of two isotopes, U235 and U238, for which the abundances are 0.7% and 99.3%, respectively. Of these isotopes, only U235 has high enough fission probability to support the self-sustaining state.
When a neutron is emitted in fission, its energy is high. The collisions to U235 and U238 are roughly divided according to the corresponding isotopic ratios. If the reactor is operated with these high-energy, or fast neutrons, the fraction of U235 has to be considerably raised. The enrichment in such fast-spectrum reactors is typically at least 20-30%.
Most nuclear reactors operate on low-energy, or thermal neutrons, taking advantage of the fact that the collision probabilities depend on neutron energy. For low-energy neutrons, the probability to collide with a fissile U235 nucleus is increased by more than a factor of 40, compared the probability of colliding with non-fissile U238. So even in natural uranium, low energy neutrons are much more likely to cause fission in U235, than to be parasitically captured by U238. This type of thermal reactor can operate with low-enriched fuel. In the Chernobyl RBMK, the enrichment was 2%.
The next question is, how to produce these low-energy neutrons? The answer lies in the neutron moderator, which is simply some low-absorbing material comprised of light elements. When the neutron collides with a hydrogen or carbon atom, it most likely bounces off, or scatters, losing some of its kinetic energy in the process.ii If the uranium fuel rods are surrounded by a sufficient amount of moderator, a significant fraction of high-energy fission neutrons are slowed down to low energies before returning back into the fuel. This increases fission probability, and the chain reaction is able to reach the self-sustaining state.
Conventional pressurized (PWR) and boiling water-type (BWR) light water reactors (LWRs) are moderated by the primary coolant that flows through the fuel assemblies. Water contributes to neutron moderation also in the RBMK, but the primary moderator is graphite. The coolant flow is divided into 1660 vertical pressure channels, inside which the uranium fuel assemblies are loaded. The channels pass through solid graphite blocks. Since the graphite moderator constitutes a large fraction of the overall core volume, the size of the RBMK is large compared to conventional LWRs. The height and diameter are 7 and 12 meters, respectively, while the corresponding dimensions for an LWR of similar output are between 3 and 4 meters.
When Legasov and Shcherbina arrive in Chernobyl, they begin do discuss the various options to extinguish the reactor fire. Legasov explains that using water is out of the question, because the fire burns too hot. It is decided to use helicopters to drop sand and boron into the reactor shaft. This operation is also based on real-life events. One of the reasons for not using water was that the resulting generation of steam would have carried more radioactive materials into the environment. The first helicopters began dropping sand on the burning reactor on Sunday morning.iii
There is also another turn of events. Khomyuk discovers that the melting fuel may eat its way through the concrete lower biological shield, and cause a steam explosion in the pressure suppression pools located a few floors below the reactor core (I think Khomyuk used the term ”thermal explosion” and ”bubbler pool”). These pools are a part of the containment function of the RBMK reactor. If one of the pressurized fuel channels bursts, the steam pressure inside the reactor vessel increases rapidly. In order to prevent structural damage, the steam is diverted through channels and blow-down pipes into a large pool of water. The pressure build-up is alleviated as the steam is cooled and condensed into water.
This story line has a fragment of truth to it, but the scale of the potential consequences is way off. Khomyuk claims, that the energy released in the event would be the equivalent of a hydrogen bomb (2-4 megatons), and that the explosion would destroy everything within 30 km radius, and leave Ukraine and Belarus uninhabitable for the next 100 years. This claim is based on a well-known Chernobyl myth, and the one that that is the most untrue.
A steam explosion occurs when high-temperature molten material comes in sudden contact with water. The impact breaks the mass into small droplets, dramatically increasing the contact area. Heat is rapidly transferred into the water, causing it to flash into steam.iv Even though this is a violent phenomenon, it does not generate any additional energy. The explosive yield comes from the heat that is stored in the molten mass alone.
To put the given values into perspective, 1 megaton of explosive energy corresponds to about 4 petajoules (PJ) of heat. A nuclear reactor with 1000 MW electric output produces about 8 PJ of fission energy per month, so 2-4 megatons is the equivalent of 1-2 months of full-power operation. Storing this amount of heat in the molten fuel is physically impossible.v
My impression is, that the real concern in Chernobyl was that the steam explosion would disperse more radioactive material into the environment, worsening the already extremely challenging conditions.vi By this time there were thousands of workers at the site, and the operation to extinguish the reactor fire was full-on. A new major release would have endangered the people, and made all previous efforts futile. To prevent this from happening, three volunteers were sent in diving suits to open the slide gate valves inside the reactor building, in order to drain the pools from water.
