What happened in Chernobyl?

 The explosion of the reactor at the Chernobyl nuclear power plant has been recognized as the greatest nuclear disaster in the history of nuclear energy in the world. The consequences were borne not only by Ukraine, Belarus, and the then Soviet Union, but all of Europe, over which a cloud of radioactive debris spread.

To understand what actually happened in Chernobyl, let's recall the diagram of a classic thermal power plant. It consists of a boiler in which the heat from the burned coal is taken over by water and transformed into high-energy superheated steam with a temperature of over 500o C. This steam rushes to the turbines, which drive electricity generators. In a nuclear power plant, this system is preserved, with one exception: instead of a boiler, we have a nuclear reactor in which a controlled chain reaction takes place, which releases enormous amounts of energy.

 The high-power RBMK channel reactor used at the Chernobyl NPP was equipped with a graphite moderator. The moderator is used to slow down the neutrons; with slower neutrons, the so-called thermal reactions, nuclear reactions with their participation are more effective. The active zone of the Chernobyl reactor is a cylinder 7 m high and 14 m in diameter, filled with nuclear fuel and graphite. This cylinder is cut by vertical pipes in which control rods slide, which strongly absorb neutrons, so that when they are fully inserted into the reactor, all reactions stop. Of course, water constantly flows through the reactor core, taking away the heat generated. If the flow of this water were to be stopped, the temperature in the core would rise and the fuel elements would then melt in what is known as the greatest hypothetical reactor catastrophe. After the Chernobyl explosion, the word "hypothetical" in this term became redundant.

On April 25, 1986 in the Chernobyl NPP the fourth block was to be stopped and it was planned to be renovated. By the way, it was intended to conduct an attempt to supply the power plant with the current generated by the turbogenerator rotor, which rotates before deceleration due to inertia. During the normal operation of the power plant, such a rotor performs 3600 rpm, so a lot of energy is stored in it, which can be recovered in the form of electricity and used in emergency situations for the needs of nuclear power plants.

Similar tests were carried out earlier, but always with the reactor protection systems turned on. Switching off the power plant from electricity is then only a theoretical assumption. This time, however, the NPP was to be completely cut off from the electrical supply, and the reactor's protections were disconnected. So it was a risky experiment and other NPPs refused to do it. Previous experiments of this type were successful because the reactor was operated under stable conditions, its control was ensured, and the protection unit was on all the time.

In Chernobyl, the ambitious management decided to go further: for the duration of the experiment, that is, for four hours, the emergency cooling of the reactor was to be completely disconnected, and no other safeguards "for the purity of the experiment" were provided for. Moreover, the operators in the control room did not realize that in the RBMK type reactor, several factors lead to an over-normative increase in reactivity (i.e. an increase in neutron power). This is especially true of the control rods. Under certain conditions, the reactor's capacity to increase power exceeds its quenching capacity. This is what happened in Chernobyl.

Thus, at 1 a.m. on April 25, 1986, the reactor's power began to be reduced in unit 4. At 2 p.m. a key mistake was made: according to the experiment program, the emergency reactor cooling system was disconnected. At the same 2 p.m., the shutdown of the block was stopped at the request of the dispatcher from Kiev. Only at 11 p.m. the power was reduced again. At midnight, the reactor control was handed over and the experiment continued with a new shift.

According to the test program, the inertial run of the generator rotor was to take place at the reactor capacity of 700–1000 MW. This should have been done while the reactor was shutting down, not while it was running. As a result of an operator's error, the reactor's power dropped below 30 MW and the so-called poisoning the reactor with nuclear decay products, mainly xenon and iodine. It is very difficult to restore normal parameters in such a situation. It would be a failure of the experiment. The scream was then raised by the deputy chief engineer for the operation of Dyatlov, who forced the young and inexperienced operator Toptunov to increase his power. At 1 am the next day, i.e. April 26, 1986, the power was stabilized at the level of 200 MW. It was decided to continue the experiment.

However, the reactor was in an unstable condition and was in danger of exploding. At 1.22 a.m. Toptunow realized that the level of reactivity required an immediate shutdown of the reactor. However, instead of manually lowering the reactor's power and stopping the experiment, it was decided to continue it. At 1.23 am, the inertial run of the generator rotor began. At the same time, the main pumps began to choke steam and the water flow through the active zone of the reactor decreased. This threatened to melt the core and the decision was made to drop the control rods immediately.

The construction of such a rod in the RBMK reactor is as follows. With the height of the active zone of the reactor equal to 7 m, the absorbing part of the rod was 5 m long, and there were empty one-meter sections below and above it. The lower end of the rod, also 5 m long, located below the core when fully lowered, was filled with graphite. Thus, when the bars are dropped into the active zone, first these graphite tips enter, giving a short-term increase in reactivity, then this empty section, and only then the absorbing part.

