REPRESENTING COMMON-CAUSE FAILURES IN THE SAPHIRE SOFTWARE

Currently, the risk analysis software SAPHIRE has implemented a common-cause failure (CCF) module to represent standard CCF methods such as alpha-factor and multiple Greek letter approaches. However, changes to SAPHIRE are required to support the Nuclear Regulatory Commission’s 2007 “Risk Assessment Standardization Project” CCF analysis guidance for events assessment. This guidance provides an outline of how both the nominal CCF probabilities and conditional (e.g., after a redundant component has failed) CCF probabilities should be calculated. Based upon user-provided input and extending the limitations in the current version of SAPHIRE, the CCF module calculations will be made consistent with the new guidance. The CCF modifications will involve changes to (1) the SAPHIRE graphical user interface directing how end-users and modelers interface with PRA models and (2) algorithmic changes as required. Included in the modifications will be the possibility to treat CCF probability adjustments based upon failure types (e.g., independent versus dependent) and failure modes (e.g., failure-to-run versus failure-to-start). In general, SAPHIRE is being modified to allow the risk analyst to define a CCF object. This object is defined in terms of a basic event. For the CCF object, the analyst would need to specify a minimal set of information, including: - The number of redundant components - The failure criteria (how many component have to fail) - The CCF model type (alpha-factor, MGL, or beta-factor) - The parameters (e.g., the alpha-factors) associated with the model - Staggered or non-staggered testing assumption - Default level of detail (expanded, showing all of the specific failure combinations, or not) This paper will outline both the theory behind the probabilistic calculations and the resulting implementation in the SAPHIRE software.