Fig. 3: Cross-sectional view of the reactor building. The damaged reactor shaft is at the center, and the two pressure suppression pools are shown in the lower two floors below it. The figure also shows structures that a part of the sarcophagus, which was constructed later.
i) This simplified description is sufficient for explaining the basic concepts, such as neutron balance, but it fails to account for the dynamic behavior of the neutron chain reaction. The time dependence is best understood using the concept of prompt fission chains, which refer to the sequence of fission reactions induced by neutrons in consecutive generations. Under normal operating conditions, all of these fission chains are finite in length. In LWRs they proceed from beginning to end in a matter of milliseconds (equivalent to a camera flash). In addition to prompt neutrons, i.e. neutrons that are emitted instantaneously as the atom is split, the radioactive decay of certain fission products releases neutrons with a considerable delay. These are called delayed neutrons, and they are emitted hundreds of milliseconds, seconds or even minutes after the fission event. The significance of delayed neutrons is that they can be thought of as the initiators of new prompt fission chains. So even though the time scale associated with fission chains is very short, the interval between two consecutive chains is relatively long. Criticality condition can be understood as the state where each fission chain produces, on the average, one new delayed neutron, and dying chains are constantly replaced with new ones. Delayed neutrons are also the key to reactor control. Without their impact on time constants, it would be impossible to raise or lower the fission power in any controlled manner.
ii) The average fractional energy loss per scattering collision in the moderator depends on the mass of the target nucleus. The lighter the target, the larger the energy loss. Neutron slowing-down in water requires some 25 collisions with hydrogen. For graphite, the same number is more than 100. Neutrons are collected in the low-energy range of the spectrum, where their energy distribution forms an equilibrium with the thermal motion of the surrounding moderator atoms. Neutrons that have reached this equilibrium are called thermal neutrons (hence the name ”thermal reactor”).
iii) Boron is a strong neutron absorber, so dropping boron compounds into the reactor core suggests that there may have been some concerns about re-criticality, even though most of the fuel assemblies were already crumbled at the bottom of the reactor vessel. Self-sustaining chain reaction requires both fuel and moderator, laid out in a somewhat optimal geometry.
iv) This is not a 100% accurate description, as the phenomena associated with steam explosions are fairly complex. The main point is, that steam explosion is an instantaneous event, and not the same thing as the rupture of a pressure vessel under high steam pressure.
v) There is also another version of this myth, where the event would have caused a nuclear explosion. This is not possible either. Even if the molten fuel would form a critical mass, low-enriched uranium can maintain a self-sustaining chain reaction with low-energy neutrons only (see the explanation on neutron moderator above). The average time between two fission events becomes relatively long when neutrons are slowed down by collisions in the moderator, which puts a physical upper limit to how fast the power can grow. The process is way too slow to achieve yields even remotely comparable to an actual nuclear explosion. When a nuclear bomb is detonated, the chain reaction proceeds at least a million times faster, releasing a massive amount of energy before the explosion breaks the critical geometry. Megaton-scale explosions are not possible even with fission bombs, but require fusion (merging of two hydrogen isotopes) to boost the yield. The yield of the Hiroshima bomb, for example, was 15 kt, or 0.015 Mt.
vi) The steam explosion scenario has brought about a lot of discussion around the Internet. My colleague pointed out in the comments of the original Finnish text, that an order of magnitude estimate for the total energy content of the molten fuel can be obtained by assuming that 100 tonnes of uranium oxide is cooled from 3000°C to 100°C. This would amount to about 200 GJ of heat. A steam explosion could convert about 1%, or 2 GJ of this heat into kinetic energy. This is equivalent to about 500 kg, or 0.0000005 megatonnes of explosive yield. This is far from an accurate estimate, but it is likely that the worst-case scenario would have been evaluated with a similar simplified calculation. In reality, the temperature of the molten fuel was way below 3000°C when it entered the pool (see Fig. 4). Also, the hot mass would not have fallen into the water as a single lump, but rather seeping sluggishly through the blow-down pipes, which would have also considerably limited the amount released energy. Obviously, the actual state of the molten fuel was not known at the time of the accident, so all preparatory actions were most likely based on some worst-case scenario.
Episode 3 – Open Wide, O Earth
The active phase of the accident lasted for ten days. During this time, more than 5000 tonnes of sand, boron carbide, dolomite, clay and lead were dropped on the burning reactor. The campaign took more than 1800 sorties. Despite the efforts, there was no significant drop in radioactive emissions, and one week after the explosion, the measured activities began to rise. The operation had to be aborted, as it was feared that the concrete structures would collapse under the weight. It has been later suspected, that the bulk of all material dropped from helicopters most likely missed its target completely.i
It was decided to switch strategy, and extinguish the fire by purging the reactor with liquid nitrogen. The original plan was to use the lower piping to inject nitrogen directly into the core via reactor cooling channels, but the idea had to be abandoned because the pipework was too damaged, and debris and high levels of radiation prevented the installation of pumping equipment.
Plan B was to use nitrogen gas instead. Holes were drilled on concrete walls, and pipes laid to pump the nitrogen into the pressure suppression pools below the reactor shaft. It was hoped that the gas would be picked up by the updraft, and carried into the burning reactor. The pumping was started on Tuesday, May 6. By this time, the active phase of the accident was already over, and the reactor fire extinguished.
The most likely course of events is that the fire was extinguished on its own. The molten fuel (also referred to as ”corium”) penetrated the lower biological shield on Monday, May 5, and flowed through steam distribution headers and blow-down pipes into the two pressure suppression pools below (see Fig. 3). As the corium was spread over a large surface area, the material cooled down and solidified. The temperature in the reactor shaft began to fall, stopping the production of combustible gases. This was accompanied by a sharp drop in radioactive emissions. The measured activities soon fell by three orders of magnitude compared to the peak values.
