Chernobyl Nuclear Power Plant - Chronology of accident

Radiation background in the Administrative building area is in average 65 microrentgens per hour, in the Shelter object's Observation pavilion area - 1,08 myllyrentgens per hour.
Controllable parameters of the Shelter object are within the limits established by Operational Regulations. According to monitoring systems data the maximal value of gamma - radiation rate inside the object has made 3403 Rontgens per hour, temperature near to Fuel Containing Materials is +29 degrees on Celsius. Exceeding of control levels of Shelter object condition parameters is not fixed.

	Home
	News
	Photos
	Download
	Links
	Contacts

Chronology of accident

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.