On April 25, 1986, reactor No. 4 was scheduled to be closed for regular maintenance. It was decided to test the opportunity of the reactor's turbine generator capability on this occasion and generate enough electricity to supply the reactor's safety system power (especially the water pump) in case of power loss. Like Chernobyl, the reactor has a pair of diesel generators that can be used as standby but cannot start instantaneously - the reactor will therefore be used to rotate the turbine, at which point the turbine will separate from the reactor and rotate under its own inertia. The goal of the test is to determine whether the turbine can adequately supply pumping power during the deactivation phase when the generator is started. The test was performed successfully in other units previously (all safety supplies started) and the result was failed (that is, the turbine generated insufficient power to supply pump power in the reduction phase), but additional improvements prompted the need for other tests. In order to perform testing at a safer and lower power, the energy output of the Chernobyl reactor 4 was reduced from 3.2 gigawatts of normal power to 700 megawatts. However, due to delays in the start of the experiment, the reactor controller reduced the energy level too quickly and the actual power output fell to only 30 megawatts. As a result, the fission product 氙-135 attracted by neutrons is increased (this product is typically consumed in a reactor at a higher power). Although the scale of the power drop is close to the maximum limit allowed by the safety regulations, the managers of the staff group chose not to shut down the reactor and continue the experiment. Later, the experiment decided to "cut shortcuts" and only increase power output to 200 megawatts. In order to overcome the neutron absorption of the remaining helium-135, far more control rods than the number of safety regulations were pulled out of the reactor. At 1:05 am on April 26th, as part of the experiment, the pump driven by the turbine generator was started; the flow of the water exceeded this by the safety regulations as a result of this action. The water flow increased at 1:19 AM - because water also absorbs neutrons, further increases in water flow require the manual removal of control rods, resulting in a very unstable and dangerous operating condition. At 1:23:04 in the morning, the experiment began. The unstable state of the reactor did not show any condition on the control board, and it appears that all reactor employees are not fully aware of the danger. The power of the water pump is turned off and driven by the inertia of the turbine generator, and the speed of the water flow is reduced. The turbine is separated from the reactor and the reactor core vapor level increases. Because the coolant is heated, individual steam is formed in the coolant duct. The special design of the RBMK Graphite Relaxation Reactor in Chernobyl has a high frontal void coefficient, meaning that the effect of neutron absorption in the absence of water increases the reactor's power rapidly, and in this case, the reactor Operations become increasingly unstable and more dangerous. At 1:23:40 am the operator pressed the AZ-5 ("Rapid Emergency Defense 5") button that ordered the "Emergency Shutdown" - full insertion of all control rods, including control rods that were accidentally removed before . Whether this is used as an emergency measure, or simply as a regular method of shutting down the reactor when the experiment is completed, is not clear (the reactor is scheduled to be shut down as a regular maintenance). This usually means that an emergency shutdown order is a reaction because of an unexpected rapid power increase. On the other hand, the chief engineer Anatoly Dyatlov, who was at the Chernobyl nuclear power station during the accident, wrote in his book: “Before 1:23:40, centralized control system... No registration can justify Any parametric change in an emergency shutdown.According to the statement of appointment... Gathering and analyzing a lot of materials, in its report, did not determine why the reason was ordered for an emergency shutdown. There was no need to look for the reason. The reactor was simply shut down at the completion of the experiment. "Because of the control rod insertion mechanism (slow completion of 18 to 20 seconds), the hollow portion of the rod and the temporary displacement of the coolant, the escape rate leads to an increase in the reaction rate. The increased energy product causes deformation of the control rod duct. The stick was stuck after it was inserted. It could only enter one-third of the pipe and it could not stop the reaction. At 1:23:47, reactor output soared to about 30 gigawatts, which is ten times the normal operating product. The fuel rods began to melt and the vapor pressure increased rapidly, causing a large steam explosion, displacing and damaging the top of the reactor, exploding coolant tubes and blasting a hole in the roof. In order to reduce costs, and its volume is too large, the reactor is built in a single protective layer. This allowed radioactive contaminants to enter the atmosphere after a steam explosion in the main pressure vessel broke. After a portion of the roof was blown up, oxygen inflow - combined with extremely high temperature reactor fuel and graphite moderator - caused a graphite fire. This fire causes radioactive material to spread and contaminate more extensive areas. Due to the inconsistency between eyewitness reports and on-site records, there are some arguments that the actual incident occurred at 1:22:30 local time. The last agreed version is described above. According to this theory, the first explosion occurred at approximately 1:23:47, and the operator ordered an "emergency shutdown" after seven seconds.
