Analyses of the OECD Main Steam Line Break Benchmark with the Codes DYN3D and ATHLET

The OECD Main Steam Line Break (MSLB) Benchmark was defined to validate the thermal-hydraulic system codes coupled with three-dimensional neutron kinetic codes. The reference problem is a MSLB in a pressurized water reactor (PWR) at end of cycle (EOC). The analyses were performed with the three-dimensional core model DYN3D, the thermal-hydraulic system code ATHLET and the coupled code DYN3D/ATHLET. The results of the DYN3D and ATHLET simulations based on the specification are compared with the results of other participants in the final OECD reports. The effect of the thermal-hydraulic nodalisation of the core, i.e. the number of coolant channels, and the influence of the coolant mixing inside the pressure vessel are studied in the paper. Calculations with a reduced number of coolant channels are performed often in coupled calculations for saving
computational time. Results of a 25-channel model were compared with the 177-channel calculation (1 channel/assembly). The results for global parameters like nuclear power show only small differences for the two models, however the prediction of local parameters such as maximum fuel temperatures requires a detailed thermal-hydraulic modeling. The effect of different coolant mixing within the reactor pressure vessel is investigated. It is shown that the influence of coolant mixing mitigates the accident
consequences when three-dimensional neutron kinetics is applied. In case of point kinetics, coolant mixing leads to an opposite effect. To profit from the three-dimensional core model, a realistic description of the coolant mixing in the coupled codes is a topic of further investigations.