SUMMARY DESCRIPTION OF THE METHODS USED IN THE PROBABILISTIC RISK ASSESSMENTS FOR NUREG-1150

This paper summarizes the methods utilized by the staff of Sandia National Laboratories in performing four probabilistic risk assessments (PRAs) in support of NUREG-1150. The emphasis is on the "back-end" analyses, that is, the accident progression, source term, and consequence analyses. The accident progression and source term analyses performed for NUREG-1150 differ from previous analyses in several ways. Some of the features of the new analysis procedures were adopted to facilitate a comprehensive treatment of uncertainty in the complete risk analyses. The use of sampling techniques to provide quantitative estimates of the uncertainty in risk required that new approaches be developed for the accident progression and source term analyses. The uncertainties in the consequence analysis were not included in the overall uncertainty estimates in this project. The PRAs performed for NUREG-1150 also devoted a large effort to obtaining expert opinion to quantify parameters and phenomena for which there is large uncertainty and divergent opinions in the reactor safety community.