In the last episodes of the series “Chernobyl” of TV HBO Russian scientists reveal the truth on what could have caused the explosion of the reactor of the 4th unit of the Chernobyl NPP, “epilepsy” subsequently with radioactive caesium territory of 17 countries in Europe with a total area of 207,5 thousand square kilometers. The Chernobyl disaster has exposed fundamental weaknesses in the RBMK-1000 reactor. Despite this, today 10 reactors of type RBMK-1000 reactors still operating in Russia. Are they safe? According to Western experts in nuclear physics, who shared his opinion with the portal Live Science, this question remains open.
After a terrible accident in the design of the operating RBMK-1000 changes were made, but they are still not as safe as most reactors, established on the Western model. Besides, there are no international guarantees and obligations, which could prevent construction of new nuclear plants with similar design flaws.
“There are a number of different types of reactors, which examines various countries, and which greatly differ from the Western standard light water reactors. Many of them have their drawbacks, which minimised their designers,” says Edwin Lyman, Project Manager for nuclear safety at the Union of concerned scientists.
“And the more things change, the more they stay the same”, adds the scientist.
What feature of the reactor of the 4th unit of the Chernobyl NPP?
The first reactor unit of the Chernobyl nuclear power plant similar to the fourth before the accident
In the center of the Chernobyl accident was a reactor of the type RBMK-1000. Rectors of similar design use only in the Soviet Union and is significantly different from most light water reactors, which is standard for most Western countries.
Light-water reactors consist of a large pressure vessel that contains nuclear material (the core or active area), which is cooled by circulating water. In nuclear fission atoms (in this case uranium) are split, which leads to generation of huge amounts of heat and free neutrons. The last hit other atoms, causing them to collapse, leading to the release of even more heat and neutrons. Heat turns circulating to the reactor water into steam, which turns turbines, producing electricity.
In light water reactors water is used as moderator, which helps to control the ongoing nuclear fission in the active zone. The water slows the movement of free neutrons to those with a greater likelihood of continued fission, thereby increasing its effectiveness. Heating of the reactor, more water turns to steam and becomes less available for this role of moderator. As a result of nuclear fission slows down. This principle of negative feedback is a key aspect of security that prevents such type reactors from overheating.
Reactors of type RBMK-1000 are different. They were created specifically to work on less-enriched fuels. As a coolant for the reactors of this type also use water, but as moderator they use graphite blocks. This role separation of coolant and moderator in the RBMK did not work the principle of negative feedback “more steam — less reactivity”. Instead, the RBMK reactors used the principle of the void coefficient of reactivity.
A part of the fluid in the reactor can evaporate, forming vapor bubbles (voids in the coolant). Increased vapor content can lead to an increase in reactivity (positive void coefficient) or decrease (negative void coefficient) depending on neutron-physical characteristics. If a positive coefficient for neutron eases the task of traffic to the graphite moderator, said nuclear physicist from Sweden Lars-Erik de Geer.
Hence grows the root of the disaster, says De Geer. With increase in reactivity, the reactor heats up, more water turns to steam, which further increases the reactivity. The process continues and continues.
What caused the Chernobyl disaster?
When the Chernobyl nuclear power plant worked at full strength, it was not a big problem, says Lyman. At high temperatures uranium fuel which powers the nuclear fission, it absorbs more neutrons, making it less reactive. But when working at reduced power reactors of type RBMK-1000, becoming very unstable.
At the station April 26, 1986 went according to plan-precautionary repair. And each repair for RBMK reactors included performance testing of various equipment, as scheduled and custom, which takes place on individual programs. This involves the testing of the so-called mode “coasting of the rotor of a turbogenerator”, proposed by the General designer (Institute Hydroproject) as an additional emergency power system.
“In the end, it was one of the reasons that happened,” said De Geer
Before the start of the shutdown the reactor was operating at 50 percent power for 9 hours. By the time the operators received permission to further reduce power in the reactor due to fission of uranium, accumulated neutron-absorbing xenon (xenon poisoning), so inside it could not be supported by the appropriate level of reactivity. When working the active zone of the rector in the full power of the xenon burned earlier than you can begin to create problems. But as the rector worked for 9 hours, only half-heartedly, so the xenon is not fading. During a scheduled gradual reduction was the intermittent failure of power to almost zero. The station staff took the decision to restore the power of the reactor, by retrieving the absorbing rods of reactor (composed of a neutron absorbing boron carbide), which are used to slow down the fission reaction. In addition, due to the lower speed of the pumps connected to “running” the generator has worsened the problem of positive steam coefficient of reactivity. In seconds the reactor power sharply increased, exceeding the level of its capacity 100-fold.
