RISMC Framework Development
Although LWRs are a mature and successful technology in the United States with an impressive track record in nuclear power plant safety and performance over the past two decades, the next wave of new plant deployment and life extension of the existing operating LWR fleet beyond 60 years is anything but certain. There is broad consensus that technical, cost, and schedule uncertainties in certification and licensing are a significant hindrance to prospective applicants for new licenses, especially for technologies other than LWR. Many discussions tacitly assign a great deal of blame for this to NRC processes.
Part of the traditional approach to licensing is to invest very substantially in margin. The concept of margin has enormous benefits in decision-making, but traditional implementation of the concept has proven to be enormously expensive. A comprehensive set of plausible safety margins will make the sustainability decision easier for both licensees and NRC. Thus, it is important to formulate a margin based safety case framework aimed at streamlining NRC review and subsequent licensee implementation. The actual technical content of a safety case is necessarily plant-specific; the framework will establish a set of plant-specific technical demonstrations whose integrated presentation to NRC should help to reduce regulatory uncertainty.
Development and demonstration of a new technology-neutral paradigm of science-based safety case development, evaluation, and acceptance will ensure predictable, streamlined, and cost-effective licensing of nuclear installations. It will be achieved through (1) a set of advanced simulation and analysis tools to enable accurate quantification of the system’s margins to safety, (2) a formalized (computerized) technology-neutral framework for safety case development, and (3) a knowledge center of previous license applications. A comprehensive, high-quality, and defensible safety analysis submitted by the license applicant is paramount to ensuring the effectiveness of the application’s regulatory review.
The proposed research is driven by the idea that reducing uncertainties facing applicants can be achieved not only by working on improved understanding of the technical factors governing particular margins, but by proactively establishing the character and rigor of the portfolio of tests, demonstrations, and commitments to be comprised in the licensing safety case. In the abstract, this idea is not new, but in the United States previous implementations of it have defaulted to licensing tradition, rather than proactively trying for an improved formulation of the safety case. In short, the proposed task will take up, from a DOE perspective, where NRC left off and identify and address technical issues within the RISMC framework.
This task was formulated with the objective of identifying crosscut case studies that support formulation and demonstration of the RISMC framework for LWRS. The work scope is accomplished largely within the RISMC working group activity.
Enabling Tool and Methods
With focus on the effect of plant aging on life extension decision making, characterization of the nuclear power plant safety margin is hindered by large uncertainties that exist in modeling and predicting behaviors of aging SSCs in a broad range of nuclear power plant operating and abnormal conditions and nuclear power plant system dynamics in accident scenarios involving SSC failure modes not studied before. Existing PRA and DSA methods ignore reliability of the plant’s passive SSCs and their failure physics, making them unsuited for capturing the essence of aging impact.
Of particular interest is identification of catastrophic system degradation scenarios (e.g., cascading failures that cannot be ruled out as “physically unreasonable”). These scenarios require measures (in nuclear power plant inspection, maintenance, and modification) to eliminate system vulnerability. This thrust focuses on advancing the PRA and DSA methods to enable their use in assessing the aging effects on nuclear power plant safety margin.
Apart from specialized application areas (such as seismic PRA), most current PRA methodology takes most passive SSCs for granted because it is believed that these components do not contribute significantly to offsite risk. Within the LWRS Program, it is important to challenge that presumption and to examine whether margin issues could emerge for SSCs whose performance is presently taken for granted.
The point of this task is to incorporate, into risk models, passive SSCs whose performance has previously been taken for granted in PRA, but whose loss of physical margin may need to be analyzed. Ultimately, the risk model that these SSCs are added to is the same risk model to be quantified in the enhanced PRA paradigm described above.