Accuracy Assessments with Fast Fourier Transform Based Method (FFTBM)

Abstract

NPP safety analysis based on the “Best Estimate” approach and best estimate codes represent the plant behavior in a realistic way. The result of best estimate system codes, however, are still affected by uncertainty; a large number of different and complex phenomena are occurring in a plant, and the model and solution methods in the codes remain approximate. System codes offer a great degree of freedom to their users, which, as downside, introduce user dependent results. Also hardware and the compiler that has been used might affect the calculations. Therefore the best estimate predictions of nuclear power plant scenario must be supplemented by proper uncertainty evaluations in order to be meaningful. UMAE (Uncertainty Methodology based on Accuracy Extrapolation) and CIAU (Code with the capability of Internal Assessment of Uncertainty) uncertainty methods derive the uncertainty by comparing experimental results with code calculations. UMAE and CIAU need an objective method, independent from expert judgment, which quantifies the accuracy of a code calculation with respect to an experiment. A methodology suitable to quantify the code accuracy has been developed based on the Fast Fourier Transform method (FFTBM). This method is easy to understand, convenient to use, user independent and it clearly indicates when a simulation needs to be improved. The FFTBM derives from the measurement and the prediction the accuracy in the frequency domain. The acceptability factor for code calculation was determined based on several hundreds of code calculations. Application of the FFTBM is not limited to support for uncertainty evaluation. It also is a convenient method to derive a figure of merit, to quantify the quality of a code prediction. In fact, the FFTBM method has been applied to various international standard problems, standard problem exercises and other experiment – simulation exercises. The purpose of the paper summarize typical applications of the FFTBM in international standard problems (ISP) or standard problem exercise (SPE) organized by CSNI or IAEA.