Benchmarking on the performance of system codes to reproduce a long SBO sequence with the actuation of a passive heat removal system

An analytical benchmark activity was launched within the OECD/NEA ETHARINUS project to assess the capabilities of system codes to simulate the relevant phenomena associated to a Station Black Out with Extended Loss of Alternate Power with the activation of the SAfety COndenser passive system. In total, 11 organizations took part in the benchmark coming from a broad number of countries and applying different system codes. The activity consisted on a blind and an open phase. The results of the blind phase showed significant discrepancies between the experimental data and the simulation outcomes, as well as among different participants' predictions. This was mainly due to the complexity of the scenario, its long duration, the strong influence of heat losses and the presence of cliff edge phenomena. During the open phase, each participant made specific adjustments to their simulation models. The challenges addressed were similar and mainly focused on improving the reproduction of heat losses and achieving a better heat balance. The ETHARINUS benchmark provided valuable insights into the challenges of simulating complex thermal-hydraulic phenomena. The findings and recommendations from this benchmark activity contribute to enhancing the predictive capabilities of system codes and the integration of passive safety systems in nuclear reactors.