The use of PSA numerical results in the regulation of nuclear safety in the UK

The Nuclear Installations Inspectorate receives a variety of probabilistic safety assessments (PSAs) for review. Each appears as part of an overall safety case, most of which is made in terms of the quality of the engineering. The PSAs are assessed against the probabilistic criteria, which are described, together with some likely features of a revision which is currently underway. The generation of low probability values in a PSA is related to the criteria adopted. The significance and use of these low values is discussed and the assessment process is illustrated by some examples. Emphasis is placed on the importance of relating the numerical results to specific engineering features of the plant, and in forming a judgment, jointly with engineering colleagues, as to whether the indications from the PSA represent real weaknesses in the plant or whether they are artifacts of the PSA itself. The question of whether reasonably practicable improvements can be made to the plant is then taken up with the licensee. An indication is given as to how the problem of the risk from external events is handled and a suggestion is made for a possible approach to the probabilistic criteria for such events which might bypass the need to estimate their frequency