Benchmarking and qualification of the NUFREQ-NPW code for best-estimate prediction of multichannel core stability margins

The work described in this paper focuses on the development, verification, and benchmarking of the NUFREQ-NPW code at Westinghouse for best-estimate prediction of multichannel core stability margins in US boiling water reactors (BWRs). The NUFREQ-NPW code can allow for generalized three-dimensional core analyses of BWRs. The code was modified at Westinghouse to allow for a mixed-fuel-type multichannel core-wide stability analysis. One of the key distinguishing features of NUFREQ-NPW over other stability codes is that the analytical model allows for the system pressure perturbation as an external forcing function. This enables direct comparisons against pressure perturbation test data, instead of inferring equivalent information via curve fitting of other transfer functions. The results of comparisons with experimental data using the Westinghouse methodology, which is based on the NUFREQ-NPW code, demonstrate the best-estimate predictive capability for BWR core stability margins. The methodology is thus suitable for design and licensing applications.