Comments, Recommendations and ConclusionsThis Section summarises the background to the Chernobyl accident, the accident itself, its consequences and implications, steps taken in the USSR following the event, and the recommendations of the International Atomic Energy Agency. Finally, it gives the Watt Committee Working Group's recommendations.
9.1BACKGROUND TO THE CHERNOBYL ACCIDENTThe first nuclear power reactor in the USSR was a graphite moderated pressure tube design cooled by pressurised water. It started generating electricity in 1954. The RBMK system was developed from this. It has the basic advantages of large scale manufacture without the need for advanced fabrication facilities and, without major development, expansion to large outputs. Currently 1500 MW(e) RBMK reactors are in operation and under construction using a reactor the same size as the Chernobyl reactors which have outputs of 1000 MW(e). Limitations in the Soviet manufacturing capacity for specialist equipment such as electronics and control, and the lack of a comprehensive quality assurance system, linked to the pressure to design and build plant that could be manufactured, constructed and commissioned quickly, led to a design which, in both concept and detail, has features which impaired ultimate plant safety. Among the features of the design which appear to be questionable, some of which contributed to the initiation and development of the accident at the Chernobyl 4 reactor, the following merit noting:
|(1) The reactor was considerably over-moderated to allow the use of low enrichment levels for the fuel elements. The water in the fuel channels therefore acts predominantly as an absorber and displacement by steam from evaporation can lead to increases in reactivity (i.e. positive void coefficient). Under normal conditions, neutron capture in absorbers in the core and in the fuel, and the Doppler effect, more than compensate for this. However, the Doppler effect depends on the fuel temperature and at low operating loads, where fuel temperatures are also low, this effect is small and the positive void coefficient predominates. When combined with the withdrawal of absorber rods from the core, this can lead to an unstable condition where an increase in steam production leads to an increase in heat generation, increasing steam production further and escalating heat output, i.e. a rapidly acting positive power coefficient. |
|(2) The rate of movement of the control rods, even when tripped to shut down the reactor quickly, was slow-0·4 m/s with a travel of 6·25 m. |
|(3) There were no 'stops' to prevent the operators withdrawing control rods to their full extent, but there were station rules which were intended to prevent this. |
|(4) The details of control rod design with an absorber and graphite 'follower' can, under some conditions, lead to an increase in reactivity as the rods enter the core. This is known as 'positive scram'. |
|(5) Graphite core cooling was by conduction across tiles in direct contact with the pressure tubes which contain pressurised boiling water. |
|(6) The automatic reactor trips could apparently be easily deactivated by the operators. |
|(7) Instrumentation and alarms to indicate unsafe operation, or operation outside laid down limits, were either inadequate or ignored. |
|(8) Start-up of a standby diesel generator was slow |
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Book title: The Chernobyl Accident and Its Implications for the United Kingdom.
Contributors: Norman Worley - Editor, Jeffery Lewins - Editor.
Publisher: Elsevier Applied Science.
Place of publication: London.
Publication year: 1988.
Page number: 89.
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