Disaster in an hour What exactly happened at the Chernobyl nuclear power plant

The accident at the Chernobyl nuclear power plant became the largest disaster in the entire history of nuclear energy. Until 2011, when an earthquake and tsunami triggered an accident at the Japanese Fukushima-1 nuclear power plant, the Chernobyl disaster remained the only one in history that was assigned a maximum danger level of seven.

Studying the consequences of the accident and all available data allowed experts to reproduce an almost second-by-second scenario of what happened at the fourth power unit of the Chernobyl nuclear power plant, although experts still differ in the details of their assessment of what happened. Below is a highly simplified version of the development of events on the night of April 25-26, 1986 and the mechanisms that caused the explosion are disassembled.

A little theory

Nuclear power plants extract energy from the decay of unstable isotopes of heavy elements and convert it into electrical energy. The fuel for nuclear power plants is most often the isotope of uranium-235 (more precisely, uranium oxide) - when its nuclei decay, heat is released, which heats the water in the reactor and turns it into steam, which rotates the turbines. The kinetic energy of the turbine movement with the help of a generator is stored in the form of electricity.

The “heart” of a nuclear power plant is the reactor - it is in its active zone that nuclear decay occurs. Heavy nuclei of uranium-235 decay into nuclei of lighter elements, and this process, in addition to the release of heat, is accompanied by the emission of free neutrons - elementary particles, which, along with protons, are part of atomic nuclei. Colliding with uranium-235 nuclei, neutrons stimulate their decay, which also releases neutrons - this cascade is called a chain reaction.

If during the decay of nuclei the number of neutrons emitted is equal to the number of neutrons that caused fission, then the same amount of energy is released in the reactor all the time. If more neutrons are produced, the amount of energy released increases, and if fewer, it decreases. For stable operation of a nuclear power plant, it is necessary that the first of the options described above be implemented. If the number of free neutrons produced during nuclear fission increases, then sooner or later this process will end in an explosion.

During a chain reaction, the number of free neutrons, by definition, will increase over time. To prevent a catastrophic outcome, the intensity of decay reactions in nuclear power plants is regulated using so-called control rods, which contain a material that absorbs neutrons well (for example, cadmium or boron). When the number of free neutrons in the reactor becomes dangerously large, the rods are immersed in the core, and the number of decays per unit time decreases.

In order for the operation of a nuclear power plant to be safe, operators need to take into account another process - the so-called xenon poisoning of the reactor and the resulting fall into the iodine pit. When uranium-235 nuclei fission, as a result of a chain of secondary decays, the isotope xenon-135 is formed, in the nuclei of which free neutrons are effectively “stuck.” When the reactor is actively operating, all the resulting xenon-135 nuclei are quickly saturated with neutrons to the maximum - they are said to burn out. In addition, some nuclei decay into nuclei of other elements. If the reactor operating power is low, xenon does not have time to burn out and accumulates in the core - this is xenon poisoning.

When a reactor is poisoned, the precursor of xenon-135 along the decay chain - the isotope of iodine-135 - begins to turn into xenon with even greater activity (this is the iodine pit). In this state, the reactor becomes unstable and reacts poorly to the movements of the control rods, which can lead to disastrous consequences.

Xenon-135 absorbs many neutrons, the number of fissile uranium nuclei per unit time remains low, and in order to increase the power of the reactor, it is necessary to extend the rods from the core. If a lot of xenon has accumulated, then with a small amplitude of movement of the rods, no noticeable changes will occur in the reactor and there may be a temptation to push them out more.

At some point, the number of nuclear decays reaches a certain threshold value, the power of the reactor (and at the same time heat generation) increases abruptly, and it is not always possible to extinguish this process by quickly lowering the rods. For this reason, if a reactor is poisoned by xenon, it must be completely shut down and wait for the natural loss of xenon - its half-life is 9 hours. When the reactor is shut down, the turbine does not spin and no electricity is generated.

Chronology

On the night of April 25-26, 1986, at the fourth power unit of the Chernobyl nuclear power plant, a new promising technique was to be tested, which would allow, in the event of an emergency shutdown of the reactor, to immediately supply it with power coming from the turbine still rotating by inertia (the so-called run-down mode) . A shutdown reactor needs electricity, since decay reactions still occur in it and, accordingly, the temperature rises. In order to cool the core, water is continuously pumped through it using pumps, and the pumps require electricity to operate. It was intended to test the operation of the technique under several initial system parameters, so the operators categorically did not want to shut down the reactor - in this case, the experiment would have to be repeated on some other day.

To test the new technique, it was necessary to reduce the reactor power to about 700 megawatts - in this case, the emergency cooling systems (ECCS) would inevitably shut it down, so the operators decided to turn them off. The experiment began at about 23:00 on April 25 - the personnel began to gradually reduce the power of the reactor, and the operators did not give the automation command to maintain the power at an acceptable level. As a result, it dropped to 30 megawatts, and xenon poisoning of the reactor began. To increase power, operators removed all control rods from the core, and they managed to accelerate the reactor to 200 megawatts, although the xenon poisoning process continued.