The subsequent processing of the explosion in this paragraph did not cause the attention of the Soviet Union. The nuclear experts in Moscow and the Soviet leaders received the message that "a fire broke out in the reactor but it did not explode." Therefore, the Soviet official response was slow. In the 48 hours after the accident, some villages close to the nuclear power plant began to evacuate, and the government sent troops to force people to evacuate. At that time, a number of reactors with a deadly amount of explosion and a hundred times more nuclear radiation were covered in the nearby villages. The radiation value was still rising. However, this did not pay attention. Experts would rather believe that it is the failure of a measuring radiation machine to believe that there will be such high radiation. However, the residents have not been informed of all the incidents. This is because official fears will cause people to panic. Many people have absorbed a deadly amount of radiation before evacuation (if the evacuation can be immediate, the number and level of victims can be greatly reduced). Three days after the accident, an investigation team dispatched by Moscow arrived at the scene, but they were unable to submit their reports and the Soviet government did not know about the incident. Finally, almost a week after the incident, Moscow received a message from the Swedish government. At this point the radiation cloud has drifted to Sweden. The Soviet Union finally understood that things were far more serious than they thought. After a few months, the Soviet government sent countless manpower and resources to finally extinguish the reactor fire and also controlled the radiation. However, these personnel responsible for cleaning are also subject to severe radiation damage; one of the reasons is the technical limitations of remote-controlled robots, and the fact that severe radiation causes the electronic loop of remote-controlled robots to fail. Therefore, the cleanup of many of the highest-pollution sites still depends on manpower. The Soviet government designated the 30-kilometer radius around the explosive reactor as a separation zone, withdrew all residents and surrounded it with barbed wire. There was a checkpoint at the entrance, and there were only regular shift supervisors and Chernobyl nuclear power stations. The nuclear reactors are still generating power in the Pripyat staff of the abandoned village near Chernobyl. In particular, with the explosion of the No. 4 reactor in the same main building, the two reactors share the No. 3 reactor that emits radioactive waste gas and high stacks, and have worked normally for 19 years. After the 20th anniversary of the accident, the external surface of the No. 4 reactor has a 750 milli-lux exposure, which is much higher than the safe value of 20 milli-luxels. The welding workers who reinforce the stone-coffins must rotate for two hours. The average degree of illumination in the area of ​​separation is still greater than 100 millirons. Outside the area of ​​separation is a heavier polluted evacuation area, with an average exposure of around 60 milli roentgens and up to 150-200 millirons in individual locations. Further out is a lightly polluted quasi-evacuation zone with an average exposure of 30 millirons.
The subsequent processing of the explosion in this paragraph did not cause the attention of the Soviet Union. The nuclear experts in Moscow and the Soviet leaders received the message that "a fire broke out in the reactor but it did not explode." Therefore, the Soviet official response was slow. In the 48 hours after the accident, some villages close to the nuclear power plant began to evacuate, and the government sent troops to force people to evacuate. At that time, a number of reactors with a deadly amount of explosion and a hundred times more nuclear radiation were covered in the nearby villages. The radiation value was still rising. However, this did not pay attention. Experts would rather believe that it is the failure of a measuring radiation machine to believe that there will be such high radiation. However, the residents have not been informed of all the incidents. This is because official fears will cause people to panic. Many people have absorbed a deadly amount of radiation before evacuation (if the evacuation can be immediate, the number and level of victims can be greatly reduced). Three days after the accident, an investigation team dispatched by Moscow arrived at the scene, but they were unable to submit their reports and the Soviet government did not know about the incident. Finally, almost a week after the incident, Moscow received a message from the Swedish government. At this point the radiation cloud has drifted to Sweden. The Soviet Union finally understood that things were far more serious than they thought. After a few months, the Soviet government sent countless manpower and resources to finally extinguish the reactor fire and also controlled the radiation. However, these personnel responsible for cleaning are also subject to severe radiation damage; one of the reasons is the technical limitations of remote-controlled robots, and the fact that severe radiation causes the electronic loop of remote-controlled robots to fail. Therefore, the cleanup of many of the highest-pollution sites still depends on manpower. The Soviet government designated the 30-kilometer radius around the explosive reactor as a separation zone, withdrew all residents and surrounded it with barbed wire. There was a checkpoint at the entrance, and there were only regular shift supervisors and Chernobyl nuclear power stations. The nuclear reactors are still generating power in the Pripyat staff of the abandoned village near Chernobyl. In particular, with the explosion of the No. 4 reactor in the same main building, the two reactors share the No. 3 reactor that emits radioactive waste gas and high stacks, and have worked normally for 19 years. After the 20th anniversary of the accident, the external surface of the No. 4 reactor has a 750 milli-lux exposure, which is much higher than the safe value of 20 milli-luxels. The welding workers who reinforce the stone-coffins must rotate for two hours. The average degree of illumination in the area of ​​separation is still greater than 100 millirons. Outside the area of ​​separation is a heavier polluted evacuation area, with an average exposure of around 60 milli roentgens and up to 150-200 millirons in individual locations. Further out is a lightly polluted quasi-evacuation zone with an average exposure of 30 millirons.
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