Technical Approach and Results from the Fuels Pathway on an Alternative Selection Case Study

Abstract

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 by improving economics and reliability, and sustaining 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, in addition to being of current interest in their own right. One such study is a comparative analysis of safety margins of plants using different fuel cladding types: specifically, a comparison between current-technology Zircaloy cladding and a notional "accident-tolerant" (e.g., SiC-based) cladding. The present report discusses the formulation of the analysis needed to support an intelligible comparison of margins in plants using the two cladding types, and what sorts of capabilities will be needed.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.This report discusses the following points.There is no particular reason to believe that regulatory acceptance criteria (e.g., peak clad temperature < 2200 F) will be the same for new cladding types as for the traditional cladding type; comparison of cladding behavior for licensing purposes needs to be carried out in light of the higher-level purposes of the acceptance criteria. The technical basis for the recently-proposed change to the ECCS rule explicitly says as much. It is clear a priori that a meaningful analysis must analyze plant-level behavior for each cladding type, as opposed to simply looking at the physical properties of cladding. In order to compare plant-level behavior keeping all but cladding the "same," it is necessary to exercise considerable care in formulating inputs to the simulation of time histories. The technique needed for this is discussed. Recent simulation work carried out for another program, while confirming the idea that "accident-tolerant...

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