Even though the explosion had already dispersed a large amount of radioactive material around the site, the most significant emissions resulted from the reactor fire, which burned for ten days. The hot gases lifted the emissions high in the atmosphere, and the plume was carried away by the prevailing winds. The most contaminated region covers a 150,000 km2 area of Ukraine, Belarus and western parts of Russia, but also northern and central Europe received radioactive fall-out.
Selected radionuclides in the emission, together with their half-lives, estimated release fractions and total activities, are listed in the table below (source: OECD Nuclear Energy Agency). The activities are given for the time of the accident, and six months later, taking into account the effect of radioactive decay.
|Radionuclide||Half-life||Release (%)||Apr. 1986||Oct. 1986|
|I131||8.0 d||50 – 60||~1760||–|
|Cs134||2.0 y||20 – 40||~54||~45|
|Cs137||30.0 y||20 – 40||~85||~84|
|Te132||3.3 d||25 – 60||~1150||–|
|Sr90||28.0 y||4 – 6||~10||~9.8|
Of these isotopes, Xe133 is a noble gas fission product, that escapes immediately after the fuel is damaged. Chemically inert noble gases do not dissolve in water or form compounds with other elements, which is why they do not contribute to fall-out or accumulate in the body either. Xe133 was most likely a major contributor to external doses received by the emergency workers during the first night of the accident, but the emission was later dispersed and diluted by the winds.
Radioactive contamination within the fall-out zone is mostly due to iodine and cesium, which are also fission products. I131 is the most severe short-term threat, and high Cs137 contamination inflicts long-term restrictions to land-use and cultivation. The reason why these emissions are so significant, is that the isotopes form volatile compounds, and escape from the damaged fuel in gaseous and aerosol form. Of the remaining nuclides, strontium is a fission product, and neptunium and plutonium are transuranic actinides. These elements are non-volatile, and most of their inventory was suspended in the molten fuel. The release fraction mainly consists of hot particles, which were formed when the nuclear fuel was pulverized by the explosion.
In the TV-show story line, the attempts to extinguish the fire with liquid nitrogen was combined with another operation. It was feared that the molten corium would eat its way through the concrete foundation, and contaminate the ground water basin. Workers recruited from coal mines started excavating tunnels under the reactor building, so that a flat-bed heat exchanger could be installed under the base plate, and the dispersion of radioactive materials stopped by freezing the ground. The operation was started a few days after the active phase of the accident, and it lasted until the end of June. In the end, the system was never put to use (this was not covered in the series).
This massive operation may seem completely futile in hindsight, but it should be noted that not much was known about the complicated phenomena associated with severe accidents back in 1986. In fact, much of what is known now about the behavior and interactions of molten fuel, comes from studies that followed the accident in Chernobyl.ii It was later discovered that the corium did little damage after penetrating the lower biological shield. The mass cooled down, and its composition changed as steel, concrete and other substances were mixed in. By the time it reached the steam distribution corridor above the pressure suppression pools, the temperature had already decreased below the melting point of stainless steel, and the consistency was that of thick molasses.
Fig. 4: Solidified corium in a steam distribution header, located in the steam distributor corridor above the upper pressure suppression pool (see Fig. 3). The fact that the header has remained intact tells that the corium temperature had at this point fallen below the melting point of stainless steel.
Also the events leading to the reactor explosion begin to unfold. When talking to Toptunov and Akimov at the hospital, Khomyuk discovers that the explosion did not occur until Akimov pressed the AZ-5 scram button. Reactor scram refers to emergency shut-down, which causes all neutron-absorbing control rods to fall into the reactor core.iii The claim that this would have caused a power surge instead of terminating the chain reaction therefore appears somewhat counter-intuitive, even absurd.
i) This assumption is supported by the fact that core samples taken from the molten fuel after the accident contain very little lead, even though large quantities were dropped on the reactor during the active phase.
ii) In 1979 there had been an accident at the Three-Mile Island power plant in Harrisburg, Pennsylvania, USA, that had lead to a partial core melt-down inside the reactor pressure vessel. Experiments with (simulated) corium-concrete interactions had been performed in Germany in the early 1980’s. Chernobyl was the first accident that involved a complete core melt-down, where the corium penetrated the reactor pressure vessel and started to eat its way through the structures below.
iii) There is a well-known anecdote that the term ”scram” would be an acronym for ”Safety Control Rod Axe Man”. This is a reference to Enrico Fermi’s Chicago Pile 1 reactor in 1942, where one of the safety measures was an emergency shut-down rod, that was suspended above the reactor by a rope. In an emergency, the rope would be cut using an axe, and the rod would fall into the core, terminating the chain reaction. Even though this is a cool story, the anecdote is unfortunately not true.
