DIPE: Determination of input parameters uncertainties methodology applied to CATHARE V2.5_1 thermal-hydraulics code
Congress title :ICONE 15th International Conference on Nuclear Engineering
Congress location :Nagoya
Congress date :22/04/2007
Since the revision of the 10 CFR50.46 in 1988, the best-estimate codes may be used in safety demonstration and licensing, provided that uncertainties are added to the relevant output parameters before comparing them with the acceptance criteria. The uncertainty of output parameters comes principally from the lack of knowledge of the input parameters (initial and boundary conditions of the calculated transient, empirical models of the code, etc.). In the safety analysis of the large break loss of coolant accident (LB-LOCA), for a best-estimate analysis, it was decided that the 95th percentile estimated with a high degree of confidence should be lower than the acceptance criteria. Best-estimate plus uncertainty (BEPU) analysis of the LB-LOCA allows for an increase in margins with regard to the criteria. Therefore, the nuclear industry has developed and uses this type of analysis for power uprating, longer fuel cycles, fuel design, etc. It appeared necessary to IRSN, technical support of the French Safety Authority, to get more insight to these strategies. Furthermore BEPU analysis will be developed not only in thermal-hydraulics but also in other fields such as in neutronics.
The application of the BEPU analysis can be made in three steps in a statistical evaluation:
1.The determination of the input parameters (IP) statistical characteristics.
2.The modelling and understanding of the output parameters.
3.The evaluation of the 95th percentile with a high degree of confidence.
The first step is generally done manually according to expert judgments and comparing experimental data. The user effect is also very important in the determination of the statistical characteristics: range of variation and probability law. To reduce this user effect and to help the experts in their evaluation, IRSN has been developing a fully automated methodology and approach. This methodology is called DIPE: Determination of Input Parameters uncertaintiEs. This paper presents the advantages and limits of the application of DIPE for the physical models input parameters of CATHARE V2.5_1, a thermal-hydraulics code used in safety demonstration. DIPE will be applied to separate effect tests data used in the code qualification such as CANON and MARVIKEN.