The night of the 25 to 26 April 1986. The drag of dreams and anticipation of the long-awaited weekend surrounds the city of Pripyat. The last preparatory measures took place at the Chernobyl nuclear power plant before the complex of scheduled tests. It was planned to carry out a number of experiments on the night of the tragedy at the ChNPP. One of the most experienced engineers Vitaly Borets was responsible for preparing the schedule for all tests at the Chernobyl NPP. He had the experience of work at many nuclear power plants of the Soviet Union.
Borets accepted with pleasure the offer to prepare a schedule of experiments. He was perfectly familiar with the work of the Chernobyl nuclear power plant. The 4th unit was the newest and, as many believed, the safest at the plant. This power unit was erected as part of the second stage of construction. Unlike the first two power units, separate standing buildings, the third and fourth were built in duplex. A production capacity of up to 1000 MW of electricity characterized both power units. To achieve this figure, they had to produce at least 3000 MW of thermal power. The third block started functioning in December 1981, the fourth – in December 1983.
Nikolai Fomin signed the report of the commission that inspected and commissioned the 4th power unit. Although the commission assessed the functioning of all systems of the power unit as acceptable, it was not without nuances. The inspectors pointed out a number of problems with the reactor that needed to be solved in the future. In particular, they recommended modifying the design of the control rods. It was indicated that when the rods are lowered into the core to a depth of less than 2 m, they are capable of increasing reactivity. As a result, this leads to an increase in the reactor power.
The nuclear scientists of the Leningrad NPP had already faced with such a problem. In 1975, a significant increase in radioactivity and the destabilization of the reactor were recorded. A positive void effect caused an increase in radioactivity inside the reactor after the loss of the coolant. The details of the Leningrad incident were traditionally hidden from workers at other nuclear power plants in the USSR. Borets, being at that time at LNPP, was personally convinced of how dangerous this situation could be.
A failed experiment at the Leningrad NPP
Being an engineer at the Chernobyl NPP, Vitaly Borets underwent training at the Leningrad NPP to work with the RBMK reactor. He became a witness of a serious accident in the history of the station almost by accident. On that day, he decided to stay after his shift to observe the behavior of the reactor. It was put into the “gear shifting” mode. It is a process when the reactor is shut down and transferred to another operation.
Borets quickly guessed that certain problems had arisen during the test. While at low rates, the power in the reactor began to rise sharply. The operator tried to slow down the process by lowering additional control rods into the reactor core. The radiation level usually drops immediately after the operator adds control rods. After all, it is in the core where the nuclear reaction of the fuel channels takes place and nuclear energy is released.
However, RBMK behaved differently at the Leningrad nuclear power plant. When the experienced operator, who impressed Borets with professionalism, manually lowered the additional rods, the increase in power level did not slow down. Vitaly Borets realized that the RBMK was actually out of control. The reactor did not work as expected. Given this experience, he realized that if the personnel did not reduce the power, it would lead to an explosion of the reactor.
“Imagine yourself driving a car,” Borets explained the next day to a KGB officer the details of the failed experiment. You turn on the engine of the car, start moving, smoothly picking up speed. Your speed is 60 km / h. You take your foot off the gas pedal, but you realize that the car automatically continues to accelerate. The speed increases first to 80 km, then to 100, 130, 150 km / h. The brakes do not work, but the speed increases. How would you feel in such a situation?”
The uncontrolled reactor of the Leningrad NPP closed up twice using the AZ5 emergency protection system. The explosion did not happen. However, the fuel channel in the RBMK core melted due to sharp power drops. This led to a leak of uranium into the core. The reactor was closed and the next day “cleaned” with nitrogen. Radioactive waste got into the environment through the chimney. The consequences of radiation pollution were classified.
Expert recommendations to improve the functionality of control rods
One can only guess how many curies dropped out near Leningrad, located 50 km from the station. Vitaly Borets has never received an explanation of the cause of the accident. Moreover, he did not have access to information about this significant flaw in the design of the reactor. All the data were classified, but the designers did not make any significant changes in the design of the RBMK.
