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NUCLEAR REACTOR ACCIDENTS
The proposed Daya Bay reactors are Pressurised Water Reactors (PWR) and there has never been an accident to a PWR that has resulted in a significant radiological hazard to the public. The accident at Three Mile Island will be described later. The lessons of that accident and of the minor faults that have inevitably occurred in other PWRs, as in any large complex equipment, have been incorporated in current designs.
This chapter describes some general features of potential PWR accidents and their possible consequences, together with a brief description of the Windscale, Three Mile Island and Chernobyl accidents.
4.1 Cooling and Containment
In normal circumstances the radioactive fission products are contained within the ceramic fuel pellets which are securely held in the fuel cladding. In order for some of the fission products to be released the fuel pins must either be subject to major mechanical damage, or they must be allowed to greatly overheat which can lead to pin failures. Overheating could occur either because of a sudden major power increase as happened at Chernobyl or due to a loss of cooling as happened at Three Mile Island. As will be discussed later, a Chernobyl-type accident cannot happen in a PWR.
Failure of one or more fuel pins would not of itself lead to any release of radioactivity into the environment. Such a failure must also be accompanied by failure of the other barriers of the containment system, viz, the primary circuit and the containment building. To some degree containment and cooling are closely linked in a PWR since a major failure of the primary circuit, which provides one of the containment barriers, may also lead to loss of coolant. A minor breach of the primary circuit would not in general lead to a release of radioactivity. The loss of coolant would be detected, the reactor would be shut down, and coolant make-up pumps would compensate for the coolant being lost via the primary circuit leak. In order to postulate a major loss-of-coolant accident it is necessary to assume that there has been a large breach in the primary circuit such that the losses from the circuit are greater than those which can be compensated for by the coolant make-up pumps. In this situation the reactor will, of course, shutdown but there will remain the residual heat from radioactive decay of the fission products which will still require cooling to be maintained for some time. A number of independent emergency cooling systems may be brought into operation, but for the purpose of this description of a reactor accident it will be assumed, unrealistically, that all these cooling systems fail.
4.2 Release of Radioactivity into the Atmosphere
If all the cooling systems should fail, then the fuel pins would increase in temperature and hence the pressure of the fission product gases inside the pins would rise. This internal pressure together with a weakening of the cladding due to the high temperatures may lead to the failure of some of the fuel pins. If this occurs some of the gaseous and volatile fission products will escape into the primary circuit and hence, via the primary circuit breach, into the containment building, where the gases and volatiles would generally be contained. Cooling sprays in the roof of the containment building would provide a deluge of water which would condense the steam and dissolve some of the fission products.
In this major accident scenario there are a number of ways by which the pressure inside the containment could be increased still further. Steam could react with the hot zirconium of the fuel cladding which would generate hydrogen. There are safety systems to take care of this hydrogen, however if these fail the hydrogen could, in some circumstances, burn rapidly leading to a temporary increase in the containment pressure. PWR containment buildings are massive pre-stressed, reinforced concrete structures, which are designed to ensure their integrity in all credible circumstances. The containment is leak tight, and this is tested periodically. Current French designs of
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