Episode 4 – The Happiness of All Mankind
The events in the fourth episode extend until the end of 1986. By this time, the active phase of the accident was behind, and the emissions to air had practically ceased. In an effort to prevent the situation from re-escalating, an improvised protective shelter, also called the sarcophagus, was constructed around the damaged reactor building. The construction of the sarcophagus is not really covered on the show, and the events are more focused on the preparatory work. This involved a difficult cleanup operation to reduce the dose rates near the reactor building to a level, that would allow the construction work to be carried out without excessive exposure to radiation.
Soon after the evacuation of Pripyat (Episode 2), the exclusion zone was extended to 30 km radius from the site. It is estimated that some 116,000 inhabitants were evacuated from this zone. In later years, the evacuations were extended to regions of Ukraine and Belarus, that received the highest amount of radioactive fall-out. As the region was abandoned, it also became vulnerable to forest fires, which could have lead to dispersion of radioactive materials. To prevent this from happening, the cleanup operation was extended to cover the entire exclusion zone. The operation was carried out in 1986-1990, and the total number of workers is estimated to have been around 600,000, about half of which were soldiers.
Clearing the rooftops from graphite, pieces of fuel rods and other extremely radioactive debris was at first attempted using remote-controlled robots. It was realized early on, that standard industrial machines were not cut out for the job, since intense radiation is damaging to electronics, especially semi-conductors. The solution was found in space technology. The radiation environment outside the atmosphere and the Earth’s protective magnetic field is extremely harsh, especially during solar events. Dose rates on the surface of the Moon, for example, may increase to lethal levels during large solar flares. Equipment designed to operate under these conditions had much better chances of surviving the radiation in Chernobyl as well.
The Soviet Union had an active space program, and in the 1970’s a series of remote-operated robotic vehicles were designed to explore the surface of the Moon. The same technology was used in Chernobyl for the cleanup work. I could not find any original references describing the operation, but two rovers were apparently built, and they operated successfully for a period of several weeks. The robots were eventually rendered inoperable by radiation damage to the electronics, although the fact that the light vehicles were not designed for continuous hard labor may also have been a contributing factor.
As portrayed on the TV-show, the work had to be completed manually, by the so-called ”bio-robots”. It maybe tempting to think that the Soviet government did not care much about the safety of the workers, but in reality, radiation dose limits were imposed, and there was an effort to follow them. During the first year after the accident, the annual dose limit for civilian workers was set to 250 mSv, which in later years was first lowered to 100 mSv, and then to 50 mSv. Military personnel were subject to a higher wartime limit of 500 mSv for the first month, but the limit was later matched with that of the civilian workers. For comparison, the current Western safety limits for radiation workers are usually based on 100 mSv accumulated dose over a period of five years, but the limits can be temporarily raised for emergency operations.i
In practice, monitoring and complying with the dose limits was not completely successful. There were not enough personal dosimeters for everyone, and some doses had to be measured at team level, or estimated based on dose rate and exposure time. This is not a trivial task, and in many cases the accuracy suffered from errors in calibration. Some records were lost, or deliberately misplaced. The situation began to improve in June, when more personal dosimeters became available.
By the time the bio-robots began their work, the conditions were no longer the same as during the first night of the accident. The air had been cleared from radioactive gases, and aerosols and hot particles had descended and adhered on the surfaces. The most high-active short-lived radionuclides had already been decayed.
Even so, the external dose rate was still dangerously high, and radioactive dust was re-suspended in air when the workers started to move the debris with shovels. In order to comply with the dose limits, the working time for a single person had to be reduced to about 90 seconds (this is the number used in the show, some other sources talk about 40 seconds). The whole operation involved almost 4000 workers.
The story line involving Khomyuk’s investigations on the cause of the accident proceeds, when Legasov tells about certain known deficiencies in the safety design of the RBMK reactor. The technical description is, again, not fully accurate, and it actually mixes together two separate concepts, that are essential for understanding the course of events. I will therefore try to rephrase the explanation in my own words.
The role of the neutron moderator was discussed in the second episode. The purpose of the moderator is to slow down fast fission neutrons to low energies, which increases their probability to collide with a fissile U235 nucleus. Without this process, the reactor cannot maintain a self-sustaining chain reaction.
The physical properties of all materials in the reactor core change along with the operating conditions. The most apparent changes occur in the coolant. Increasing fission power increases the temperature of water, which after some point begins to boil. In reactor physics, the small steam bubbles in the coolant channel are called ”voids”, since from the neutrons’ perspective they appear as holes in the water. The volume fraction of steam in the coolant flow is consequently referred to as ”void fraction”.
The coolant plays a dual role in all water-cooled reactors. On one hand, water moderates neutrons, which is essential for maintaining the chain reaction. On the other hand, water also absorbs neutrons, removing them from the fission chains. The absorbing effect is not very strong compared to fuel or control rods, but still noticeable. Whichever effect becomes dominant, determines how the reactor behaves when the coolant starts to boil. This, in turn, has an important effect on reactor stability.
Conventional boiling water reactors are moderated with the same water that cools the nuclear fuel. The coolant boils while passing through the reactor core. When the fission power is increased, the boiling is accelerated, and more voids are formed between the fuel rods. This reduces neutron moderation, and the fraction of neutrons that reach the region of high fission probability. The result is a drop in fission rate and heat production. In other words, there is a strong inherent physical effect, that resists the change in fission power. Such effects are called negative feedbacks. A boiling water reactor is stable with respect to coolant boiling. The reactor regulates itself, and shuts down if the coolant flow is compromised.
