Quantification of input uncertainties based on veera reflooding experiments

Propagation of input uncertainties through code runs and statistical analysis of the results has come to be one of the two prevailing methods when performing Best-Estimate Plus Uncertainty (BEPU) analyses with thermal-hydraulic system codes. To use the method, the uncertainties of the inputs need to be defined. A difficult task in this respect is the quantification of the uncertainties of the constitutive equations of the code. For this reason several methods to estimate the uncertainties of reflooding related correlations have been under review in the OECD/NEA-endorsed PREMIUM benchmark, in which VTT has also been taking part. In this paper the same methodologies that were utilized by VTT in PREMIUM for quantification of the uncertainties, the FFTBM and CIRCE methods, have been used with data from the VEERA reflooding experiments. The experiments were performed in the 1990's at the Lappeenranta University of Technology VEERA facility in Lappeenranta, Finland in partnership with VTT. The code used to perform the analyses was the APROS thermal-hydraulic system code, which has been developed by VTT in partnership with Fortum Ltd since 1986. The quantification results obtained with the VEERA data were furthermore compared to the VTT results obtained with the FEBA facility reflooding data during the PREMIUM benchmark. Some issues with the VEERA data were identified during the analysis, due to which a part of the data was excluded. Even then it was possible to successfully perform a process of quantification of the uncertainties with the methodologies involved. The comparisons with the PREMIUM results revealed that even with the issues with the data intriguing results could be reached.