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Risk Informed Safety Margin Characterization (RISMC) Advanced Test Reactor Demonstration Case Study

Safety is central to the design, licensing, operation, and economics of Nuclear Power Plants (NPPs). Consequently, the ability to better characterize and quantify safety margin holds the key to improved decision making about light water reactor design, operation, and plant life extension. A systematic approach to characterization of safety margins and the subsequent margins management options represents a vital input to the licensee and regulatory analysis and decision making that will be involved.

The purpose of the Risk Informed Safety Margin Characterization (RISMC) Pathway research and development (R&D) is to support plant decisions for risk-informed margins management with the aim to improve economics, reliability, and sustain safety of current NPPs. Goals of the RISMC Pathway are twofold: (1) Develop and demonstrate a risk-assessment method coupled to safety margin quantification that can be used by NPP decision makers as part of their margin recovery strategies. (2) Create an advanced “RISMC toolkit” that enables more accurate representation of NPP safety margin.

In order to carry out the R&D needed for the Pathway, the Idaho National Laboratory is performing a series of case studies that will explore methods- and tools- development issues. One of the initial case studies that has been proposed is to demonstrate the RISMC approach using the Advanced Test Reactor (ATR) as a test. This report describes the RISMC methodology demonstration for the ATR. As part of the demonstration, we describe how both the thermal-hydraulics and probabilistic safety calculations are integrated and used to quantify margin management strategies.

Completing the ATR case study has pointed to several additional areas of promising R&D related to risk-informed margin management. First, the current NRC Significance Determination Process is focused on core damage frequency, but we showed how the concept of safety margin provided additional information, both from a quantitative aspect but more importantly from an engineering physics understanding. Further, additional applications seem to be possible including NPP risk monitor enhancements; a general decision support capability for operational decisions; and an integrated and holistic framework to account for aging effects during the NPP lifetime.

Several successful outcomes have resulted from performing the ATR case study, including the development of an improved plant physics approach and an enhanced risk- analysis capability featuring a unique suite of simulation methods that builds upon traditional PRA approaches. The approach and lessons learned from this case study will be included in future Technical Basis Guides produced by the RISMC Pathway. These guides will be the mechanism for developing the specifications for RISMC tools and for defining how plant decision makers should propose and evaluate margin recovery strategies.