In the RBMK, neutrons are moderated with solid graphite. Increasing core temperature expands the graphite blocks, but the amount of moderator in the reactor core is not changed. Neutrons are also slowed down in the coolant, but there is so much graphite between the fuel channels, that the moderating effect of water is not needed for maintaining the chain reaction. This type of reactor is called over-moderated.
If the water does not act as a neutron moderator, it acts as a weak absorber. Accelerated boiling and increased void fraction leads to reduced absorption, and increase in fission power. As the temperature increases, boiling is accelerated even more, leading to further increase in fission power, and so on. This type of effect is called positive feedback (Legasov refers to this feedback using term ”positive void coefficient”, which is a measure of how strong the effect is). The RBMK is unstable with respect to coolant boiling. Local perturbations in fission power tend to amplify, and the reactor requires constant active regulation.ii
The second effect explained by Legasov is related to a design flaw in the control rod structure of the RBMK reactor. The control rods are comprised of a movable neutron absorber, and a graphite follower attached below the absorbing part. Legasov calls this a ”graphite tip”, which is slightly misleading, since the follower is 4.5 meters long. The core height, however is 7 meters, so when the absorber part is pulled all the way up, the follower sits at the core center, with 1.25 meters of empty water channel above and below it.
Fig. 5: The control rod design used in RBMK reactors before and after the Chernobyl accident (from INSAG-7).
The problem with this design is that when the reactor is scrammed, the graphite follower replaces the water in the channel below it. Since water in the RBMK acts as a weak neutron absorber, the result is that the chain reaction is accelerated at the lower part of the core, even though the absorber is pushed between the fuel channels from above. This effect is sometimes referred to as ”positive scram”.
The positive feedback and instability associated with graphite-moderated water-cooled reactors is a well-know fact in reactor physics. This is also the reason why similar reactor type never became popular in the West. The instability of the RBMK had caused problems before, and lead to an accident at the Leningrad NPP in Sosnovy Bor (near St. Petersburg) in 1975. The positive scram had been observed in Ignalina in Lithuania in 1983. These issues were known by the reactor designers, but no precautions were made, and the safety implications were not addressed in operator training. These problems in safety culture materialized in the worst possible way in Chernobyl in 1986.
i) The lethal radiation doses received by the emergency workers during the first night of the accident were as high as 16 Sv, or 16,000 mSv. A dose of 1 Sv received over a short period of time can lead to acute radiation syndrome, and doses exceeding 5-6 Sv are usually fatal.
ii) There are also other feedback-effects in nuclear reactors. Increase in fuel temperature increases the probability of parasitic neutron absorption in U238, which leads to a decrease in fission power. This is called the fuel temperature, or Doppler-feedback, and it becomes important especially in fast power excursions, where mechanical control rods cannot be moved fast enough to terminate the chain reaction. The fuel temperature coefficient is negative in practically all reactor types, including the RBMK. In the TV-show, the Doppler coefficient is confused with the reactor power coefficient, which is actually a measure of all feedback-effects combined.
Episode 5 – Vichnaya Pamyat
It is a very common misconception that the accident in Chernobyl was the result of a reckless experiment, where the reactor power was pushed beyond its safe operating envelope. The events leading to the explosion, including the notorious turbine test, were covered in the final episode of the series, centered around the trial where Dyatlov, Fomin and Bryukhanov are held accountable for the accident.
The technical reports I have used as the reference for this writing are focused on the physical course of events, so it is very difficult to make any comments about the actions of individual operators. I will instead try to provide the physical background and technical description of the events that lead to the explosion, in a similar way as portrayed in the testimonies of Shcherbina, Khomyuk and Legasov on the show.
When a nuclear power plant is disconnected from the electricity grid, the load on the generator falls, and the turbine begins to pick up speed. To prevent damage to mechanical components, the reactor is automatically shut down, or its power is reduced to a level where the production matches on-site demand. If the controlled power-down fails, the reactor trips, and fission power falls to zero.
To prevent a complete station black-out, diesel-powered generators are automatically started, and they begin to supply power for the emergency core cooling systems. Maintaining coolant flow in the reactor core is important, since the nuclear fuel produces a significant amount of decay heat (see the comments under Episode 1), even after the fission power has been shut down.
The start-up sequence for the diesel-generators takes time, but on the other hand, the power supply from the main turbo-generator does not fall to zero immediately either, due to the large inertia of the rotating mass. The purpose of the safety test carried out in Chernobyl was to verify that the forced circulation of coolant through the reactor core could be maintained in the event of a total power loss, i.e. during the period when the power supply was switched from one system to another. The test procedure is pretty well described in Shcherbina’s testimony, but what may have not been entirely clear, is that load rejection tests involving similar procedures are routinely performed in all power plants, both nuclear and non-nuclear.
