A multi-state physics modeling for estimating the size- and location-dependent loss of coolant accident initiating event probability

Multi-State Physics Modeling (MSPM) integrates multi-state modeling to describe a component degradation process by transitions among discrete states (e.g., no damage, micro-crack, flaw, rupture, etc.), with physics modeling by (physic) equations to describe the continuous degradation process within the states. In this work, we propose MSPM to describe the degradation dynamics of a piping system, accounting for the dependence on the size and location of the Loss of Coolant Accident (LOCA) initiating event of the Reactor Coolant System (RCS) of a Pressurized Water Reactor (PWR). Estimated frequencies of LOCA as a function of break size are used in a variety of regulatory applications and for the Probabilistic Risk Assessment (PRA) of Nuclear Power Plants (NPPs). Traditionally, two approaches have been used to assess LOCA frequencies as a function of pipe break size: estimates based on statistical analysis of field data collected from piping systems service experience and Probabilistic Fracture Mechanics (PFM) analysis of specific, postulated, physical damage mechanisms. However, due to the high reliability of NPP piping systems, it is difficult to construct a comprehensive service database based on which perform statistical analysis. On the other hand, it is difficult to utilize PFM models for calculating LOCA frequencies because many of the input variables and model assumptions are over-simplified and may not adequately represent the true plant conditions. We overcome these challenges and propose a size- and location-dependent LOCA initiating event frequencies estimation by resorting to the novel MSPM modeling scheme. Benchmarking is done with respect to the results obtained with the Generic Safety Issue (GSI) 191 framework that makes use of field data for LOCA initiating event probability calculation.