The use of probabilistic analysis in the Sizewell 'B' design process

The Sizewell 'B' PWR is being designed against safety guidelines set by the Central Electricity Generating Board (CEGB). These guidelines include numerical targets for the frequencies of fault conditions. This chapter outlines how probabilistic analysis has been applied by the National Nuclear Corporation under contract from the CEGB, to assist the development of the Sizewell 'B' design from the time of the Pre-Construction Safety Report (April 1982) to the end of 1984, and how the methodology is being developed for the assessment of the final design. The chapter describes the CEGB guidelines and the methodology used to assess the design against them. The two major assessments which have been carried out are described and their results are summarized. The chapter discusses how the important areas of plant, with respect to the probabilistic analysis, are identified and any benefits obtained from modifications to the design or analysis assumptions in these areas assessed. The final section emphasises the relationship between the probabilistic and transient analyses. The intent to extend the fault tree analysis to produce input data for probabilistic safety assessments is also outlined. (author)