205 control rods, and according to other data 193, fell on the core, but instead of entering full length, they almost immediately wedged. Under the influence of temperature, the channels were already deformed and the bars were blocked in such a position that the graphite tips were in the active zone, which increased the reactivity. Three seconds later, the reactor's power reached 530 MW, the temperature in the core jumped, the main pump valves shut, the flow of water through the active zone stopped, steam production increased, and the pressure increased at a rate of 15 atmospheres per second.

The next few seconds are a cascade of violent events leading to a tragic end. The fifteen-meter-diameter disc protecting the reactor from the top, consisting of two thousand blocks, placed on channels with fuel rods, behaved like crazy. The blocks, each weighing 350 kg, bounced like lids on pots in which soup boils. It is in the active zone that the zirconium shields in the fuel rods decompose; the violent para-zirconium reaction and other reactions produced hydrogen and oxygen which formed an electrifying mixture. There was a powerful burst of steam, which the reactor's main safety valves could not withstand and were destroyed. The immense pressure also severed the lower water and upper steam pipelines. At 1.24 a.m. without two seconds, the reactor and block 4 building were blown up by explosions of the lightning mixture. The fire from the reactor reached a height of 170 meters above the ground.

Just before the explosion, a computer monitoring the state of the core showed that the upper part of the core had crumbled, melted and vaporized the nuclear fuel. This part, weighing some 50 tons, was ejected into the atmosphere by the blast and blown over Belarus, the Baltic republics and further over Scandinavia. The heavy fractions fell close to the power plant, but the rest, a suspension of uranium dioxide particles, highly radioactive isotopes of iodine 131, plutonium 239, neptunium 139, cesium 137, strontium 90 and many others, continued to travel over Europe until mid-May. A further 70 tons of fuel from side explosions dispersed across the ruins of Block 4 and the adjacent areas of the power plant. The radiation of the ejected fuel ranged from 15 to 20 thousand. x-rays per hour (the lethal dose is 100 x-rays per hour).

The remaining 50 tons of nuclear fuel plus about 800 tons of graphite remained in the reactor. Over the next few days, this graphite burned out completely, adding its share to the post-explosion emission. The power of the explosion was estimated at 10 Hiroshima bombs, not counting the 70 tons of fuel and 700 tons of radioactive graphite scattered around the battlefield.

The carelessness and incompetence of the Chernobyl NPP management and operators is often contrasted with the heroism of firefighters and NPP workers fighting immediately with the consequences of the accident. It was possible to prevent the fire from spreading to the neighboring blocks, but at the cost of the death of many rescuers. They did not realize that they were not fighting an ordinary fire. In a short time they were killed by the enormous radiation, and their skin turned brown and black. They were buried in soldered zinc coffins.

Grigory Medvedev, whose "Chernobyl Report" provided me with most of the information, believes that many factors contributed to the incident, both immediately before the disaster and earlier. First of all, the wrong reactor was selected for the Chernobyl NPP, located only 130 km from Kiev. A water-pressurized one, with a lower radioactive emission, and therefore safer, would be more suitable. The leadership of the Chernobyl NPP were not NPP specialists, they were made up of turbine engineers, electricians, nuclear physicists at best, but without reactor practice. Also in the three institutions supervising NPPs, hydropower engineers, coal-fired specialists, etc. dominated. All of them were attracted to nuclear energy, because there was prestige, modernity, open paths of promotion. The training of operators focused on ad hoc coping in the control room, without understanding the essence of the processes taking place in the reactor, which was considered a kind of furnace. "Nuclear reactors are ordinary furnaces, and the operators controlling them are stokers," said one party dignitary. There was also a "Russian" attitude to the matter: hooray, war terminology was in use - we will defeat, conquer, overcome. In such an atmosphere, sooner or later a large-scale disaster would have to happen.

Poland also experienced the effects of the Chernobyl disaster. On April 28 at 7 a.m. in Mikołajki, radioactive activity in the air over half a million times higher than normal was recorded. During the hours that followed, many reports were received indicating that a radioactive cloud was passing over Poland from the east and covering its northeastern area.

On April 29, a government commission decided to give Lugol's fluid to children and adolescents in eleven northeastern provinces over which a radioactive cloud had passed. The action started on the same day in the evening. On April 29, it was extended to the entire country. As a result of the iodine action, 18.5 million people ingested Lugol's fluid, including over 95 percent. children and adolescents. It was one of the cases in the People's Republic of Poland when the Polish authorities, despite official Soviet denials, took actions in the interest of their own citizens. The publication by the newspapers of the communiqué of the government commission together with the contamination table was a sensation. Over time, testing for contamination encountered difficulties, and at the end of May, the Central Laboratory for Radiological Protection received an order to end monitoring of the amount of iodine-131 in the thyroid gland of children.

The radioactive cloud from Chernobyl traveled across Europe from north to south, and only from there it swung over Poland. The pattern of the winds was happy for us. The Sudetes stopped the inflow of contamination over our territory; the local maximum of contamination was detected in the Opole region between Nysa and Niemodlin. It is believed that as a result of natural factors, we have avoided the absorption of dangerous doses.