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Version of accident

Assumptions of accident.

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 turbo generators rotation mechanical energy (so-called stop way) for electric power generation in conditions of two emergencies 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 turbo generators (TG) due to stop way, is supplied for the pumps starting, included in RECS that would provide secured reactor cooling. The proposal on TG stop way 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 stop way 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 stop way, 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.

Version of accident

During last decade, numerous attempts to understand essence of Chernobyl accident and the reasons of its occurrence have been made. The completed and experimentally confirmed version of Chernobyl accident not exists till now.

Versions of accident occurrence and development

Objective of the events investigation connected to the accident at Unit 4 ChNPP occurrence and development began on April, 27-28 1986, when the information on key parameters of unit 4 operation before the accident and in its first phase registered by measurement systems till the moment of their destruction became accessible to experts.

Version of the Interdepartmental Commission

The version developed at place of accident was, that accident has occurred as a result of technological channels of a core scalding because of circulation failure in FCC circuit. Circulation failure occurred due to discrepancies of the feed water discharge and the coolant rate in FCC circuit. The subsequent in-depth analysis of MCP heat-hydraulic operating mode, carried out at the end of May, 1986 by MCP developer, has not confirmed the assumption on MCP failure and cavitation. It has been ascertained, that the least resource before cavitation took place 40 seconds prior to accident, but it was higher than at that at which MCP failure could occur.

Version of USSR Energy Ministry on the basis of AUSRINPP calculations.

At the end of May, 1986 after the available data studying and calculations in All-Union scientific research institute of nuclear power plants (AUSRINPP) the group of experts from USSR Energy Ministry has developed addendums to the act in which the reasons of accident have been identified:
- In principle imperfect design of PCS roads
- positive steam and fast power reactivity factor
- high discharge of the coolant at the low discharge of a feed water
- violation of procedural value of operational reactivity resource (ORR) by the personnel, low level of capacity
- insufficiency of protection and operational information for the personnel
- Absence of instructions in design and technological regulation on established level ORR violation hazard.

Version of Interdepartmental STC

At the meetings of Interdepartmental scientific and technical council (STC), held 02.06.86 and 17.06.86, it was not paid proper attention to AUSRINPP calculations results, shown, that defects in reactor design mainly were a reason of accident. In essence, all reasons of accident have been reduced purely to mistakes in the personnel actions.

Version of USSR experts for IAEA session.

In July, 1986 in process of preparation for special IAEA session the first design analysis of accident on the simplified mockup has been performed. In the report presented by the Soviet experts at this session in August, 1986, the prime cause of accident had been named "the extremely improbable combination of operation order and the mode violations by the unit’s personnel". It was also noted, that "accident has got the disastrous scale in connection with that the reactor has been put by the personnel in such unregulated condition in which influence of positive reactivity factor on capacity increase has significantly reinforced.

The following committed violations have been specified in the same report:
- decrease in an operational reactivity resource was significantly lower than allowable value;
- connection to a reactor of all MCP with discharge excess on some MCP, established by the regulations;
- reactor protection blockage on a signal of two TG stoppage;
- reactor protection blockage on a water level and steam pressure in a drum - separator;
- reactor protection system cutoff from MDA (maximal design accident) (RECS cutoff)

Version of Kurchatov Institute

By October, 1986 the analysis of the versions explaining explosive character of accident has been carried out in NEI:
1. Hydrogen explosion in suppression-pool
2. Hydrogen explosion in the bottom tank of a PCS cooling circuit
3. Diversion (shot explosion with destruction of pipelines of a circulation circuit)
4. Break of MCP pressure collector or a dispensing group collector
5. Break of a steam - separator or water - steam communications
6. Effect of positive reactivity stopway from PCS roads displacers
7. Malfunction of an automatic regulator
8. A gross error of the operator at manual regulation roads control
9. MCP cavitation, resulting in a steam-and-water mix supply in technological channels
10. Cavitation on throttle -regulating valves
11. vapor holdup from a steam - separator in exhaust turbodrives
12. Zirconium-steam reaction and hydrogen explosion in a core
13. Ingress of a compressed gas from RECS cylinders in a reactor

The analysis has been developed on contradictions revealing between expected effect of the considered version of accident and the available objective data fixed by special program. As a result of the carried out investigations it became evident, that the unique hypothesis, not contradicting to the objective data is the version connected to PCS roads displacers effect

Version of the First International Working Group on heavy accidents and their consequences.

In October - November, 1989 various aspects of Chernobyl accident have been discussed in details at the First Meeting of International Working Group on heavy accidents and their consequences (Dagomys, USSR). The reason of accident had been unanimously recognized ' "the reactor instability caused by defects of a reactor design, and mode of its operation". Accident has achieved disastrous scales because of positive steam reactivity effect and defects of an absorbing roads design. Actions of the personnel before accident were those, which promoted this reactor defects event. Having violated some procedural restrictions (on ORR value and coolant discharge), the personnel has practically put a reactor in "white spot" area where the reactor behavior has not been investigated and turned out nuclear-unstable.

Александр Сирота

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