Towards a Better Understanding of Reflood Thermal-Hydraulics: A Summary of the OECD/NEA RBHT Project

Reflood thermal-hydraulics remains a difficult and complex subject and understanding the physical phenomena that occur during a reflood transient are important to nuclear safety. The OECD/NEA Rod Bundle Heat Transfer (RBHT) project was designed to provide unique experimental data for code assessment and model development. Participants, which came from 21 international organizations, used analysis codes including APROS, ATHLET, CATHARE, CTF, MARS, RELAP5, TRACE, and SPACE to simulate the tests performed in the RBHT facility. The experimental campaign carried out within the OECD/NEA RBHT project, produced data for a total of sixteen reflood tests conducted in two test series. An “open” test series consisted of eleven experiments and a “blind” test series five experiments. In the “blind” tests only the initial and boundary conditions were provided to participants prior to simulation of those experiments. Reflood rates ranged from 0.5 cm/sec to 15 cm/sec thus producing data applicable to dispersed flow film boiling and inverted annular flow film boiling. Inlet subcooling was ranged from 2.8 K to 80 K. Tests with variable reflood rates and oscillatory reflood rates were included in the test matrix. This paper describes the project and presents a summary of major experimental and analytical findings.