At the same time, the experts gave recommendations. It was necessary to improve the functionality of the control rods without explaining the reasons. Their instructions could not fail to find their reflection in the recommendations of the commission that inspected the 4th power unit of the Chernobyl NPP. In fact, few people understood that the absolutely critical question of rods. People have not learned the lessons of the Leningrad accident. There were many options for improving the operation of the reactor. However, the main task of the ChNPP personnel was to generate energy. The last thing they worried about was the development of new reactors or the modification of already invented ones. The repair would wait.
The KGB did not take their eyes off the Chernobyl NPP. In general, they had satisfaction with the state of the two new power units. The agents were tracking the progress of construction. They reported that the 3rd and 4th power units were safer than the 1st and 2nd. In 1984, the committee members summed up: despite serious security problems, the situation at the ChNPP is stable. The last couple of years became the point of comparison. In 1982, 16 equipment breakdowns and 3 accidents took place at three power units. And in the first nine months of 1984, there were only 10 breakdowns and not a single serious accident.
4th power unit on the eve of the accident
Neither Vitaly Borets, nor anyone else, preparing for the test, had any reason to recall the Leningrad incident. Borets did everything he was responsible for. The engineer collected information on all the proposed experiments, made the appropriate schedule. He handed it over to a group of specialists responsible for preparing for the shutdown of the 4th reactor. Borets suggested starting a full stop on Thursday, April 24, at 10 p.m. The entire set of tests, including modeling the reactor failure, had to be completed on April 25 at 1 p.m.
A responsible team made their adjustments and stated that the tests should run out by 10 a.m. This ensured that the level of radioactivity in the reactor would not drop to unacceptable levels. There were no objections. Nikolai Fomin, Chief Engineer, signed all the necessary documents. The station administration has not submitted the test program for approval by the Ministry of Energy and reactor manufacturers. Although the instructions assumed this, in practice, it happened extremely rarely.
The ChNPP was going to meet the last weekend in April with a successful shutdown of the 4th power unit. Preparations for shutdown of the 4th reactor began NOT in the evening of April 24, as Borets had suggested. In the early morning of April 25, a new shift of operators took over and launched it.
By 04:48 a.m., the reactor power level had halved and stabilized at 1600 MW. Igor Kazachkov, the head of the morning shift at the 4th unit, continued the shutdown procedure. He started work at 8:00 a.m. Kazachkov, who was 30 years old, was one of the most experienced shift supervisors at the ChNPP. He was highly regarded for his “exceptional thoroughness and work discipline.”
Kazachkov received a reactor in the state of the beginning of the experiment from the night shift. Almost all of the controllable rods were inserted into the reactor core to reduce the intensity of the nuclear reaction. Less than fifteen rods remained unused. They were at Kazachkov’s disposal to regulate the work. The manufacturer’s instructions noted the need to shut down the reactor with just this number of rods.
Experiment permission was late
However, neither the night shift personnel, nor Kazachkov began the reactor shutdown procedure. Such a decision would become an elementary violation of the experiment program, approved by the management. The test of the reactor turbine, like numerous other tests and measurements, had not run out yet. In addition to an emergency, only the permission of the management of the NPP could stop the reactor and turn it off. In turn, it depended on official confirmation from the power grid inspector in Kiev.
Kazachkov remarked later: “Why did none of us stop the reactor when the number of control rods decreased? Because we could not even imagine that this would lead to a nuclear accident. We knew that the manufacturer prohibited pulling. But we didn’t think about it. At the same time, if I stopped the reactor, I would definitely get scolded. In the end, we followed the plan.” When we asked what the consequences of the shutdown of the reactor could be, Kazachkov answered: “I think I would have been fired. Yes, they would have fired me for sure. Of course, they would have found a reason. None of us considered this particular parameter, the number of rods, significant.”
Thus, not particularly concerned about the insufficient number of unused rods, Kazachkov continued to carry out the adopted program. He was preparing for the test by disconnecting the emergency water supply system of the reactor. The reactor shutdown procedure had to last only a few hours. Kazachkov believed that the probability of failure of the emergency water supply system was equivalent to the probability of the plane falling on someone’s head.
