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In the new HBO series entitled "Chernobyl", Russian scientists explained the reason for the explosion of reactor 4 at the Chernobyl nuclear power plant, which spewed radioactive material into northern Europe.
This reactor, of design RBMK-1000, is revealed to be basically defective after the Chernobyl accident. And yet, 10 reactors of the same type are still in service in Russia. How do we know if they are safe?
The short answer is that we do not do it. Experts say these reactors have been modified to reduce the risk of another Chernobyl-type disaster, but they are still not as safe as most Western-type reactors. And there are no international guarantees that would prevent the construction of new factories with similar defects. [Images: Chernobyl, Frozen in Time]
"There are a large number of reactor types being considered in different countries that differ significantly from the standard light water reactor, and many of them have security flaws that designers minimize," said Edwin Lyman, a researcher. Principal and Acting Director of the Nuclear Safety Project of the Union of Concerned Scientists.
"The more things change," Lyman told Live Science, "the more they stay the same."
Reactor 4
The RBMK-1000 reactor, used only in the Soviet Union, was at the center of the Chernobyl disaster. The reactor was different from most light water nuclear reactors, the standard design used in most Western countries.
Light water reactors consist of a large pressure vessel containing nuclear material (the core), which is cooled by a circulating water reserve. In nuclear fission, an atom (uranium, in this case), splits, creating heat and free neutrons, which turn into atoms, causing them to split and release heat and more neutrons. . The heat turns the circulating water into steam, which then turns a turbine, generating electricity.
In light water reactors, water also serves as a moderator to help control ongoing nuclear fission in the heart. A moderator slows the free neurons so that they are more likely to continue the fission reaction, making the reaction more efficient. When the reactor warms up, more water turns into steam and there are fewer resources left to play this role of moderator. As a result, the fission reaction slows down. This negative feedback loop is an essential safety feature that prevents reactors from overheating.
The RBMK-1000 is different. He also used water as coolant, but with graphite blocks as a moderator. Variations in the design of the reactor allowed it to use a fuel less enriched than usual and to be refueled in operation. But with the separate coolant and moderator roles, the negative feedback loop "More Steam, Less Reactivity" has been broken. Instead, RBMK reactors have what is called a "positive vacuum coefficient".
When a reactor has a positive vacuum coefficient, the fission reaction accelerates when the cooling water turns into steam rather than slowing down. Lars-Erik De Geer, a nuclear physicist who retired from the Swedish Defense Research Agency, said Lars-Erik De Geer, a nuclear physicist who has retired.
From there, the problem is created: the fission becomes more efficient, the reactor becomes hotter, the water becomes hotter, the fission becomes even more efficient and the process continues.
On the road to disaster
When the Chernobyl plant was running at full power, it was not a big problem, said Lyman. At high temperatures, the uranium fuel that powers the fission reaction tends to absorb more neutrons, making it less reactive.
However, at low power, the RBMK-1000 reactors become very unstable. In the run-up to the Chernobyl accident on 26 April 1986, operators were carrying out a test to determine whether the plant's turbine could run backup equipment in the event of a power failure. This test required to run the facility at reduced power. While power was reduced, Kiev authorities ordered operators to suspend the process. A conventional power station was out of order and Chernobyl power generation was needed.
"It was really the main reason why everything finally happened," De Geer said.
The plant operated at partial power for 9 hours. When the operators were allowed to reduce most of the current, there was a build up of neutron absorbing xenon in the reactor and they could not maintain the proper fission level. The power has fallen to almost nothing. In trying to boost it, the operators removed all control rods, made of neutron absorbing boron carbide and used to slow down the fission reaction. Operators also reduced the flow of water into the reactor. This exacerbated the problem of positive vacuum coefficient, according to the Agency for Nuclear Energy. Suddenly, the reaction became very intense. In a few seconds, the power was multiplied by 100, to the capacity of the reactor. [Chernobyl Nuclear Disaster 25 Years Later (Infographic)]
There were other design flaws that made it difficult to regain control of the situation once it started. For example, the control bars were covered with graphite, explains De Geer. When the operators saw that the reactor was starting to break down and tried to lower the control rods, they found themselves stuck. The immediate effect was not to slow the fission, but to improve it locally, because the additional addition of graphite to the ends initially increased the efficiency of the nearby fission reaction. Two explosions quickly followed. Scientists are still discussing the cause of each explosion. It may be a steam explosion due to the rapid increase in pressure in the circulation system, or one may be steam and the second a hydrogen explosion caused by chemical reactions in the reactor Out of order. According to the detection of xenon isotopes in Cherepovets, 370 km north of Moscow after the explosion, De Geer believes that the first explosion was actually a jet of nuclear gas penetrating several kilometers of the atmosphere.
