CASA Grande

CASA Grande

There are approximately 104 nuclear power plants in the United States. These plants are some of the most sophisticated and complex energy systems ever created. Despite the painstaking engineering that has gone into such plants, no system is failure-proof. Calculating the risk of damaging the fuel by all possible failure paths is important to minimize the chance of a catastrophic event.

The Nuclear Regulatory Commission (NRC) has been striving to resolve what it calls Generic Safety Issue (GSI) 191. GSI-191 involves the following scenario: a high-energy pipe breaks, creating debris that finds its way to the plant’s emergency recirculation sump strainers. Debris passing through the strainer can also accumulate in fuel channels, adversely impacting reactor cooling.

To improve and refine how best to address GSI-191, Bruce Letellier and his team at Los Alamos National Laboratory (Los Alamos) established a Cooperative Research and Development Agreement with the University of Texas to assist South Texas Project Nuclear Operating Company. The partners developed CASA (Containment Accident Stochastic Analysis) Grande, a software code that automates the evaluation of a single postulated accident so that thousands of possible scenarios can be assessed. CASA Grande enables generation of a spectrum of possible outcomes that range from successful performance of the plant’s safety systems to various “failure” states defined by regulatory levels of concern. CASA Grande statistically samples probability distributions defined for each factor, propagating uncertainty on the input into an assessment of uncertainty on the measures of failure. Non-uniform 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 methods. A prototype of CASA Grande was exercised for the first time in December of 2011.

Los Alamos filed a copyright disclosure for CASA Grande and soon after established an exclusive commercial licensing agreement with Alion Science and Technology Corporation in 2013. Alion has made numerous improvements to the code to support their consulting services and is actively developing a market for users in the nuclear utility community.

Under the terms of the exclusive license, Alion can modify, market, and apply the CASA Grande code for revenue generation so long as periodic improvements to the code revert back to Los Alamos National Security, LLC for noncommercial, government use. From a corporate perspective, CASA Grande constitutes a business investment for Alion that will help the company grow its market base among commercial nuclear utilities. Alion has made substantial improvements to CASA Grande, including user-accessibility features and model-fidelity enhancements. The commercial version, now under software quality control, provides the foundation for GSI-191 resolution at the South Texas Project nuclear power plant.