The NEA Advanced Thermal-hydraulic Test Loop for Accident Simulation (ATLAS) project focuses on the analysis of the thermal-hydraulics in accident scenarios in existing and future nuclear power plants. With three project phases completed, the current one (ATLAS-4) seeks to advance international understanding of complex accident scenarios in water-cooled reactors, including small modular reactors (SMRs), with particular emphasis on the performance of passive safety systems within the reactor coolant system (RCS) and the containment.
The second meeting of the NEA ATLAS-4 project was held in Lyon, France, on 22–24 October 2025. Hosted by EDF, the event brought together 52 participants from nine countries, marking a milestone in collaborative thermal-hydraulic research under the NEA framework.
Significant progress was achieved in the experimental programme, with the completion of first ATLAS-4 tests in the ATLAS-CUBE facility. These tests focused on the behaviour of spray systems in containment and provided valuable data for understanding multi-dimensional thermal-hydraulic phenomena. Conducted in two runs, the experiments revealed unexpected spray performance characteristics that will be further explored and modelled using state-of-the-art thermal-hydraulic codes.
Participants also approved the specifications for the next series of tests scheduled for late 2025 and 2026, as well as the selection of tests for codes benchmarking.
The meetings further provided an opportunity to strengthen collaboration across NEA thermal-hydraulic projects. Discussions focused on organising future joint analytical workshops with projects such as PANDA, POLCA, and SYSTHER, to foster cross-project learning and model validation.
Looking ahead, participants also discussed plans for a joint workshop uniting all NEA thermal-hydraulic experimental projects within the context of the NEA Advanced Thermal-Hydraulic Experiment for Nuclear Analysis and Safety (ATHENA) framework. This initiative follows the recommendations of the Strategic Roadmap for Nuclear Reactor Safety Research, which calls for a co-ordinated framework supporting scaled experimental programmes for advanced reactor concepts.
