Assumptions: Accident at Unit 4 Chernobyl NPP occurred on April, 26, 1986 in 01.23 minutes 40 sec. (Moscow time) during one of a safety systems design tests implementation. This safety system provided usage of staying turbogenerators rotation mechanical energy (so-called stopway) for electric power generation in conditions of two emergency situations superposition. One of them - full loss of NPP electro supply, including MCP and pumps of reactor emergency cooling system (RECS); another - maximal designed accident (MDA) in the design the big diameter pipeline of a reactor circulating contour break has been considered as a reason of which. It was stipulated by the design, that at external power supplies switching-off the electric power, generated by turbogenerators (TG) due to stopway, is supplied for the pumps starting, included in RECS that would provide secured reactor cooling. The proposal on TG stopway usage has been initiated in 1976 by NIKIET – Main Designer of RBMK reactor. This concept has been recognized and included in civil-engineering designs of NPPs with such type reactors.
However Unit 4 of ChNPP, as well as other power units with RBMK, has been accepted in operation without this mode approbation, though such tests should be a component of preoperational tests of the basic design modes of unit. Except for Chernobyl, at NPPs with RBMK- 1000 reactors after their commissioning, design tests on TG stopway use were not carried out. Such tests have been carried out at the Unit 3 of Chernobyl NPP in 1982. They have shown, that requirements on electric current characteristics, generated due to TG stopway, during set time were not observed and completion of TG excitation regulation system was necessary.

It was stipulated by tests programs for 1982-1984 to connect to stopwaied TG one of each of two reactor circulation loops, and by programs for 1985 and April, 1986 - two MCP. Thus emergency situation modelling was stipulated at switched - off by manual RECS valves. It was planed to carry out test at Unit 4 in the afternoon on April, 25 1986 at reactor’s thermal capacity 700 mwt, after that it was planed to stop reactor for scheduled repair works implementation. It should be noted, that the tests program corresponded to requirements effective at that time. Thus, tests should be carried out in a lowered capacity mode for which increased relatively to nominal coolant consumption through a reactor, insignificant coolants underheating up to boiling-point on an input in a core and minimal steam-content are characteristic. These factors have direct impact on accident scale.

 

April, 25, 1986.

01.06. The beginning of Unit capacity decrease (operational reactivity margin was equal to 31 roads).
03.47. Thermal reactor capacity was reduced and stabilized at a level of 50 % (1600 mwt).
07.10. TRM is equal to 13,2 roads.
13.05. HG-7 is disabled from grid (first of two HG, being part of Unit)
14.00. RECS was cut off from coolant circuit. A delay in tests program performance on the requirement of Kievenergo dispatcher (RECS hasn’t been put into operation, the reactor continued to work on thermal capacity 1600 mwt).
15.20 - 23.10. Preparation of the Unit for tests is started.
Decrease in Unit capacity is continued. Thermal capacity of a reactor is reduced up to 720 mwt, uniform unloading of the Unit proceeds. Reactor control operation personnel change takes place.

April, 26.
00.28. At thermal reactor capacity about 500 mwt during transition from local capacity regulation system on an automatic capacity regulator of the basic range on unprovided by the program decrease in thermal capacity approximately up to 30 mw has been committed. Capacity rise is started.
00.39.32' - 00.43.35'. The personnel according to the tests regulations has blocked an emergency protection signal on two HG stopping
00.41 - 01.16. Cut off TG -8 from grid for vibrating characteristics removal on idling (the second HG, being part of Unit)
01.03. Thermal capacity of a reactor was increased up to 200 mwt and stabilized (it was decided to carry out tests at this capacity)
01.03. The seventh MCP was turned on operation in addition to six working MCP
01.07. Eighth MCP was turned on operation (last from ensuring circulation in a reactor).
01.09. Water discharge was sharply reduced up to 90 t/h on the right side and up to 180 t/h on left at the total discharge on a circuit 5600-5800 t/h. As a result temperature on MCP suck made 280 degrees on Celsius.
01.22.30'. System "Skala "recorded reactor parameters on a tape.
01.23.04'. The tests beginning. On TG-8 shutoff-regulating turbine valves were closed, stopway of four MCP began
01.23.10'. MDA button specially mounted for tests carrying out with the purpose of MDA signal imitation was pushed
01.23.40'. Reactor’s emergency protection button EP-5 is pushed; emergency protection roads have begun movement in a core.
01.23.43'. Emergency signals on the runaway period and on reactor capacity exceeding have appeared
01.23.46'. The first pair of "stopway" MCP is switched - off
01.23.46,5'. The second pair of "stopway" MCP is switched - off
01.23.47'. Sharp (on 40 %) decrease in MCP discharges not participating in stopway, and uncertain indication of MCP discharges participating in stopway, sharp increase in pressure and rise of a level in drum-separators; signaled "Malfunction in a measuring part " in both automatic regulators of the basic range (1AP, 2AP)
01.23.48'. Discharges recovery on MCP, not participating in stopway up to the values close to initial on stopwaied MCP of left side recovery of discharges on 15 % lower than initial; recovery of discharges on right side of stopwaied MCP on 10 % lower than initial for MCP-24 and "uncertainty" for MCP-23; the further increasing of pressure and a level in drums - separators; snapping into action high-speed reducing devices of steam discharge in the turbine condenser
01.23.49'. An emergency protection signal "Increase of pressure in reactor space (the technological channel break) "; the signal " there is no voltage=48V" (power supply from muffs servo-motors of a reactor’s control roads and protection system was removed; signals "Malfunction of an actuating part of automatic regulators 1AR, 2AR"
01.24. (From record in log book of the senior supervisor of a reactor control). " Strong blows, roads of a control system and protection have stopped, not having reached bottom ends. The key of muffs power supply is removed".
On eyewitnesses evidence, two powerful explosions with a partial destruction of reactor’s compartment and a turbines hall has occurred at this time, a fire has occurred at the Unit 4 ChNPP. On April, 26 1986 by 3 p.m. it has been reliably ascertained, that the reactor is destroyed, and huge amounts of radioactive substances are released in an atmosphere from its ruins.