Degraded Core Analysis for the Pressurized-Water Reactor

An analysis of the likelihood and the consequences of `degraded-core accidents' has been undertaken for the proposed Sizewell B PWR. In such accidents, degradation of the core geometry occurs as a result of overheating. Radionuclides are released and may enter the environment, causing harmful effects. The analysis concludes that degraded-core accidents are highly improbable, the plant having been designed to reduce the frequency of such accidents to a value of order 10<sup>-6</sup> per year. The building containing the reactor would only fail in a small proportion of degraded-core accidents. In the great majority of cases the containment would remain intact and the release of radioactivity to the environment would be small. The individual risk of accidental death is about 3 x 10<sup>-4</sup> per year in the U.K. For an individual living very close to the reactor, degraded-core accidents would raise this risk of accidental death by only 1 part in 100000. They would lead to an even smaller increase in that individual's risk of death due to cancer some years later. In the extremely unlikely event of the worst degraded-core accident (frequency about 3 x 10<sup>-8</sup> per year), the calculated expectation is that, on average, 130 people would die soon afterwards and 3300 would die between 10 and 40 years later from cancer attributable to the accident. A more realistic estimate of the amount of radioactivity released reduces the figure to 7 accidental and 1300 cancer deaths. Based on this more realistic estimate of release there is 1 chance in 100 that, should the worst accident actually occur, about 100 early deaths could ensue giving a frequency of about 3 x 10<sup>-10</sup> per year for this occurrence (less than once in the lifetime of the Earth). Consequences as severe as those exemplified above are not typical of the predicted outcome of degraded-core accidents: indeed the balance of probabilities is that a degraded-core accident would lead to no immediate fatalities at all and to cancer deaths measured in single figures or tens. Although these estimates are approximate, studies to investigate the uncertainties, and sensitivities to different assumptions, show that potential errors are small compared with the very large `margin of safety' between the risks estimated for Sizewell B and those that already exist in society.