5
CONTINGENCY PLANNING
The Daya Bay reactors cannot explode like atomic bombs. There is however a very small risk of an accident at one of the reactors which could result in the release of radioactive fission products into the environment. The reactors incorporate a series of safety barriers and safety devices to provide a defence in depth against such a release of radioactivity. The greater the potential hazard the more stringent are the safety precautions. The risk to the general public is thus made extremely small. But although the risk is small, it is not zero and it is clearly sensible to be prepared to take any action that may be necessary by having a contingency plan.
5.1 Scale of Accident
Any emergency planner involved with making contingency arrangements for natural disasters, aeroplane or train crashes, fires, etc., must decide upon the magnitude of the potential emergency that the plan will need to cater for. Having made this decision flexibility will be built into the plan so that if necessary it may be extended by, perhaps, involving external resources which may take a little time to arrive. The same is true for contingency arrangements for an accident at a nuclear power station.
Reactor designers consider possible accident sequences which may be caused by the failure or partial failure of the safety systems and barriers. These are often referred to as the design-based, or design-basis, accidents. The
design-basis accident which leads to the largest off-site consequences is termed the 'reference accident'. In the UK contingency plans are designed to cope with the consequences of the reference accident and are capable of extension to deal with even more improbable beyond design-basis accidents (ref 7).
The radiological consequences of a major release of radioactivity can be substantially reduced by the introduction of a number of often, in principle, quite simple countermeasures, for example banning the distribution and sale of contaminated foodstuffs. It is prudent therefore to have a contingency plan which will give details on when and how countermeasures should be introduced.
It is usual for contingency plans to be drawn up, rehearsed and if necessary executed or coordinated, at least initially, by the operators of the nuclear facility. The Government of Hong Kong, in making contingency arrangements for a potential reactor accident at Daya Bay in the People's Republic of China, is in a different situation to a nuclear facility operator. An operator must make arrangements for dealing with the more serious consequences which might occur near to the facility, and also the lesser consequences which may occur at greater distances from the reactor site. The Government of Hong Kong clearly needs to consider only the consequences which could arise 25 km or more from the reactor.
The distances out to which plans should be made, the so called emergency planning zones, vary a little from country to country, and to some extent from reactor to reactor, (ref 8) See Table 5.1. This reflects, amongst other things, plant locations and plant designs. At 25 km from the reactor there is a consensus of international opinion that there should only exist a need for monitoring and perhaps control of foodstuffs. Plans for evacuation, sheltering and issue of stable iodine are not generally considered necessary at this distance. It is true that following the accident at Chernobyl people were evacuated out to a distance of 30 km, however this RBMK reactor accident represents an extreme accident of a type which could not occur in a PWR. Notwithstanding this, mention will be made in this report to some of the more serious countermeasures. This is for completeness only. The probability of, for example, sheltering being required in the north-eastern sector of Hong Kong due to an accident at the Daya Bay reactor is extremely small.
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