Uncertainty and sensitivity analysis of QUENCH experiments using ASTEC and RELAP/SCDAPSIM codes

To prevent total meltdown of the uncovered and overheated core the reflooding with water is an unavoidable accident management measure. Because these actions lead to the generation of hydrogen, which can cause further problems, the related phenomena are investigated performing experiments and computer simulations. One of the most widely known is the QUENCH test-program, performed in Forschungszentrum Karlsruhe. In this program QUENCH experiments are modelling the loss of coolant accidents in Light Water Reactors and are used by the computer code developers and users for the code validation and verification purposes. Unfortunately any deterministic computer simulation is not free from the uncertainties. There are many sources of possible uncertainties: nodalization development, uncertain initial and boundary conditions, approximate correlations used in the computer code and etc. To receive the realistic calculation results, the best estimate computer codes should be used for the calculation with combination of uncertainty and sensitivity analysis of calculation results. In the Lithuanian Energy Institute different Best Estimate computer codes for the analysis of thermal-hydraulic processes and codes for the analyses of severe accidents in nuclear reactors are employed: RELAP5, ATHLET, COCOSYS, RELAP/SCDAPSIM, ATHLET-CD and ASTEC. For the uncertainty and sensitivity analysis very widely is used the SUSA 3.5 package, based on GRS (Germany) best estimate methodology. Also, for the evaluation of uncertainties of calculation results, the SUNSET package developed by IRSN (France) and RELAP/SCDAPSIM Mod4 with integrated uncertainty package are used. In this paper the QUENCH-03 experiment is modelled using ASTEC and RELAP/SCDAPSIM codes. ASTEC (Accident Source Term Evaluation Code) was jointly developed since several years by the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) and the German Gesellschaft für Anlagen und Reaktorsicherheit mbH (GRS) to simulate all the phenomena that occur during a severe accident in a water-cooled nuclear reactor. RELAP/SCDAPSIM computer code is designed to describe the overall reactor coolant system thermal hydraulic response and core behaviour under normal operating conditions or under design basis or severe accident conditions. For the uncertainty and sensitivity analysis the above mentioned tools are used. The performed benchmark allowed analyze different modelling features and to present the recommendations for the model development. (author)