Even though the turbine safety test can be considered a major contributing factor to the accident, the underlying cause is not straightforward. The reactor was inadvertently driven into an extremely unstable state during the period of 24 hours prior to the test. This was related to the so-called xenon poisoning, as explained by Khomyuk and Legasov, although the description was missing a few key elements. In his testimony, Legasov also referred to the concept of ”reactivity”, which is associated with the rate of change in fission power. The higher the reactivity, the faster the increase in power. Reactivity can be regulated by movable control rods, but also various physical effects, including feedbacks, can be thought to inflict the neutron chain reaction via changes in reactivity.
As described under the previous episode, the RBMK reactor is unstable with respect to coolant boiling. This is related to the fact that the reactor core is over-moderated by graphite, which effectively makes water a neutron absorber. The absorbing effect is not particularly strong, since neutrons are absorbed in fuel and control rods as well, but strong enough to cause a noticeable increase in reactivity when coolant boiling is accelerated. The absorbing effect of water becomes more pronounced when the contribution of other components, such as control rods, is reduced. This, in turn, amplifies the positive feedback, since the reactivity increase inflicted by coolant boiling is also amplified.
The test sequence involved tripping the turbine when the reactor was running at approximately 30% fission power, and measuring the voltage supplied by the generator during the coast-down phase. Reactor run-down to the planned 30% power level began at 1 am on Friday, April 25, and the plan was to perform the test sometime during the afternoon. However, at 2 pm there was a request from the Kiev power grid controller to postpone the test, and maintain production until late evening. The run-down was halted, and the reactor remained at 50% power until about 11 pm. When the preparations for the test continued, the day shift had been replaced by the night shift. The operators had not received any briefing on the test procedures.
As explained by Khomyuk, the fact that the reactor was left at 50% power level for a period of several hours, was the first step towards the disaster. When a nuclear reactor is operated at power, various fission products are accumulated in the fuel. Some of these isotopes, called fission product poisons, have very high probability of capturing low-energy neutrons. The most significant poison is xenon isotope Xe135. The concentration of xenon begins to increase when the reactor is started, and it reaches an equilibrium within the first 48 hours of operation. However, if the power level is then reduced (or the reactor is shut down), there is a temporary peak in the concentration, before it saturates at a new, lower equilibrium. This phenomenon, called xenon poisoning, is strong, but relatively slow. The peak concentration is reached after about 10 hours from the reduction in fission power.i
When the reactor was left at reduced power, xenon poisoning began to slowly consume the reactivity, which had to be compensated for by withdrawing control rods from the reactor core. This was carried out over a period of several hours.
At 00:28, a transfer was made in the automatic power control system, which caused the fission power to shut down. The root cause of this event is not fully clear, but is was most likely a combination of xenon poisoning and operator error. In any case, to bring the reactor back to power, the operators had to withdraw a significant number of control rods from the core. The reactor was stabilized at 200 MW fission power 30 minutes later, and the power could not be raised any further. The minimum power level specified in the test program was 700 MW.
The test was to be carried out with all 8 main circulation pumps running. In the normal operating mode, core flow was maintained by 6 pumps, with 2 in reserve. Some of the operating pumps had been shut down in the afternoon when reactor power reached the 50% level. The remaining pumps were engaged, which increased coolant flow into reactor core.ii This, in turn, suppressed coolant boiling. The amount of steam in the reactor core was reduced, and the amount of water correspondingly increased. Since water acts as a weak neutron absorber, reactivity was reduced, which again had to be compensated for by control rod withdrawal.
By this time, the reactor was already in an extremely unstable state. Positive reactivity reserve had been gradually shifted from absorbers to coolant by withdrawing control rods and increasing the amount of water in the reactor core.iii Even though water is not a particularly strong neutron absorber, its contribution to total absorption was already dangerously high. The operators had no longer control over this excess reactivity either. Water temperature was below the boiling point almost throughout the core, and all that positive reactivity was just waiting to be released when the boiling started to accelerate.
The test was started at 01:23:04 by closing the turbine stop valves. Four main circulation pumps connected to the generator began to coast down, and coolant flow into the reactor core was reduced. Water temperature began to rise. When the boiling started, the positive reactivity reserve bound to water was gradually released, as the coolant channels began to fill with steam. Positive feedback started to kick in, and boiling and increase in fission power began to accelerate each other.
The power excursion was noted at the control room, and Akimov rushed to press the AZ-5 scram button at 01:23:40. Control rods began to move inside the reactor core, but instead of terminating the chain reaction, the transient was accelerated. At 01:23:43, emergency power protection system signals were actuated, as power exceeded 530 MW. Two seconds later, the reactor was destroyed by an explosion.iv The last recorded reading showed 33 gigawatts, but it has been estimated that the peak fission power could have been as high as 1.3 terawatts. For a brief moment, the reactor at Chernobyl NPP unit 4 produced more fission power, than all than the other nuclear power plants in the world combined.v
The positive reactivity insertion following reactor scram was explained in the previous episode. In the RBMK control rod structure, a graphite follower is attached below the neutron-absorbing part. This follower, however, is shorter than the core height, so when reactor scram is actuated using control rods that are pulled all the way up, the follower part replaces neutron-absorbing water in the channel below it, causing a local reactivity increase at the bottom of the core.
