RELAP5 and TRACE simulations of reflood experiments at the RBHT facility

Abstract

The reflood phase of a loss-of-coolant accident in a nuclear power plant is crucial for safety, as it determines the peak cladding temperature. A high accuracy in the prediction of this parameter by thermal-hydraulic system codes like RELAP5 and TRACE is essential to ensure compliance with regulatory safety limits. Experimental programs, such as the Rod Bundle Heat Transfer (RBHT) project, provide benchmark data for evaluating and improving these models. This paper assesses the ability of TRACE to simulate the reflood phase, focusing on postcritical heat flux phenomena in the upper rod bundle regions. A TRACE model was developed, incorporating key features like the spacer grid model and a droplet field. The results were compared with experimental data and previous RELAP5 simulations. Additionally, a sensitivity analysis examined which parameters have a stronger influence on the PCT and the quench time. The findings indicate that TRACE performs well in high reflood rate scenarios but struggles with low or variable flooding rates, particularly in capturing heat transfer mode transitions. The sensitivity analysis highlighted key factors affecting simulation accuracy, such as the quench temperature and the heat transfer coefficient during dispersed flow film boiling. Despite some limitations, TRACE demonstrated potential in modeling certain phenomena, including heat transfer enhancement from spacer grids. Further improvements could enhance its predictive capabilities, increasing confidence in its use for safety assessments of nuclear reactor reflood scenarios.

We would like to thank the Consejo de Seguridad Nuclear (CSN, Spain) that has partly financed this research. In addition, we would like to thank the OECD Nuclear Energy Agency (NEA) for providing technical secretariat to this activity and to the participating organizations for their support.

Peer Reviewed

Postprint (published version)