It was initially planned that the experiment would take place at a reactor power of 700 to 1000 megawatts, but despite the fact that it was not possible to bring the reactor to this value, the staff decided to continue testing the methodology. At about one o'clock in the morning, operators turned on all the main circulation pumps (MCPs) of the power unit, with the help of which water is pumped through the reactor, to carry out the necessary tests. This load turned out to be excessive - there was not enough water to operate all the pumps, steam generation in the reactor, washed by huge volumes of H2O, decreased, and the automation completely removed the control rods from the core.

Fearing an emergency shutdown of the reactor and failure of the experiment, the operators turned off the system that shuts down the reactor at extremely low water levels and steam pressure. The reactor power still remained low, and at 01:19 personnel removed the manual control rods that were still there from the core. As a result, operators lost all leverage over the system. Chernobyl NPP employees turned off some of the pumps, but despite the external stability of the reactor, the instruments issued a message that it was consuming an unreasonable amount of water and (most importantly) that the processes in the core were almost impossible to regulate from the outside (this is called low reactivity). In such cases, safety instructions require the reactor to be shut down immediately, but the operators decided to continue the experiment.

Moreover, in order to avoid an emergency shutdown of the reactor during experiments, the operators blocked the system for shutting it down in the event of a loss of steam supply to the second turbine, if the first one had already been turned off, which is strictly prohibited. One of the turbines was supposed to be turned off in order to test the technique being studied. After this was done, the MCPs sharply reduced their activity, and the flow of water through the core also became less intense. As a result, vaporization began to increase in the reactor.

Some of the control rods automatically began to move into the core, but their capacity was not enough to reduce the power of the reactor. Since the steam supply to the turbine was turned off, it rotated more and more slowly, and, accordingly, the amount of water in the reactor was reduced, since all the H2O in the system is “common”. At 01:23:40, the shift supervisor ordered the AZ-5 button to be pressed, which causes the control rods to move into the core as quickly as possible.

This order became fatal due to the so-called end effect of the rods. The absorbent substance does not occupy the entire volume of the rod - at the very bottom there is a displacer (in the case of the Chernobyl nuclear power plant it was graphite), which should “remove” water from the path of movement of the absorber. When the AZ-5 button was pressed, the first to enter the active zone were the displacers, which, like pistons, pushed out some of the water from there. The vaporization in the reactor increased further, and the rods “stuck” on the steam layer, without immersing the absorber into the core. The operators resorted to a last resort and turned off the electromagnets that hold the rods to the reinforcement, but this did not help - the steam was too dense. At 01:23:43, the reactor “went crazy” (several more emergency systems managed to work, but they all gave the AZ-5 command), and at 01:23:44 a thermal explosion of a nuclear nature occurred, destroying the reactor core.

The zirconium shell of the fuel rods began to react with the steam, as a result, hydrogen began to be released (the so-called steam-zirconium reaction), which formed an “explosive mixture” with oxygen in the air, which exploded at 01:23:46. A heavy concrete cover was torn off the reactor, which flew into the air and fell next to the fourth power unit. A huge amount of radioactive substances from the reactor core was released into the atmosphere, and “contaminated” hot debris was scattered throughout the station. Numerous fires started. The first fire brigade under the command of Lieutenant Pravik left for the Chernobyl nuclear power plant at 01:30 and largely thanks to his actions, it was possible to prevent the widespread spread of the fire.

Who is guilty

Immediately after the accident, a special commission was formed in the USSR to investigate the causes of what happened. She reconstructed the chronology of events and came to the conclusion that the cause of the disaster was the actions of the station operators. Later (but also in 1986), this opinion, based on data provided by the Soviet side, was supported by an IAEA expert group called INSAG (International Nuclear Safety Advisory Group).

In 1991, the USSR Gosatomnadzor commission re-examined the available information and concluded that the accident was the result of personnel actions, but their consequences acquired such catastrophic proportions due to some design flaws in the Chernobyl reactor. Such disadvantages, in particular, include the end effect of the rods and the so-called positive reactivity under certain operating conditions. Positive reactivity is the property of a reactor to increase power in response to certain conditions, and during the experiment at the Chernobyl Nuclear Power Plant on April 26, 1986, a situation was created where these conditions were constantly maintained and even intensified, causing a rapid and ultimately uncontrollable increase in power. INSAG also revised its conclusions and again agreed with its colleagues from the USSR.

Currently, most experts adhere to this point of view. The accident at the Chernobyl nuclear power plant was the result of thoughtless and illiterate actions of operators who consistently did everything possible to lead the situation to a disaster, but, firstly, some of their decisions, which now seem crazy, were not considered dangerous then (there was simply no relevant data) and were not prohibited by the regulations, and, secondly, the catastrophic outcome was made possible due to imperfections in the design of the station and its safety systems (in particular, they allowed for complete shutdown of protections).

After the Chernobyl accident, nuclear energy development programs were frozen in many countries, but gradually this reaction of rejection weakened. Analysis of what happened allowed specialists to identify many shortcomings of reactors and other nuclear power plant systems, which, as it turned out, could play a fatal role, and eliminate them when designing new types of reactors.