In reality, there is a bit more physics behind this positive scram. When the reactor was poisoned during the hours prior to the safety test, the accumulation of Xe135 also caused a distortion in the axial power distribution. The production rate of xenon follows fission power, so the poison concentration peaked at the core center, where the power density used to be at maximum. When the reactor was operated at half power, and xenon poisoning began to take effect, the distribution was pushed down at the center, disconnecting the upper and lower halves from each other.
By the evening, there were essentially two independent chain reactions producing fission power in the 7 meter-high reactor core (this is not an exact description, but it illustrates the effect). When the control rods were pushed down, the absorber quenched the chain reaction in the upper part of the core. The positive scram had the opposite effect on the lower power peak, which initiated the explosive power excursion.
Fig. 6: Illustration of the physical principle behind the positive scram in the Chernobyl accident. Most of the control rods had been pulled all the way up (a), and the accumulation of high-absorbing Xe135 at the core center had separated the axial power distribution into two peaks. When the control rods started moving downwards (b), the absorber part (in red) was pushed into the core from above, terminating the chain reaction in the upper part. The effect on the lower peak was the opposite, since the graphite follower (gray) replaced neutron-absorbing water in the channel, leading to a local reactivity insertion, and increase in fission power.
Investigations on the cause of the explosion started in Soviet Union soon after the accident. The results were presented to the IAEA in Vienna in August 1986. It was concluded, that the reactor was destroyed by a reactivity-driven power excursion, caused by operator error and gross negligence of safety protocols.
This conclusion was based on the fact that the reactor was operated at a low power level (200 MW), way below the minimum safe limit dictated by the operating regulations (700 MW). The simultaneous operation of 8 circulation pumps was also prohibited, and in addition, the operators had switched off the emergency core cooling systems (ECCS), and two important scram signals. It was noted, that an automatic scram triggered by the turbine trip would have saved the reactor, and that this signal was disconnected right before starting the test. The role of the positive scram was not addressed at all.
Since the international expert group had no means to verify or disproof this course of events, the Soviet view was also adopted in the first official INSAG-1 report by the IAEA. It was noted, however, that: ”It would be surprising indeed if the report, issued after a short period of preparation and at a time when many questions remain to be settled by analysis, were found to be correct in every detail.”
As new evidence began to unfold, also the view on the root causes of the accident began to change. The international expert group published the updated INSAG-7 report in 1992, six years after the accident and INSAG-1. By this time, the effect of the positive scram was fully understood. Also the role played by operator actions was completely revised.
It was noted that many of the actions, listed in the previous report as safety violations, were in fact not prohibited by the operating regulations. There was no mention of a minimum safe power level, or limitations on the simultaneous use of all circulation pumps.vi Disconnecting the ECCS and the scram signals was allowed under special circumstances. The turbine test was exactly such case, since it would have been impossible to conduct the test according to the program without these actions. More importantly, they did not even make a difference for the final outcome. Reactor scram triggered by the turbine trip, for example, would have lead to the explosive power excursion a few seconds earlier, before Akimov triggered the scram manually by pressing the AZ-5 button.
The INSAG-7 report did not give the operators a complete absolution. It is stated that: ”When the reactor power could not be restored to the intended level of 700 MW(th), the operating staff did not stop and think, but on the spot they modified the test conditions to match their view at that moment of the prevailing conditions.” It should be noted, however, that deviating from procedures was not uncommon in the operation of RBMK reactors at the time. There were several institutional problems and major flaws in the safety culture, and the deficiencies of the RBMK design were simply not addressed in the regulations and operator training. The operating staff was most likely genuinely unaware of these problems, and could not anticipate the fact that the reactor was driven into an unstable state before starting the test.vii
The Chernobyl accident naturally raises the question: could the same thing happen in LWRs? As described above, light-water moderated BWRs are stable with respect to coolant boiling over the entire power regime, and the same applies to PWRs. This results from the fact that water acts as the primary moderator, and not as a neutron absorber as in the RBMK. Accelerated boiling leads to a reduction in reactivity (negative feedback), and the reactor shuts itself down if the coolant flow is compromised.
Several modifications were made in the remaining RBMK reactors after the accident. The graphite followers in the control rods were extended from the bottom (see Fig. 5), which eliminated the positive scram at the lower part of the core. The magnitude of positive feedback was reduced by increasing the number of fixed absorber rods.viii The impact on neutron economy had to be compensated for by increasing the fuel enrichment from 2% to 2.4%.
i) The physics behind xenon poisoning is not trivial. The isotope is produced primarily from the radioactive decay of another fission product, I135. When the concentrations of these isotopes are in equilibrium, the production rate of Xe135 matches the rate at which the isotope decays and is transmuted by neutron capture. When there is a reduction in reactor power, the capture loss rate is also reduced. The production rate, however, follows the change with a considerable delay, characterized by the half-life of I135 (6.6 hours). The resulting imbalance between the source and loss rates causes the concentration of Xe135 to increase at first, until the excess I135 reserve has fallen to the new equilibrium level.
