Development and assessment of a method for evaluating uncertainty of input parameters

Best-estimate plus Uncertainty Evaluation methods are gaining increased interest in the licensing process. Besides, lessons learnt from the OECD/NEA BEMUSE benchmark and other projects show that improvements of the present methods are necessary. In particular, methods to properly estimate input parameter uncertainties need to be developed and assessed. An Input Parameter Range Evaluation Methodology (IPREM) has been proposed and developed at University of Pisa for the quantification of the variation ranges of the input parameters through comparison of sensitivity calculations results of a selected system thermal-hydraulic code with experimental data, utilizing the mathematical apparatus of the FFTBM (Fast Fourier Transform Based Method). Within the framework of OECD/NEA PREMIUM benchmark, IPREM has been applied to the quantification of the uncertainty of the RELAP5 code models related to the simulation of reflood phenomena. Calculations of an experimental test of FEBA facility (KIT, Germany) have been performed with RELAP5 Mod3.3 code and the uncertainties of reflood-influential models (e.g. wall-to-fluid heat transfer) were quantified. The obtained model uncertainties were verified by performing blind calculations of various tests from FEBA and PERICLES test facilities, performing the uncertainty analysis of obtained results and verifying that the uncertainty band envelopes the experimental data. Within EC-funded NURESAFE project same methodology has been applied to quantify uncertainty of reflood-related models of CATHARE2 code and obtained ranges have been verified against experimental data of FEBA and ACHILLES facilities. Comparison of the RELAP5 and CATHARE2 calculated results with the FEBA, PERICLES and ACHILLES test data shows that the uncertainty bands envelope the experimental data in the majority of the measurement points. This supports the validity of obtained ranges of reflood-related models of RELAP5 and CATHARE2 codes. Based on the selected validation matrix, the IPREM has proved to be test, facility and code independent.