The essence of tests at 4 reactors in 1986
It was planned to stop the Chernobyl NPP Unit 4 for scheduled repairs on April 25, 1986. In connection with this, a decision was made to conduct a test during which it was possible to verify the ability of the plant equipment to provide the necessary electricity to operate the reactor core cooling system and protection system equipment from the moment the main power supply systems until the backup power from diesel units is turned on.
As a result of uncoordinated actions and an insufficient level of safety culture for plant employees, operators carry out a number of operations that did not comply with established safety instructions and created a potentially dangerous situation.
The situation is compounded by serious flaws in the design of the reactor, which makes the nuclear installation potentially unstable and can easily lead to accidents in the event of operator errors.
The combination of these factors causes a sharp increase in the energy field, which leads to almost complete destruction of the reactor. The consequences of this event are further complicated by the fire that engulfed the graphite masonry of the reactor and other materials, which starts in the building and generates the release of radioactive materials into the environment.
The accident at Unit 4 of the Chernobyl NPP occurred on April 26, 1986 at 01 h 23 min 40 s during the design tests of one of the safety systems included in the RBMK-1000 power unit. This safety system provided for the use of mechanical rotation energy of stopping turbogenerators to generate electricity under the conditions of two emergency situations. One of them is the complete loss of power supply to nuclear power plants, including coolant pumps and emergency reactor cooling system pumps; the other is the maximum design basis accident, which is considered a rupture of a pipeline of large diameter of the reactor circulation loop.
According to the test program, when external power is turned off, the electricity generated by turbogenerators due to run-out is supplied to start the pumps of the emergency reactor cooling system, which ensures guaranteed cooling of the reactor. The proposal to use the generator run-out came from the chief designer of RBMK and was included in the construction projects of nuclear power plants with reactors of this type. However, power unit No. 4 of the Chernobyl nuclear power plant, like other power units with RBMK, was put into operation without testing this mode, although such tests should be an integral part of pre-operational testing of the main design modes of the power unit. None but the Chernobyl nuclear power plant with RBMK-1000 reactors, after putting them into operation, design tests on the use of the generator run-out were carried out.
Similar tests at Unit 3 of the Chernobyl NPP, held in 1982, showed that the requirements for the characteristics of the electric current generated due to the run-out of the turbine were not maintained for a given time, and it was necessary to refine the system for regulating the excitation of the turbogenerator. Additional tests with the upgraded system were carried out in 1984 and 1985. In this case, emergency modeling was provided for when the emergency reactor cooling system was turned off by manual valves.
Tests at the 4th power unit were scheduled for April 25, 1986 during the daytime, with a reactor thermal power of 700 MW, until the reactor stopped to carry out scheduled repair work. It should be noted that the test program met the requirements in force at that time. They should have been carried out in a reduced power mode, which is characterized by an increased (compared to the nominal) flow rate of the coolant through the reactor, a slight heating of the coolant to the boiling point at the entrance to the core and a minimum vapor content. These factors had a direct impact on the scale of the accident.
In accordance with the planned program, it was necessary to turn off the emergency reactor core cooling system, which provided water for cooling nuclear fuel in emergency situations. As the reactor shutdown procedure continued, the latter worked at about half the capacity, and the power system dispatcher did not give permission to further reduce the reactor power. In accordance with the test program, about an hour after that, the emergency core cooling system of the reactor was turned off. While the reactor continued to operate at a 50 percent power level. Only at about 11 p.m. on April 25, the power system manager gave permission to further reduce the reactor power.
To conduct the test, the reactor had to stabilize at a thermal power of about 1000 MW, however, due to an error that arose during operation, the reactor power dropped to 30 MW. The operators tried to raise the power to 700 - 1000 MW by turning off the automatic regulators and manually releasing all the control rods. Only at about 1 a.m. on April 26, the reactor stabilized at about 200 MW. Although under the operating conditions of the nuclear reactor the technological regulations established the requirement that there should be at least 30 control rods in its core, in the course of the experiment only 6 to 8 control rods were involved. Most of the rods were removed from the core to compensate for the poisoning of the reactor. Despite this, a decision is made to continue the test program.
As a result of an increase in the coolant flow, the vapor pressure drops. An automatic system that shuts down the reactor at excessively low vapor pressure has been shut down. To maintain reactor power, operators are forced to remove almost all the remaining compensation rods, after which the reactor becomes extremely unstable and operators have to make adjustments every few seconds to maintain constant power. Around the same time, operators reduce the flow of coolant in order to maintain vapor pressure. At the same time, pumps powered from a stopped turbine begin to supply a smaller volume of coolant through the reactor. The loss of coolant exacerbated the unstable state of the reactor and increased the productivity of the steam in the cooling channels, and the operators could no longer prevent a surge in energy, which, according to estimates. exceeded the rated power of the reactor by 100 times. An unexpected increase in heat production destroys part of the nuclear fuel, and the smallest hot fuel particles react with water, which leads to a steam explosion that destroyed the reactor core, as well as to the destruction of the roof of the reactor building.
At 01 h 23 min 40 s, the reactor control operator presses the manual emergency stop button of the reactor (the reason for pressing the button has not been reliably established), and after three seconds the alarm signals appear along the reactor acceleration period, as well as when the power is exceeded. Within approximately three seconds, displacers of emergency rods of the reactor control and protection system at a design speed of rods of 0.4 m / s travel 1.2 m and completely displace the water columns located under them. The “displacer effect” is triggered, as a result of which, according to the calculations, positive reactivity is introduced and uncontrolled acceleration of the reactor in its lower part begins.
As a result of the explosion, hot radioactive particles and graphite are released into the atmosphere; the destroyed active part of the reactor remains unprotected. A radioactive cloud of smoke, radioactive fission products and particles of nuclear fuel rises into the air at a distance of 1 km. Heavier particles of the radioactive cloud settle on the territory in the immediate vicinity of the emergency reactor, and lighter components, including fission products and almost the entire set of noble gases resulting from the accident, are the prevailing winds in the direction north-west of the nuclear power plant.
A fire starts on the equipment and destroyed structures of the power unit, which causes clouds of steam and dust, and the fire also covers the roof of the turbine hall, reserves of diesel fuel and combustible materials. Approximately 100 members of the fire brigades located both on the territory of the nuclear power plant and those called from the city of Pripyat arrived to extinguish the fires that occurred, and it was these people who received the highest doses of radiation. The fires were extinguished by 5.00 the same day, but at the same time the burning of the graphite masonry of the reactor began. Intensive burning of graphite causes a dispersion of radioisotopes and fission products that have risen high into the atmosphere. The emission lasts about 20 days, but its intensity decreases significantly on the tenth day, when the combustion of graphite was finally stopped.