In Russia, 10 Chernobyl-type nuclear reactors are still operating. Are they safe?

In the last series of the series "Chernobyl"TV scientists HBO Russian scientists discover the truth about the cause of the explosion of the reactor of the 4th power unit of the Chernobyl nuclear power plant, which later “pollinated” radioactive cesium in 17 European countries with a total area of ​​207.5 thousand square kilometers. The disaster at the Chernobyl nuclear power plant revealed fundamental shortcomings in the RBMK-1000 reactor. Despite this, today 10 RBMK-1000 type reactors are still operating in Russia. Are they safe? According to Western experts in nuclear physics, who shared their opinions with the portal Live Science, this question remains open.

After a terrible accident in the design of workingThe RBMK-1000 was modified, but still not as safe as most Western-designed reactors. In addition, there are no international guarantees and obligations that could prevent the construction of new NPPs with similar design flaws.

"There are a number of different types of reactors,projects which are considered by different countries and which differ significantly from Western standard light-water reactors. However, many of them have flaws that are minimized by their designers, ”says Edwin Lyman, head of the Nuclear Safety Project at the Union of Interested Scientists.

“And the more everything changes, the more it remains the same,” adds the scientist.

What is the peculiarity of the reactor of the 4th power unit of the Chernobyl NPP?

The reactor of the first power unit of ChNPP is similar to the fourth one before the accident

In the center of the Chernobyl disaster wasreactor type RBMK-1000. Rectors of similar design used only in the USSR and were markedly different from most light-water reactors, which are the standard for most Western countries.

Light water reactors consist of a large vessel.under pressure, which contains nuclear material (core or core), which is cooled by a circulating water source. In nuclear fission, atoms (in this case, uranium) are split, which leads to the generation of a huge amount of heat and free neutrons. The latter hit other atoms, causing their decay, which leads to the release of an even greater amount of heat and neutrons. Heat converts the water circulating to the reactor into steam that turns turbines that produce electricity.

In light water reactors, water is used inas a moderator that helps control ongoing nuclear fission in the active zone. Water slows the movement of free neutrons so that they are more likely to continue the fission reaction, thereby increasing its efficiency. As the reactor heats up, more water turns into steam and less becomes available for this moderator role. As a result, nuclear fission slows down. This principle of negative feedback is a key safety aspect that prevents reactors of this type from overheating.

RBMK-1000 reactors are different. They were created specifically to work on less enriched fuel. Reactors of this type also use water as a coolant, but graphite blocks are used as a moderator. Because of this separation of the roles of the coolant and moderator, the principle of negative feedback “more steam less reactivity” did not work in RBMK. Instead, RBMK-type reactors used the principle of void reactivity coefficient.

Part of the coolant in the reactor can evaporateforming vapor bubbles (voids in the coolant). An increase in the vapor content can lead to both an increase in reactivity (a positive vapor coefficient) and a decrease in reactivity (a negative vapor coefficient); this depends on the neutron-physical characteristics. With a positive neutron coefficient, the task of moving to a graphite moderator is made easier, says nuclear physicist from Sweden, Lars-Erik de Geer.

Hence the root of the disaster grows, says DeGeer As the reactivity increases, the reactor heats up, more water turns into steam, which further increases the reactivity. The process goes on and on.

What was the cause of the disaster at the Chernobyl nuclear power plant?

When the Chernobyl nuclear power plant was fully operational,it was not a big problem, says Lyman. At high temperatures, uranium fuel, which drives nuclear fission, absorbs more neutrons, making it less reactive. But when operating at reduced power, RBMK-1000 type reactors become very unstable.

At the station on April 26, 1986 was walkingscheduled preventive maintenance. And each such repair for an RBMK-type reactor included tests of various equipment, both routine and non-standard, carried out according to separate programs. This stop involved the testing of the so-called “run-down mode of the turbogenerator rotor” proposed by the general designer (Institute Hydroproject) as an additional emergency power supply system.

“In the end, this was one of the reasons for what happened,” says De Geer

Before the scheduled shutdown, the reactor operated at50 percent power over 9 hours. By the time when the station's operators received permission to further reduce the power, xenon absorbing neutrons (xenon poisoning) accumulated in the reactor due to the splitting of uranium, so the appropriate level of reactivity could not be maintained inside it. When the core of the rector is operating at full power, xenon is burned before it can start creating problems. But since the rector worked only for half a hour for 9 hours, therefore xenon did not burn out. With the planned gradual decrease, there was a short-term power failure almost to zero. The station personnel decided to restore the reactor power by extracting the absorber rods of the reactor (composed of neutron absorbing boron carbide), which are used to slow down the fission reaction. In addition, due to the reduction in the speed of the pumps connected to the “running out” generator, the problem of a positive steam reactivity coefficient exacerbated. In seconds, the reactor power has increased dramatically, exceeding its level by 100 times.

