A CASE STUDY IN FAULT TREE APPLICATION FOR TARGET SET SELECTION

This paper reviews the evolution of Regulatory Guide (RG) 5.81 (Reference 3) and the fundamental shift from focusing on Safety-Related Equipment and Vital Equipment as the starting point for identifying target elements to the direct use of the site-specific Probabilistic Risk Assessment (PRA) and presents a case study in fault tree application for target set selection based on firsthand experience. While Revision 1 of RG 5.81 (Reference 3) now instructs the user to start with the PRA, there is no direct guidance on how to accomplish this. Simply starting with the cutsets of the PRA does not provide a means for successful desirability screening. PRA cutsets are based on reliability and consequence, where an adversary attack is based on accessibility and consequence. Directly using the PRA model will lead to missed equipment and incorrect desirability conclusions. In 2016, Westinghouse created the first target set methodology, coined Fault Tree Target Set Analysis (FTTSA), to use the Boolean Logic Model of the site’s PRA and base the quantification on desirability instead of reliability. The FTTSA method completely removes reliability data at the basic event level and instead applies location weighting factors based on equipment locations; where the equipment locations are based both the physical component locations and other potential component impact locations using cable routing from the Fire PRA and flooding propagation paths from the Internal Flood PRA. The resulting cutsets reflect combinations of equipment and rooms that can lead to core damage and are prioritized based on desirability (product of location weighting factors) and not reliability. Using the site’s PRA in this way was a paradigm shift in the industry. Westinghouse has successfully implemented the FTTSA at two facilities, Barakah Nuclear Power Plant and Vogtle Plant Units 3 and 4; the application of the PRA at these two sites for target set selection will serve as the focus for this paper. The results from these two facilities demonstrate that the FTTSA method provides robust documentation which is auditable, searchable, repeatable and meets the guidance presented in Revision 1 of Regulatory 5.81 (Reference 3). As new concepts such as Reasonable Assurance of Protection Time (RAPT) and Security Bounding Time (SBT) are introduced in the industry, the target set selection process using the PRA will continue to advance. Implementation of these new concepts will be explored in this paper and observations on the future of the risk informed approach to target set selection will be presented.