Realizing the danger of the situation, the shift supervisor of the 4th power unit gave the team the senior engineer of reactor control, press the emergency button of the damping reactor A3-5. The signal this button in the active zone was introduced the emergency protection rods. However, due to the design flaws of the reactor to the lower end of these rods failed by the vapor pressure in the reactor is detained them at a height of 2 meters (the height of the reactor is 7 meters). Thermal power has continued to grow rapidly, began samarason reactor. There were two powerful explosions, which resulted in the 4 reactor unit was completely destroyed. Also was destroyed the walls and ceiling of the turbine hall, have pockets of fire. Employees began to leave jobs.
Scientists are still arguing what might be the cause of each explosion. According to some opinions, both explosions could be steam caused by the sharp increase in pressure in the circulation system. According to another version, one explosion could be steam. As a result, the second exploded hydrogen in the course of chemical reactions inside the crumbling reactor. However, the definition after the explosion of xenon isotopes in Cherepovets, in 370 km from Moscow, indicates according to De Geer that the first explosion was in fact a release of radioactive gas, pellet leaving the barrel at several kilometers into the atmosphere.
That changed in RBMK reactors since the Chernobyl disaster?
The disaster in Chernobyl was a real blow for the Soviet Union, says Jonathan Coopersmith, a historian of technology from the Texas A&M University, the former in Moscow in 1986. A real incident because of the slowness of the authorities and also negligence on the ground the society has learnt not at once.
The Soviet media did not immediately report the accident. The first information about the consequences of the explosion appeared in the Swedish media after the country was born radioactive cloud. In the absence of reliable information and clear comments from the authorities of the foreign press began to spread unverified information, based on hearsay. Soviet Newspapers responded by accusing “certain circles” abroad in an attempt to escalate the situation.
Mikhail Gorbachev addressed Soviet citizens only may 14, nearly three weeks after the disaster.
“Did the publicity real,” says Coopersmith, meaning that this event laid the Foundation for the emerging policy of transparency in the Soviet Union.
In addition, this marked the beginning of a new era of international cooperation on nuclear security. In August 1986, the International atomic energy Agency held a conference in Vienna, where Soviet scientists have shown unprecedented for that time level of openness, giving details of the incident, says De Geer, who was also present at that conference.
“Still surprising that they are so much told us,” says Swedish scientist.
After a terrible accident in the design of the operating RBMK-1000 has been amended: was used more enriched fuel, has increased the number of control rods, an additional inhibitors to avoid losing control of the reactor at low power.
The three remaining Chernobyl reactor was in operation until 2000. 15 December 2000, was permanently shut down the reactor last, the 3rd unit. Lithuania also had two RBMK reactors, which were subsequently closed on demand after the country became a member of the European Union. To date, four operating RBMK is Kursk, three at Smolensk and three in St. Petersburg (the fourth was closed in December 2018).
“These reactors are not as good as European,they have become less dangerous,” said De Geer.
“There are fundamental design feature of the RBMK-1000, which is not correct. It is hardly possible to improve the safety of RBMK reactors in General to a level that can be expected from a similar reactor to that of a Western,” adds Edwin Lyman.
In addition, De Geer notes that these reactors do not require protective systems full localization, which is available in Western reactors. These systems represent a shield of lead and steel and intended to retain the radioactive gas or vapour from atmospheric emissions in the event of an accident.
Needs tighter control
Despite the potential accident consequences at nuclear power plants for the global community, there is still no international agreements that have clearly defined what can be considered “safe” nuclear power, says Lyman.
He noted that the Convention on nuclear safety requires countries full transparency of the adopted safety measures of nuclear power plant operation and allows for expert evaluation of these systems, but legally there are no enforcement mechanisms and sanctions for compliance with these requirements. Individual countries have their own independent governing bodies, however, their independence is limited by how they provide local authorities, says Lyman.
“How can you expect competent operation of an independent regulator in countries with high corruption and lack of good governance?”, — asks Lyman.
Despite the fact that in addition to the USSR, nobody has built reactors of type RBMK-1000, some countries proposed new projects of reactors, which also is the void coefficient of reactivity. For example, this principle is used in reactors-breeder fast reactors (breeder reactor) in which the growth of power produces more fissile material. Such reactors are constructed, for example, in China, Russia, India and Japan.in the latter case the reactor is not functioning and is scheduled to be fully decommissioned. India is behind schedule commissioning of the reactor for 10 years. In Canada, there are also reactors that use the effect of a small positive void coefficient.
“Designers say that if you take everything into consideration, in General, such reactors are safe, so it does not really matter. But these designers are too overestimate their system,” says Lyman.
“This kind of thinking eventually led the USSR to disaster. And we could be in trouble, if we with negligence refers to what you do not know or understand”.