The Risk-Informed Safety Margin Characterization (RISMC) pathway is a set of activities defined under the U.S. Department of Energy Light Water Reactor Sustainability Program. The overarching objective of RISMC is to support plant life-extension decision-making by providing a state-of-knowledge characterization of safety margins in key systems, structures, and components (SSCs). The methodology emerging from the RISMC pathway is not a conventional probabilistic risk assessment (PRA)-based one; rather, it relies on a reactor systems simulation framework in which physical conditions of normal reactor operations, as well as accident environments, are explicitly modeled subject to uncertainty characterization. RELAP 7 (R7) is the platform being developed at Idaho National Laboratory to model these physical conditions.
Adverse effects of aging systems could be particularly significant in those SSCs for which management options are limited; that is, components for which replacement, refurbishment, or other means of rejuvenation are least practical. These include various passive SSCs, such as piping components. Pacific Northwest National Laboratory is developing passive component reliability models intended to be compatible with the R7 framework. In the R7 paradigm, component reliability must be characterized in the context of the physical environments that R7 predicts. So, while conventional reliability models are parametric, relying on the statistical analysis of service data, RISMC reliability models must be physics-based and driven by the physical boundary conditions that R7 provides, thus allowing full integration of passives into the R7 multi-physics environment. The model must also be cast in a form compatible with the cumulative damage framework that R7 is being designed to incorporate.
Primary water stress corrosion cracking (SCC) of reactor coolant system Alloy 82/182 dissimilar metal welds has been selected as the initial application for examining the feasibility of R7- compatible physics-based cumulative damage models. This is a potentially risk-significant degradation mechanism in Class 1 piping because of its relevance to loss of coolant accidents. In this report a physics-based multi-state model is defined (Figure ES-1), which describes progressive degradations of dissimilar metal welds from micro-crack initiation to component rupture, while accounting for the possibility of interventions and repair. The cumulative damage representation of the multi-state model and its solutions are described, along with the conceptual means of integration into the R7 environment.