The German precursor study

Results of a first plant-specific precursor study are presented. Using plant-specific operating experience, precursors to potential severe core damage accidents were assessed. The main steps of the study are: assessment of the plant-specific frequencies of the initiating events; estimation of the probabilities of system failures using plant-specific unavailabilities of the system trains, evaluation of the operating experience (16 years) of Biblis A and B on actual or potential system failures and multiple failures, evaluation of operator errors based on the plant-specific operating experience, performance of a parameter uncertainty analysis. The quantification of the precursors to potential severe core damage accidents is based on the minimal requirements for the systems laid down in the licensing process for full load operation. Using a realistic evaluation there are often fewer system trains necessary than specified by these minimal requirements. A value of 5x10-5 per year of reactor operation was estimated to be the overall result of the mean frequency of potential severe core damage accidents corresponding to the observed precursors. The parameter uncertainty analysis yielded an upper subjective 95% confidence limit of 5x10-4 per year of reactor operation. Analysis of the time-dependent trend showed a clear decrease in frequency as a function of increasing operating time. (orig./HP)