Swedish scientists have come to the conclusion that a weak nuclear explosion occurred during the accident at the Chernobyl nuclear power plant. Experts analyzed the most likely course of nuclear reactions in the reactor and simulated the meteorological conditions for the propagation of fission products. "Lenta.ru" tells about an article by researchers published in the journal Nuclear Technology.
The accident at the Chernobyl nuclear power plant occurred on April 26, 1986. The disaster threatened the development of nuclear power throughout the world. A 30-kilometer exclusion zone was created around the station. Radioactive fallout fell even in the Leningrad region, and cesium isotopes were found in increased concentrations in lichen and deer meat in the Arctic regions of Russia.
There are various versions of the causes of the disaster. Most often, they indicate the wrong actions of the Chernobyl nuclear power plant personnel, which led to the ignition of hydrogen and the destruction of the reactor. However, some scientists believe that there was a real nuclear explosion.
Boiling hell
A nuclear chain reaction is maintained in an atomic reactor. The nucleus of a heavy atom, for example, uranium, collides with a neutron, becomes unstable and decays into two smaller nuclei - decay products. In the process of fission, energy and two or three fast free neutrons are released, which in turn cause the decay of other uranium nuclei in nuclear fuel. The number of decays thus increases exponentially, but the chain reaction inside the reactor is under control, which prevents a nuclear explosion.
In thermal nuclear reactors, fast neutrons are not suitable for exciting heavy atoms, so their kinetic energy is reduced using a moderator. Slow neutrons, called thermal neutrons, are more likely to cause the decay of the uranium-235 atoms used as fuel. In such cases, one speaks of a high cross section for the interaction of uranium nuclei with neutrons. Thermal neutrons themselves are called so because they are in thermodynamic equilibrium with the environment.
The heart of the Chernobyl nuclear power plant was the RBMK-1000 reactor (a high-power channel reactor with a capacity of 1000 megawatts). Basically, it is a graphite cylinder with many holes (channels). Graphite acts as a moderator, and nuclear fuel is loaded in fuel elements (fuel rods) through the technological channels. The fuel rods are made of zirconium, a metal with a very small neutron capture cross section. They allow neutrons and heat to pass through, which heats the coolant, preventing the leakage of decay products. Fuel rods can be combined into fuel assemblies (FA). Fuel elements are characteristic of heterogeneous nuclear reactors in which the moderator is separated from the fuel.
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RBMK is a single-loop reactor. Water is used as a heat carrier, which partially turns into steam. The steam-water mixture enters the separators, where the steam is separated from the water and sent to the turbine generators. The spent steam is condensed and re-enters the reactor.
RBMK reactor cover
There was a flaw in the design of the RBMK, which played a fatal role in the disaster at the Chernobyl nuclear power plant. The fact is that the distance between the channels was too great and too many fast neutrons were inhibited by graphite, turning into thermal neutrons. They are well absorbed by water, but steam bubbles are constantly formed there, which reduces the absorption characteristics of the heat carrier. As a result, the reactivity increases, the water heats up even more. That is, RBMK is distinguished by a sufficiently high vapor reactivity coefficient, which complicates the control over the course of a nuclear reaction. The reactor should be equipped with additional safety systems; only highly qualified personnel should work on it.
Broke firewood
On April 25, 1986, a shutdown of the fourth power unit was planned at the Chernobyl nuclear power plant for scheduled repairs and an experiment. Experts from the Hydroproject Research Institute proposed a method for emergency power supply to the station's pumps using the kinetic energy of a turbine generator rotating by inertia. This would allow, even in the event of a power outage, to maintain the circulation of the coolant in the circuit until the backup power is turned on.
According to the plan, the experiment was to begin when the thermal power of the reactor dropped to 700 megawatts. The power was reduced by 50 percent (1600 megawatts), and the process of shutting down the reactor was postponed for about nine hours at a request from Kiev. As soon as the decrease in power resumed, it suddenly dropped to almost zero due to erroneous actions of the nuclear power plant personnel and xenon poisoning of the reactor - the accumulation of the xenon-135 isotope, which reduces the reactivity. To deal with the sudden problem, the emergency neutron absorbing rods were removed from the RBMK, but the power did not rise above 200 megawatts. Despite the unstable operation of the reactor, the experiment began at 01:23:04.
ChNPP reactor diagram
The introduction of additional pumps increased the load on the run-out turbine generator, which reduced the volume of water entering the reactor core. Together with the high steam reactivity, this rapidly increased the power of the reactor. The attempt to introduce absorbing rods due to their poor design only made the situation worse. Just 43 seconds after the start of the experiment, the reactor collapsed as a result of one or two powerful explosions.
Ends in water
Eyewitnesses claim that the fourth power unit of the nuclear power plant was destroyed by two explosions: the second, the most powerful, happened a few seconds after the first. The emergency is believed to have arisen from a burst of pipes in the cooling system caused by the rapid evaporation of water. Water or steam reacted with the zirconium in the fuel cells, causing large amounts of hydrogen to form and explode.
Swedish scientists believe that two different mechanisms led to the explosions, one of which was nuclear. First, the high steam reactivity coefficient increased the volume of superheated steam inside the reactor. As a result, the reactor burst, and its 2000-ton top lid flew up several tens of meters. Since the fuel elements were attached to it, there was a primary leak of nuclear fuel.
The destroyed 4th power unit of the ChNPP
Secondly, the emergency lowering of the absorber rods led to the so-called “end effect”. On the Chernobyl RBMK-1000, the rods consisted of two parts - a neutron absorber and a graphite water displacer. When the rod is introduced into the reactor core, graphite replaces the neutron absorbing water in the lower part of the channels, which only enhances the vapor coefficient of reactivity. The number of thermal neutrons increases and the chain reaction becomes uncontrollable. A small nuclear explosion occurs. The streams of fission products, even before the destruction of the reactor, penetrated into the hall, and then - through the thin roof of the power unit - entered the atmosphere.
For the first time, experts started talking about the nuclear nature of the explosion back in 1986. Then scientists from the Khlopin Radium Institute analyzed the fractions of noble gases obtained at the Cherepovets factory, where liquid nitrogen and oxygen were produced. Cherepovets is located a thousand kilometers north of Chernobyl, and a radioactive cloud passed over the city on April 29. Soviet researchers found that the ratio of the activities of the 133Xe and 133mXe isotopes was 44.5 ± 5.5. These isotopes are short-lived fission products, indicating a weak nuclear explosion.
Swedish scientists calculated how much xenon was formed in the reactor before the explosion, during the explosion, and how the ratios of radioactive isotopes changed up to their fallout in Cherepovets. It turned out that the ratio of reactivities observed at the plant could arise in the event of a nuclear explosion with a capacity of 75 tons in TNT equivalent. According to the analysis of meteorological conditions for the period April 25 - May 5, 1986, xenon isotopes rose to a height of up to three kilometers, which prevented its mixing with the xenon that was formed in the reactor even before the accident.