Changes made
The immediate consequences of the accident were "a very troubling time" in the Soviet Union, said Jonathan Coopersmith, a technology historian at Texas A & M University, which was in Moscow in 1986. In the beginning, Soviet authorities kept the information confidential; the state press buried the story and the rumor took over. But far from Sweden, De Geer and his scientific colleagues were already detecting unusual radioactive isotopes. The international community will soon know the truth.
On May 14, Soviet leader Mikhail Gorbachev delivered a televised speech in which he explained what had happened. This was a turning point in Soviet history, Coopersmith told Live Science.
"This has made glasnost real," said Coopersmith, citing the nascent transparency policy in the Soviet Union.
It has also opened a new era of cooperation in nuclear safety. In August 1986, the International Atomic Energy Agency held a summit in Vienna on the aftermath of the accident and Soviet scientists tackled it with an unprecedented sense of openness, said De Geer, also present.
"It was amazing what they told us," he said.
Among the changes made as a result of Chernobyl, there were modifications made to the other operating RBMK-1000 reactors, 17 at the time. According to the World Nuclear Association, which promotes nuclear energy, these changes include the addition of inhibitors to the nucleus to prevent low-power packaged reactions, increasing the number of control rods used in operation and increasing fuel enrichment. The control rods have also been modernized so that the graphite does not move to a position that can increase responsiveness.
The other three Chernobyl reactors operated until 2000, but have since been shut down, as have two other RBMK reactors in Lithuania, which have been closed as a precondition to the entry of this country into the European Union. Four RBMK reactors are in operation at Kursk, three at Smolensk and three at St. Petersburg (a fourth was decommissioned in December 2018).
These reactors "are not as good as ours," said De Geer, "but they are better than before."
"There were fundamental aspects of the design that could not be corrected whatever they did," Lyman said. "I would not say that they were able to increase the safety of the RBMK to the height of the standard that is expected from a Western-style light-water reactor."
De Geer also pointed out that the reactors were not built with complete containment systems as in Western-type reactors. Containment systems are lead or steel screens designed to prevent radioactive gas or vapor from escaping into the atmosphere in the event of an accident.
Supervision neglected?
Despite the potentially international effects of a nuclear power plant accident, there is no binding international agreement on what constitutes a "safe" power plant, said Lyman.
The Convention on Nuclear Safety requires countries to be transparent about their safety measures and allow for peer review of plants, he said, but there are no mechanisms for ensuring the safety of nuclear power plants. 39, application nor sanctions. Countries have their own regulatory agencies, which are independent only to the extent that local governments allow them, Lyman said.
"In countries characterized by widespread corruption and a lack of good governance, how can you expect any independent regulator to be able to function?" Said Lyman.
Although no one other than the Soviet Union has manufactured RBMK-1000 reactors, some new designs of proposed reactors involve a positive vacuum coefficient, Lyman said. For example, fast neutron reactors, which generate more fissile material as they generate energy, have a positive void coefficient. Russia, China, India and Japan have all built such reactors, although Japan is not operational and is scheduled to downgrade, and India is 10 years behind schedule. opening. (There are also positive low vacuum reactors operating in Canada.)
"The designers argue that if you take everything into account, they are generally safe, so it does not matter," Lyman said. But designers should not be too confident in their systems, he said.
"It's this kind of thinking that has caused problems for the Soviets," he said. "And that's what can get us into trouble by not respecting what we do not know."
Originally published on Science live.
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