ii) This part is a bit sketchy. Some references state that running with all 8 pumps was a part of the test program, while from others I got the impression that the excess pumps were engaged to manage with low water level in the steam separator drums. In any case, the when the test was started, all pumps were running.
iii) To be precise, some reactivity reserve was shifted from control rods to xenon, as the reactor was being poisoned, and then released by xenon burnout, when the power excursion began to take effect. Xenon burnout can be understood as an opposite effect to xenon poisoning. The loss rate (neutron capture) increases faster than the production rate (I135 decay), and the imbalance causes a temporary drop in the concentration.
iv) There were actually two explosions. The first explosion lifted the reactor cover plate, and severed all the fuel channels. The sudden pressure loss caused all water to flash into steam, which resulted in an increase in reactivity, and a second power excursion. Another theory is that the second explosion was caused by hydrogen, produced by the oxidation of zirconium-metal cladding tubes at high temperature. The hydrogen caught fire as air entered the damaged reactor vessel after the first explosion.
v) By the end of the power excursion, the chain reaction crossed a line into another dynamic state, called ”prompt super-criticality”. In this state, some of the prompt fission chains (see footnote i under the 2nd episode) become infinitely long. The reactor time constants are no longer dominated by delayed neutron emission, but rather the average time between two fission events, which in reactor physics is referred to as the prompt neutron lifetime. Prompt super-critical power excursions are terminated by Doppler feedback (see footnote ii under the 4th episode), but if the reactivity insertion that triggered them is strong enough, they can cause severe damage to the nuclear fuel before the chain reaction is terminated (this is an important safety design criterion for LWRs). Prompt super-criticality is very often confused with a nuclear explosion. These are not the same thing. While the power excursion in a nuclear explosion proceeds in the prompt super-critical state, the associated time scale is orders of magnitude shorter compared to any reactor transient. Prompt neutron lifetime in thermal reactors (including LWRs and RBMK) is limited by the fact that neutrons have to be slowed down to low energies in order to continue the fission chain, which puts an upper limit to how fast the fission power can grow (see also footnote v under the 2nd episode).
vi) The 700 MW limit came from the test program, not the regulations defining the safe operating regime of the RBMK reactor. Consequently, the safety implications of performing the test at a lower power level were not understood in the control room. Some limitations were imposed on the total coolant flow-rate, but the operation of all 8 circulation pumps was not prohibited.
vii) In one of the scenes, Toptunov and Akimov are trying to explain Dyatlov that the SKALA computer is recommending the reactor to be shut down. In reality, this was not such an automated system. The computer was used to evaluate the so-called operating reactivity margin (ORM), which provides the effective reactivity worth of control rods inside the core (hence also indirectly reflecting the magnitude of the positive feedback). The ORM was clearly below the safe limits at the time of the accident. However, similar to several other safety-related parameters, the significance of the ORM was not properly addressed in operator training. The computer was not located in the control room either, but in another room 50 meters away. Determining the ORM required approximately 10 minutes of automated measurements and computations, so the relevant information was not immediately at the operators’ disposal.
viii) The total amount of positive void reactivity was reduced below the level of prompt super-criticality, which essentially eliminates the possibility of a fast power excursion triggered by coolant boiling alone.
All things considered, I enjoyed the show. The story line was not a completely accurate account of real events, but more like a mixture of historical facts and a few popular Chernobyl myths.
There were a few anachronisms. The evacuation of Pripyat happened before the fall-out was detected in Sweden, not after it. The operation to release water from the pressure suppression pools was carried out one week after the explosion, several days later that portrayed on the show. The IAEA meeting where Legasov presented the Soviet view on the cause of the accident took place already in August, and also the cleanup operation at the site was carried out at a somewhat faster pace (the sarcophagus was already completed by November).
Some technical terms were used incorrectly. Radioactivity, radiation dose and dose rate are different quantities, which were consistently mixed up. For, example, when Legasov compares the radioactive emissions to the Hiroshima bomb, the numbers are given in Roentgen, which is a measure of radiation exposure. From a technical point of view, this does not make any sense. There was also some similar mix-up of terms in the explanations of fission, chain reaction, reactivity, etc. Since this is not a documentary, these errors did not bother me that much, but I wonder if it was really possible for someone without a technical background to get a grasp on what is going on? (Or does it seem more like generic science-stuff, that just belongs in the story line, which I guess is OK as well.)
I honestly do not know what to make of some of the main characters. Legasov’s suicide two years after the accident was real. He did play a major role in the operations during the active phase of the accident, and lead the Soviet delegation at the IAEA meeting in August 1986. He was also actively pursuing for safety improvements for RBMK reactors after the accident.
Dyatlov was obviously portrayed as the bad guy (every story needs one). I feel that the INSAG-7 report puts less responsibility on operator action, and more on institutional failures in safety culture. Dyatlov died in 1995, and since he was the last person alive who was in the control room that night, it is no longer possible to obtain any first-hand accounts on what was said and done. He wrote a long article on the operator’s perspective to the accident before his death, which I think is at least worth the read. (Update June 19, 2019: It appears I was too hasty to make the conclusion that Dyatlov was the only operator in the control room who survived the accident. There were, in fact, several eye-witnesses that survived.)