Shutting down the emergency water supply is a long and difficult process. Operators manually tighten the valves of huge pipes. So, it took 40-45 minutes to complete, with teams of two to three people working on each valve. The staff turned off the system for about 14 hours. The shift had another 15-20 minutes before the start of the actual shutdown of the reactor. However, a call came from the station administration. They insisted that the reactor remain at its fixed power level of 1,600 MW. The shutdown and testing of the reactor, planned to be carried out at a power level of 700 MW, had to take place a bit later.
Interventions in the test by the Electricity Distribution Office
The management’s plans changed after a call from the Office of Electricity Distribution and Consumption in the Kiev region. In fact, the Office was the only customer for the services of the Chernobyl nuclear power plant. Unless an emergency took place at the station, the personnel had to follow the instructions of the power grid operator. As it turned out, one of the power units suddenly went out of order at the South Ukrainian NPP. As a result, the operator of the Office made a decision to maintain the power level of the 4th power unit of the ChNPP until late in the evening. The demand for electricity decreases at night. As a result, it will be possible to continue the process of shutting down the reactor.
None of the employees of the 4th power unit was happy with such a request. The call came in 15 minutes before the start of the test. And the personnel had already turned off the emergency water supply system. But the workers of the ChNPP had no choice but to simply follow the instructions of the Kiev operator. The electricity produced by only one power unit of the Chernobyl NPP was enough to ensure the functioning of the whole Kiev. Therefore, the station could not independently turn off the reactor, even for the purpose of routine tests. The NPP personnel had already complained about similar incidents before, but to no avail.
The KGB reports noted the repeated dissatisfaction of the personnel of the ChNPP with the Kiev operators of the power grid. In 1985, 26 cases happened when the operators of the ChNPP had to change the capacity of power units. In the first three weeks of 1986, there were already nine such cases. This led to a general drop in power output.
Reactor operators complained about the RBMK-1000 design to operate at a stable output power level. Such fluctuations can cause system failure. Moreover, changes in power output result in the release of radioactive material into the atmosphere. However, Moscow’s response showed that the KGB could not influence the state of affairs. The ministries in Moscow and those responsible for the nuclear power industry, simply took note of this problem.
The night shift was waiting for Dyatlov
The control panel of the 4th power unit took over the evening shift at 4 p.m. Its boss, Yuri Tregub, was not familiar with the test program. The process of shutting down the reactor should have run out before the start of his duty. Therefore, Tregub had no satisfaction with the demand of the Kiev power grid dispatcher, considering it unjustified.
Nevertheless, Tregub and his engineers had once again come to terms with the situation. By the time he took over the shift, the reactor’s power level had halved. Yuri was warned that the power system dispatcher had to give permission to shut down the reactor only at about 6 pm. Accordingly, the experiment with a steam turbine, which required shutting down the emergency water supply system, was postponed, but not canceled.
The night shift went on nervously. The operators monitored the state of the reactor every second. At 6 p.m. no news from the dispatcher from Kiev arrived. At about 8 p.m. Tregub contacted the head of the evening shift of the nuclear power plant. The head advised Tregub not to start the reactor shutdown procedure until the arrival of Anatoly Dyatlov, the deputy chief engineer. It was Dyatlov who was responsible for the operation of all the reactors of the ChNPP. When the personnel solved the question of stopping them, he acted as the highest authority for the staff.
Everyone felt a barely concealed nervous tension. The shift was part of the night watch. Late in the evening, the Chernobyl NPP dispatchers received the “long-awaited” permission from Kiev to shut down the reactor. The procedure for artificial simulation of an emergency could start at 10:00 p.m. Tregub immediately called Dyatlov. His wife replied that Anatoly was on the way. The shift was ready to start a complex of tests. In theory, this should have taken about two hours. Tregub hoped to complete the experiment before midnight. However, Dyatlov was still not at work.
Anatoly Dyatlov, the deputy chief engineer for the operation of the Chernobyl NPP, appeared at the control panel only after midnight. Before his arrival, no one really knew whether the experiment would take place or not. Where was he, what questions did he decide, from whom did he receive the task to shut down the reactor at night? Why, having received permission to test, did he delay the start of the experiment for several hours? These questions remained unanswered….