Members of the NEA's nuclear safety joint project Rod Bundle Heat Transfer – Phase 2 (RBHT-II) met at the laboratories of CEA (the French Alternative Energies and Atomic Energy Commission) in Saclay, France. The meeting was held from 23-25 June 2025 and assembled 30 international experts to share results of the project’s experimental, modelling and simulation work.
About the RBHT-II project
Modelling reactor core behaviour under accident conditions with delayed re-introduction of cooling water (commonly referred to as reflood) is a challenge for safety analysis computer codes. Reflood thermal-hydraulics (e.g. post-critical heat flux flow and heat transfer, entrainment, quench) remains a major contributor to code uncertainties in the simulation of many accident scenarios and must be better understood to enhance nuclear safety.
The Rod Bundle Heat Transfer Phase II (RBHT-II) project aims at improving understanding of reflood. Building on the success of phase I, RBHT-II is generating new experimental data under various water flow rates. These conditions are especially relevant for small modular reactors (SMRs) and long-term cooling scenarios. RBHT-II is a collaborative international effort that involves regulators, industry and research organisations.
The Rod Bundle Heat Transfer (RBHT) facility is located at Pennsylvania State University (PSU) and operates under the United States Nuclear Regulatory Commission (NRC) co-ordination. The objective of Phase II is to conduct new experiments and evaluate and improve the accuracy of system and sub-channel thermal hydraulics codes in the simulation of reflood tests in a full height rod bundle for complex inlet flows.
Third meeting of the RBHT-II project
During the meeting, PSU and NRC teams presented outcomes of all the experimental campaigns, while participants from the different member organisations shared modelling and simulation results. The group then discussed how to consider user effects and provide uncertainty analysis. Analytical benchmarking activities aim to enable participants to compare modelling results through both open and blind simulations based on RBHT-II experiments. The group also agreed on plans to complete the benchmark modelling and simulation contributions and the final report.
The meeting also included a visit to the Thermal-hydraulics and Fluid Mechanics Section (STMF) at CEA.
Read more about the NEA’s work on nuclear power plant accident management on the WGAMA webpage.
