Recent progress in probabilistic safety assessment of nuclear power plants

From the reactor safety study in 1975 to the second draft of NUREG-1150 in 1989, the technology of probabilistic safety assessment (PSA) has advanced remarkably, and its application to actual plants has been carried out many times. Particularly in the last 10 years, great development was achieved in the techniques of assessment for source term, external events and fuzziness. In this report, the present status of the technical development in this field and the state of carrying out and utilizing PSA in Japan and foreign countries are explained. The PSA is the techniques of quantitatively evaluating the frequency of occurrence and the effect at the time of occurrence of all the accidents which can occur in atomic energy facilities, and indicating the degree of safety by the magnitude of risk, that is, the product of those. The procedure of PSA for nuclear power plants is explained. The progress of PSA technology is shown. In June, 1989, the second draft report of NUREG-1150 was published in USA, which is the summary of the state of PSA technology and the results of reevaluating the risk of severe accidents. (K.I.)