Chronology
On April 25, 1986 was scheduled stop of the 4-th unit of the Chernobyl nuclear power plant for the next scheduled preventive maintenance. During these stops are usually held a variety of routine procedures and test equipment. This time to one of them was to test the possibility of using the kinetic energy of the rotor turbine generator to provide power supply nutrients (PEN) and the main circulation pumps (MCP) in case of blackout auxiliary station. When such accidents occur off feed water pumps that feed water reactor (flow stopped within 0.5 seconds), and the main circulation pumps to circulate the coolant through an active zone (due to under flywheel mass decrease pump flow can last for 2 minutes). At the same time triggered reactor protection and stops flow of steam to the turbine. Despite the cessation of the steam turbine rotor has continued for some time to rotate by inertia, which allows, in principle, the generator for some time to give electric shocks, which can maintain the pumps and thus avoid an immediate shutdown. Such a regime would be significantly increased possibility of emergency cooling system (ECCS), making it the third channel, fed by water from PENov, operational in the blackout auxiliary unit. However, the time of testing on April 26 mode was not full-time, has not been perfected and never used. Similar experiments were conducted at the Chernobyl NPP in 1982, 1984 and 1985, ended in failure: prevailing characteristics of excitation system on the pumps did not provide the required time (to include diesel generators). The tests did not agree with the chief designer and supervisor, as the leadership of CNPP wrongly believed their electric.
Tests were conducted at power 700-1000 MW (thermal) April 25, 1986. Approximately one day before the accident (about 3-4 hours of April 25), the reactor power was reduced by about 50% (1600 MW), but further reduction in power was forbidden Manager Kyivenergo due to breakage at the South Ukrainian nuclear power plants. Continued reduction of the power unit was allowed to manager in 23 hours, so for a long time active zone was in the mode of poisoning by xenon. For approximately two hours the reactor power was reduced to the level required program (about 700 MW thermal), and then, for unknown reasons, up to 500 MW. Following this, staff made a mistake, which resulted in the reactor power began to decline rapidly uncontrollably. The power recorded by sensors intraband energy release has decreased to a value not exceeding 30 MW (in accordance with the characteristics of these sensors, for such values of the error of the readings exceeded the displayed value, and therefore determine the minimum level of difficulty), the power recorded by lateral ionization chambers - to zero. Staff has been on the MCR-4, has decided to restore the power of the reactor, and (removing the absorber rods of the reactor) in a few minutes has started its growth in the future - for stabilization at 160-200 MW (thermal). With the rapid decline of power and subsequent work at less than 200 MW intensified by poisoning of the reactor core (see iodine pit "), which led to the need to further remove control rods from the reactor core. The majority of rods were in the upper limit switch.
After reaching 200 MW thermal power included additional primary coolant pump, which, together with two additional pumps working PEN, were to serve the load for the generators during the experiment. Increase coolant flow through the reactor led to a reduction of vaporization. At the same time, consumption of relatively cold feedwater remained small, the corresponding power 200 MW, which caused the rise in coolant temperature at the entrance to the active zone and is close to boiling temperature of the onset of water. This has created an unfavorable environment, which could manifest itself particularly strongly positive void coefficient of reactivity.
At 1:23:04 the experiment began. Due to lower speed pumps connected to the "running out" generator, and a positive void coefficient of reactivity of the reactor experienced a trend towards a higher power (introduced positive reactivity), but for most of the time of the experiment management system successfully counteract this by immersing control rods into the core . Around 1:23:39 registered team to stop the reactor. It is known that has been pressed emergency protection AZ-5, but now it is a pressing matter of contention. It is suggested that depression was caused by an emergency situation - started dispersing and occurred during or after the destruction of the reactor, on the other allegations, it was stipulated in advance and carried out in a calm atmosphere. With INSAG, evaluating pressing AZ, does not make any rigorous conclusions about the time you press it, nor the purpose of pressing. It should be noted that the control system of the reactor is not intended for recording of fast processes, so by the recorded data is difficult to determine whether the dispersal of the reactor started to enable the operator of an emergency protection or after.
According formed team of emergency protection of reactor (no matter what its root cause) absorber rods began to move into the active zone, but because of their unsuccessful designs (see end effect) and low (no regular technical) operational reactivity margin reactor has not been muted: the reactor power after seconds to reduce the start to increase rapidly, tipped the scales for all instrumentation. Emergency dispersal accompanied by sound effects (recurrent attacks with increasing amplitude), powerful blows, turning out the lights (including emergency lighting). According to various testimonies came from one to several powerful blows (the majority of witnesses pointed to the two powerful explosions), and to 1:23:47-1:23:50 reactor was completely destroyed.
The exact sequence of processes that led to the bombings, there is no single representation. In the process of uncontrolled dispersal of the reactor, which was accompanied by increasing temperatures and pressures, were destroyed fuel elements (cartridges), and part of the technological channels (see RBMK), in which the fuel elements are. Steam from the damaged canals began to flow in the reactor space, which caused its partial destruction, separation and recovery ("flight") of the upper plate (scheme "Elena") of the reactor and further development of a catastrophic accident, including discharges of core materials.
On the root cause of the uncontrolled dispersal of the reactor voiced several opinions. It is stated that such could be the "end effect" off "running out" the main circulating pumps or other events. There were also suggestions that the explosion destroyed the reactor, has a chemical nature, ie hydrogen explosion, which was formed in the reactor at high temperature as a result parotsirkonievoy reaction and a number of other processes. According to another hypothesis, an explosion of pure nuclear nature, ie a thermal explosion of the reactor as a result of its crackdown on the prompt neutron induced complete dehydration of the core. Large positive void coefficient of reactivity makes this version of the accident is quite probable. Finally, there is a version that the explosion - only steam. According to this version all the destruction caused the flow of steam, emissions from the mine a large part of graphite and fuel. A pyrotechnic effects in the form of fireworks flying and burning incandescent fragments ", who observed the witnesses - the result" of parotsirkonievoy and other chemical exothermic reactions.