Participants from two NEA activities, the Rod Bundle Heat Transfer Phase II (RBHT‑II) joint project and the Working Group's on Analysis and Management of Accidents International Standard Problem ISP‑53 (WGAMA ISP-53), gathered at the French Nuclear Safety Authority's (ASNR) Cadarache facilities between 16 and 20 March 2026 for a series of final project meetings and technical exchanges. The programme also included laboratory tours of the ASNR Cadarache experimental facilities, supporting continued collaboration in thermal‑hydraulics safety research.
The WGAMA ISP‑53 activity is based on the ASNR COAL reflooding experiments and built around comparative simulations of full‑length 7×7 electrically heated rod bundles representative of a 17×17 PWR design. The programme includes open and blind benchmarking phases assessing code‑to‑experiment predictive capability under both undeformed (B0) and deformed (B2) fuel‑bundle conditions. More than 75 COAL reflooding experiments have been conducted since 2020, providing an extensive experimental basis for the ISP.
Throughout the closing meeting, participants reviewed outcomes from all benchmark phases, including comparisons of modelling approaches, treatment of fuel‑rod deformation effects, and uncertainty‑propagation exercises. As the last gathering of the ISP‑53 activity, the discussions marked the completion of this multi‑year international effort to strengthen confidence in thermal‑hydraulic code predictions for LOCA reflooding scenarios.
Participants of the ISP‑53 meeting
The NEA RBHT‑II project, operated by the US Nuclear Regulatory Commission (NRC), builds upon the earlier phase of the Rod Bundle Heat Transfer programme and continues to focus on challenges in modelling reflood behaviour, especially the complex heat‑transfer mechanisms that dominate accident sequences involving delayed re‑introduction of coolant. These include post‑critical heat‑flux regimes, entrainment, quench behaviour and associated uncertainties in codes.
The project generates new experimental data under a range of inlet‑flow conditions, including scenarios relevant for small modular reactors and long‑term cooling phases, using the RBHT facility at Pennsylvania State University under US NRC co-ordination. During the final meeting, participants exchanged results from recent blind test campaigns, discussed simulation outcomes, and reviewed progress toward completing the project’s concluding report.
Participants of the RBHT‑II project meeting
Looking ahead: Opportunities for follow‑up activities
Participants also explored potential follow‑up activities, building on synergies between RBHT‑II’s and ASNR's COAL experimental programme and ISP‑53’s code‑comparison framework. The complementary nature of the two initiatives provides a natural basis for future collaborative efforts in advancing thermal‑hydraulics code validation and safety research.
The week‑long exchanges at ASNR Cadarache highlight the continued importance of international co-operation in improving the predictive accuracy of nuclear safety analysis tools and ensuring robust technical bases for reactor accident management strategies.
