Tuesday 26 August, Working Groups
The meeting now divided into four Working Groups, Working Groups 1 and 2 convened in Session 2A: Groups 3 and 4 in Session 2B.
Session 2A, 10.00 to 18.00 hours: Detailed Presentations on Plant Design, Safety Analysis, Accident Description
Cause of the accident, sequence of events, radioactive releases, short term stabilization and longer term arrangements.
The presentations began with a historical review of the development of nuclear power in the USSR. This was straightforward, starting with the usual claims for the world's first atomic power station which produced 5MW of industrial power at Obninsk, near Moscow in 1954. Of more interest was an outline of the expansion envisaged for nuclear electricity generation during the next 5 year plan. It was intended to utilize nuclear heating, not only for the generation of electricity but also thermal energy by making use of the 'waste' steam. Combined electricity/thermal power plant and purely thermal output (for urban space heating) were planned. The development and implementation of fast breeder reactors was envisaged during this period with a suggestion that an 800 MW plant was being considered.
The presentations followed closely the text of the first two Annexes to the Russian report. Annexe 1 describes operating experience with the RMBK reactors. Annexe 2 the design of these reactors. Particular attention was paid to the void coefficient (which has a value of 2 x 10-4% steam). It is positive and so an increase in the volume of steam voids (bubbles) in a channel leads to an increase in the amount of heat being given off by the uranium fuel in that channel.
As a result even more steam will be generated, and the resultant voidage will further increase the rate of heat generation. Ultimately, if this circular process is not halted by the automatic reactor control and protection systems, the rate of steam production will be so great that it will damage the reactor. This was what happened in the Chernobyl accident. The void coefficient had maximum effect because there were only six control rods in the reactor and the automatic system had been turned off.
A "local automatic regulating system" is used to control the power output of the reactor as a whole. A decision is taken about the amount of power required. It can lie anywhere in the range from 10% to 100% of the maximum possible output. Then the automatic system maintains the power within one percent of the selected level. The system embodies twelve independent local regulators.
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