Having understood the danger of the situation, the shift supervisor of the 4thThe power unit instructed the senior reactor control engineer to press the emergency shutdown button for the A3-5 reactor. At the signal of this button, emergency protection rods were to be inserted into the active zone. However, due to the design flaws of the reactor, these rods could not be completely lowered - the vapor pressure in the reactor held them at a height of 2 meters (the height of the reactor was 7 meters). Thermal power continued to grow rapidly, the reactor began to self-accelerate. Two powerful explosions occurred, as a result of which the reactor of the 4th power unit was completely destroyed. The walls and ceilings of the engine room were also destroyed, and fires appeared. Employees began to leave jobs.

Scientists are still arguing what could servecause of each explosion. According to some opinions, both explosions could be steam and caused by a sharp increase in pressure in the circulation system. According to another version, one explosion could be steam. And as a result of the second hydrogen exploded during chemical reactions inside the collapsing reactor. However, the definition after the explosion of xenon isotopes in Cherepovets, which is 370 kilometers from Moscow, indicates, according to De Geer, that the first explosion was in fact a release of radioactive gas that had fired several kilometers into the atmosphere.

What changed in the RBMK reactors after the Chernobyl disaster?

Chernobyl disaster was a real blow toThe Soviet Union, said Jonathan Coopersmith, a technology historian from Texas A & M University, who was in Moscow in 1986. The society did not immediately know about the real scale of the incident due to the slowness of the authorities and also the negligence on the ground.

The Soviet media did not immediately report a catastrophe. The first information about the consequences of the explosion appeared in the Swedish media after a radioactive cloud appeared over the country. In the absence of reliable information and intelligible comments from the authorities, foreign publications began to disseminate unverified data based on rumors. In response, Soviet newspapers accused "certain circles" abroad of trying to escalate the situation.

Mikhail Gorbachev appealed to Soviet citizens only on May 14, almost three weeks after the disaster.

“It made publicity real,” says Coopersmith, meaning that this event laid the foundation for the emerging transparency policy in the Soviet Union.

In addition, it marked the beginning of a new erainternational nuclear security cooperation. In August 1986, the International Atomic Energy Agency held a conference in Vienna, where Soviet scientists displayed an unprecedented level of openness at that time, providing details of the incident, said De Geer, who also attended that conference.

“It is still surprising that they told us so much,” says the Swedish scientist.

After a terrible accident in the design of workingChanges were made to RBMK-1000: more enriched fuel was used, the number of control rods was increased, additional inhibitors were introduced to avoid loss of control over the reactor at low capacities.

The three remaining reactors of the Chernobyl nuclear power plantwere in operation until 2000. On December 15, 2000, the reactor of the last, third power unit was permanently shut down. In Lithuania, two RBMKs also remained, which were subsequently closed on demand after the country became a member of the European Union. To date, four of the operating RBMK is in Kursk, three in Smolensk and another three in St. Petersburg (the fourth was closed in December 2018).

“These reactors are not as good as the European ones, although they have become less dangerous,” notes De Geer.

"There are fundamental properties of the designRBMK-1000, which can not be fixed. It is hardly possible to increase the safety of the RBMK as a whole to the level that can be expected from a similar Western-style reactor, ”adds Edwin Lyman

In addition to this, De Geer notes that thesethe reactors do not provide for the presence of full containment protective systems that are available to Western-type reactors. These systems are shields of lead and steel and are designed to keep radioactive gas or steam from emissions into the atmosphere in the event of an accident.

Tighter controls needed

Despite the potential consequences of a nuclear accidentfor the entire world community, there are still no international agreements that clearly state what can be considered a “safe” nuclear power plant, says Lyman

He notes that the Convention on Nuclear SafetyIt requires countries to be fully transparent with regard to the measures taken to ensure the safety of nuclear power plants and allows for expert assessment of these systems, but there are no legislative mechanisms for enforcing these measures. Individual countries have their own independent regulatory bodies, but their independence is limited by how much they are provided by local authorities, says Lyman.

“How can we expect the competent functioning of an independent regulatory body in countries with developed corruption and lack of good governance?”, Asked Lyman.

Despite the fact that besides the USSR no one elsebuilt RBMK-1000 type reactors, in some countries new reactor designs have been proposed, where they also have a void reactivity coefficient. For example, this principle is used in fast neutron breeder reactors (RRBN), in which more fissile material is produced as power grows. Such reactors are built, for example, in China, Russia, India and Japan. Although in the latter case, the reactor is not working and it is planned to be completely decommissioned. India is 10 years behind its commissioning schedule. In Canada, there are also reactors that use the effect of a small positive void coefficient.

"The designers argue that if you take everything intoattention, then in general such reactors are safe, so it does not really matter. But these designers overestimate their systems too much, ”says Lyman.

“Such thinking ultimately led the USSR to trouble. And we can also be in trouble if we treat with negligence what we don’t know or don’t understand. ”

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