Approach used for the Advanced Test Reactor probabilistic risk assessment of shutdown operation

Following completion of the full-power operation probabilistic risk assessment (PRA) for the Advanced Test Reactor (ATR), a shutdown PRA was initiated to provide coverage of operating configurations not previously analyzed. As has been typical, full-power operation was the primary focus of previous PRA activities; however, recent nuclear industry interest in non-full-power operation prompted the effort. Because of the many variables affecting fuel safety during shutdown, such as fewer automatic safety systems, reduced safety system availability, less emphasis on emergency procedures and operator training, open confinement, and the conduct of various maintenance, construction, or reconfiguration activities, it is difficult to grasp the issues that may be dominant contributors to risk without making a concerted effort. This paper describes the approach taken to perform the shutdown PRA; results of the analysis are not provided as they have not yet received complete review.