QUANTIFICATION OF THE PROBABILITY OF CONTAINMENT FAILURE CAUSED BY ANIN-VESSEL STEAM EXPLOSION FOR THE SIZEWELL-B PWR

AEA Technology has provided an assessment of the probability of alpha- mode containment failure for the Sizewell B PWR. After a preliminary r eview of the methodologies available it was decided to use the probabi listic approach described in the paper, based on an extension of the m ethodology developed by Theofanous et al. (Nucl. Sci. Eng. 97 (1987) 2 59-325). The input to the assessment is 12 probability distributions; the bases for the quantification of these distributions are discussed. The a-mode assessment performed for the Sizewell B PWR has demonstrat ed the practicality of the event-tree method with input data represent ed by probability distributions. The assessment itself has drawn atten tion to a number of topics, which may be plant and sequence dependent, and has indicated the importance of melt relocation scenarios. The al pha-mode failure probability following an accident that leads to core melt relocation to the lower head for the Sizewell B PWR has been asse ssed as a few parts in 10 000, on the basis of current information. Th is assessment has been the first to consider elevated pressures (6 MPa and 15 MPa) besides atmospheric pressure, but the results suggest onl y a modest sensitivity to system pressure.