United States Nuclear Regulatory Commission - Protecting People and the Environment

Quantitative Code Assessment with Fast Fourier Transform Based Method Improved by Signal Mirroring (NUREG/IA-0220)

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Publication Information

Date Published: December 2009

Prepared by:
A. Prošek, B. Mavko,
Jožef Stefan Institute
Jamova cesta 39
SI-1000 Ljubljana, Slovenia

A. Calvo, NRC Project Manager

Published by:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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In recent years, the number of quantitative comparisons between experimental data and calculated data in the area of nuclear technology has increased. The fast Fourier transform based method (FFTBM) is the tool most widely used to quantify the accuracy of thermalhydraulic code calculations, which are continuing to improve. However, in preliminary applications of the original FFTBM to a severe accident test, the need for further optimization became evident. Analysts observed that FFTBM favors certain trends (e.g., monotonic increasing function). Thus, the purpose of the present study was to improve this aspect of FFTBM. Improvement was achieved by signal mirroring. Among other improvements, an index for the detection of the time shift between the compared signals was proposed. For the demonstration of improved FFTBM by signal mirroring, the analysis used the Loss of Fluid Test (LOFT) L2-5 test (large-break loss-of-coolant accident). Both qualitative and quantitative analyses were performed, and the conclusions about the quality of reference calculations were compared with those of the Best-Estimate Methods Uncertainty and Sensitivity Evaluation (BEMUSE). The results show that, with improved FFTBM, the analyst can achieve a true picture of the contribution of each discrepancy to the accuracy as judged by FFTBM. Analysis that considers the improved FFTBM tool as a “black box” obtains optimal information, which greatly aids the analyst in making a final judgment about the calculation. “Black box” here is meant that no information of the curve shape is input by the user to the FFTBM tool. The easy-to-use method is best suited to the automated comparison of several calculations of the same test.

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