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2012 Software Available for Licensing

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LA-CC Number Title Description

2012-091 CASA Grande (Containment Accident Stochastic Analysis GSI Resolution And Evaluation), Version 1.x CASA Grande was developed to assist the nuclear utility industry to achieve a risk-informed resolution to Generic Safety Issue (GSI)-191 with the Nuclear Regulatory Commission (NRC). GSI-191 involves the postulated break of a high-energy pipe with subsequent creation and transport of debris to the emergency recirculation sump strainers. Debris that passes through the strainer can also accumulate in fuel channels and lead to adverse impacts on reactor core cooling. Traditional analysis methods select conservative or bounding values for the many dozens of factors that are involved with the hand calculation needed to assess impacts to the plant from the postulated scenario.

The CASA Grande software tool automates the evaluation of a single postulated accident so that thousands of possible scenarios can be assessed to generate a spectrum of possible outcomes that range from successful performance of the plant safety systems to "failure" states that are defined by regulatory levels of concern. Because all input factors have ranges of uncertainty, CASA statistically samples probability distributions defined for each factor and propagates the uncertainty on input factors into an assessment of uncertainty on the measures of failure. Nonuniform latin hypercube sampling (LHS) is used to sample and propagate uncertainty through a basic event scenario that includes: debris generation, debris transport, and debris accumulation. Inclusion of plant-state timing in the uncertainty sampling is a novel adaptation of LHS that generates randomized event sequences that are not easily handled by traditional probabilistic risk assessment (PRA) methods.

The key capability of CASA for supporting a risk-informed resolution that is acceptable to the NRC is the ability to sample probability distributions defined for the annual frequency of pipe breaks as a function of break size. The frequency of the initiating event (a broken pipe) provides a weighting factor that places each postulated scenario on a comparative scale with all other events analyzed in the random spectrum. "Risk" is often defined as the product risk = outcome x frequency, so the fraction of all events that lead to failure is now properly weighted by their very low frequency of occurrence. CASA also provides diagnostic capabilities to help judge which factors of the analysis are dominating the scenarios that lead to failure. This information is of vital interest for improving plant safety with respect to this particular generic issue. Estimates of failure probability generated by CASA are designed to interface with existing plant PRAs so that traditional estimates of core damage frequency and large early release frequency can be updated with new failure paths that are represented by the debris-related phenomenology addressed in GSI-191.

A complete CASA analysis of a specific power plant is data intensive, but all plant-specific information is provided through input files that are never embedded in the native code. This separation enables protection of proprietary information and avoids many information security issues involved with distribution and licensing of the tool itself. Plant information begins with a geometric representation of the piping, insulation and concrete barriers throughout the containment building. Complete CAD models can be imported into CASA for the purpose of computing location-specific debris volumes associated with random breaks at any location. Additional required information includes descriptions of temperature histories associated with various break sizes, and descriptions of the number of strainers and pumps that would participate in the accident response. Industry-standard information on debris behavior is imported as a default, but plant-specific, proprietary test data could also be used.

Because of its development history, CASA Grande was designed to integrate a number of independently performed analysis activities that include thermal hydraulic calculations, test campaigns involving debris penetration, chemical product formation, debris-induced pressure loss through porous beds, and operations statistics that involve variations in water levels, reservoir temperatures, and variability of human actions during an accident response. All of these activities are necessary to quantify with defensible bases the uncertainties that are ultimately manipulated by CASA. The ultimate value of this preparation is a comprehensive description of plant features that can be interrogated by CASA to not only quantify absolute failure risk, but also, to facilitate plant operability decisions when technical specifications are challenged by unexpected discoveries. A properly constructed CASA Grande analysis will support plant management decisions long after GSI-191 has reached satisfactory closure.

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