This paper presents the results and the main lessons learnt from Phase V of BEMUSE, an international programme promoted by the Working Group on Accident Management and Analysis (GAMA) of OECD to address the issue of the capabilities of best-estimate computational tools and uncertainty analysis. The scope of Phase V is the uncertainty analysis of a Large Break Loss-Of-Coolant-Accident (LBLOCA) in a Pressurized Water Reactor. Fourteen participants from twelve organizations and ten countries participated in the Phase V of BEMUSE.The paper starts with a general description of the BEMUSE programme including the objectives, structure, and the outline of the Phase V specification. Then it summarizes some general aspects on the uncertain model parameters and the results for the uncertainty analysis and for the sensitivity evaluation. To end with, general recommendations and conclusions are presented as practical guidance for uncertainty analysis performance. (C) 2011 Elsevier B.V. All rights reserved.

Phase IV of BEMUSE Program is a necessary step for a subsequent uncertainty analysis. It includes the simulation of the reference scenario and a sensitivity study. The scenario is a LBLOCA and the reference plant is Zion 1 NPP, a 4 loop PWR unit. Thirteen participants coming from ten different countries have taken part in the exercise. The BEMUSE (Best Estimate Methods plus Uncertainty and Sensitivity Evaluation) Program has been promoted by the Working Group on Accident Management and Analysis (WGAMA) and endorsed by the Committee on the Safety of Nuclear Installations (CSNI). The paper presents the results of the calculations performed by participants and emphasizes its usefulness for future uncertainty evaluation, to be performed in next phase. The objectives of the activity are basically to simulate the LBLOCA reproducing the phenomena associated to the scenario and also to build a common, well-known, basis for the future comparison of uncertainty evaluation results among different methodologies and codes. The sensitivity calculations performed by participants are also presented. They allow studying the influence of different parameters such as material properties or initial and boundary conditions, upon the behaviour of the most relevant parameters related to the scenario.

This paper presents the results and the main lessons learnt from the phase 3 of BEMUSE, an international benchmark activity sponsored by the Committee on the Safety of Nuclear Installations [CSNI: Committee on the Safety of Nuclear installations (NEA, OECD), 2007. BEMUSE Phase III Report. NEA/CSNI R(2007) 4, October 2007] of the OECD/NEA. The phase 3 of BEMUSE aimed at performing Uncertainty and Sensitivity Analyses of thermal-hydraulic codes used for the calculation of LOFT L2-5 experiment, which simulated a Large-Break Loss-of-Coolant-Accident (LB-LOCA). Eleven participants coming from ten organisations and eight countries took part in this benchmark.In the first section of this paper, the context of BEMUSE is described as well as the methods used by the participants. In the second section, the results of the benchmark are presented. The majority of the participants find uncertainty bands which envelop the experimental data fairly well, however the width of these bands is much diverged. A synthesis of the sensitivity analysis results has been made and is expected to provide a useful basis for further uncertainty analysis dealing with LB-LOCA. Finally, recommendations are given both for uncertainty and sensitivity analysis. (C) 2008 Elsevier B.V. All rights reserved.

The revision of the 10 CFR50.46 in 1988 has made possible the use of best-estimate (BE) codes in safety demonstration and licensing, provided tha t uncertainties are added to the relevant output parameters before comparing them with the acceptance criteria. In the safety analysis of the large break loss of coolant accident, for a BE analysis, it was agreed that the 95th percentile estimated with a high degree of confidence should be lower th an the acceptance criteria. It appeared necessary to IRSN, technical support of the French Safety Authority, to get more insight into these strategies. The application of the BE plus uncertainty analysis can be made in three steps in a statistical evaluation: - The definition and the evaluation of uncertainties of the input parameters; - The modelling and understanding of the output par ameters; - The evaluation of the 95th percentile with a high degree of confidence. As a general rule the global sensitivity analysis ( GSA) is done with linear correlation coefficients. This paper presents a new approach to perform a more accurate GSA to determine and to classify the main input parameters. Such inf ormation is difficult to obtain directly by Monte-Carlo methodologies using the thermal-hydraulic code, because it is rather timeconsuming. Therefore, it is natural to replace it w ith a simpler model called a surrogate model too. We suggest Kriging methodology for its constru ction and the SOBOL methodology for the GSA. The paper presents the application of the previousl y described methodology on the LOFT (Lossof-Fluid Test) loss of coolant experiment L2-5 whic h simulated a double-ended offset shear of a cold leg in the primary coolant system. LOFT L2-5 has been the subject of the BEMUSE international problem. The output is the first maxi mum Peak Cladding Temperature of the fuel. The best estimate code used is CATHARE2 V2.5.