Simulation of Post-LOCA Heat Transfer in Nuclear Fuel Bundle : An evaluation of thermal hydraulic codes for reactor core safety

A Loss-of-Coolant Accident (LOCA) represents a critical scenario for the safety of a nuclear reactor, as the loss of the reactor coolant can result in overheating and irreversible damages to the core. For the licensing of a nuclear reactor or fuel to be approved, multiple safety criterions are required to be fulfilled. One criteria is that the peak cladding temperatures for fuel rods can’t exceed the upper limit of 1204.4 degrees Celsius to prevent any damage on the fuel and dispersal of radioactive particles. In collaboration with Westinghouse Electric Sweden AB, the aim of this report is to update and improve Westinghouse’s thermal hydraulic codes used for calculating and conducting safety analysis of a reactor core environment during a hypothetical LOCA. The Westinghouse LOCA experiment G-2 were simulated in the thermal hydraulic codes GOBLIN and CHACHA, to evaluate if the codes can produce results that are similar to the G-2 results. Accurate models and methodologies for post-LOCA environments are necessary for the licensing process of the newly developed TRITON11-fuel. With GOBLIN and CHAHA the G-2 test were simulated and the surface temperatures of the fuel rods between the codes and the G-2 data were compared. CHACHA manages to simulate the initial heat-up process well, with 83 % of the simulated runs having an accepted surface temperature at 60 seconds into the test sequence. However, the cooling transient phase were difficult for CHACHA to provide results that were similar to G-2, as only one of the six simulated runs managed to fulfill the set requirements of the surface temperatures. The setup parameters for the six simulated runs were observed to have a large impact on the results, as the runs with the largest upper plenum pressure provided the best results. Several points of possible causes to the results were identified for further improvements of the thermal hydraulic codes, namely the calculation of heat transfer coefficients, the cooling calculation