The BEMUSE Programme : Best - Estimate Methods Uncertainty and Sensitivity Evaluation - Phase

The BEMUSE (Best Estimate Methods – Uncertainty and Sensitivity Evaluation) Programme has been promoted by the Working Group on Accident Management and Analysis (GAMA) and endorsed by the Committee on the Safety of Nuclear Installations (CSNI) [1]. The high-level objectives of the work are: • To evaluate the practicability, the quality and the reliability of Best-Estimate (BE) methods including uncertainty evaluation in applications relevant to nuclear reactor safety; • To promote the use of BE-Methods by the regulatory bodies and the industry. Operational objectives include an assessment of the applicability of best-estimate and uncertainty methods to integral tests and their use in reactor applications. The present paper deals with the activities performed by the participants during the Phase II of BEMUSE. It is connected with the re-analysis of the Experiment L2-5 performed in the LOFT facility using different thermal-hydraulic system codes. The technological importance of the activity can be derived from the following: a) LOFT is the only nuclear reactor where safety experiments have been performed; b) The ISP-13 was completed more than 20 years ago and open issues remained from the analysis of the comparison between measured and calculated trends The consideration of the BE codes and uncertainty evaluation for Design Basis Accident (DBA), by itself, shows the safety significance of the proposed activity. End users for the results are expected to be the industry, the safety authorities and the research laboratories. Main achievements of the Phase II can be summarized as follows: Almost all performed calculations appear qualified against the fixed criteria; Dispersion bands of reference results appear substantially less than in ISP-13.