Conference Agenda

NEA has recently organized several large events (2023 Joint Nuclear Safety Research Projects event week, 2024 FRAME workshop) to discuss with its members the benefits of NEA collaborative nuclear safety projects, with, in 2025, close to 60 projects completed and more than 60 years of nuclear safety research. Following these events, the NEA has undertaken the development of a high-level nuclear safety research roadmap to give directions for future nuclear safety research. The roadmap reflects regulators key messages to policymakers, highlighting key nuclear safety research capabilities needs and challenges, and at the same time provides deeper technical insights and research directions in major nuclear safety technical areas to help project operators developing project proposals addressing priority nuclear safety issues. A key recommendation formulated through these initiatives is that NEA should develop a framework for securing and organizing resources for advanced thermalhydraulic experiments for safety assessment of advanced reactor designs including designs relying on passive safety systems and small modular reactors, with scaled experimental infrastructures able to generate high-quality data for the development and validation of state-of-the-art codes used in thermalhydraulic analyses. The framework should also include transverse tasks related to data preservation and knowledge transfer. Relevant roadmap insights and the development status of the new framework will be presented. A companion paper will provide main insights gained from recently concluded NEA projects in the thermalhydraulic area.

The Working Group on the Analysis and Management of Accidents (WGAMA) addresses OECD Nuclear Energy Agency (NEA) activities related to potential design-basis accident (DBA) and beyond design-basis accident (BDBA) in nuclear reactors and related technologies. The group addresses safety issues of existing nuclear reactors and related technologies as well as emerging challenges on evolutionary and innovative reactor designs and nuclear technologies, including Small Modular Reactors (SMRs). For these purposes, the WGAMA has coordinated workshops, technical publications and research activities in the fields of thermal-hydraulics (T/H), computational fluid dynamics (CFD) and severe accidents (SAs) to improve knowledge of accidents and of the confidence of the scientific calculation tools used in safety studies, namely safety analysis codes or tools.

The paper aims to review and summarize the recent activities and outcomes such as the ongoing effort on the extension and update of the CSNI Code Validation Matric (CCVM) and recently completed activities on the applicability of uncertainty quantification methodologies to CFD in the context of nuclear reactor safety studies, design extension condition without significant fuel degradation (DEC-A), reactor pressure vessel integrity assessment for in-vessel retention and the state-of-art-report on behavior of combustible gases in severe accidents.

The Experimental Thermal Hydraulics for Analysis, Research and Innovations in Nuclear Safety (called ETHARINUS) project developed in the frame of the OECD Nuclear Energy Agency (NEA), serves the objectives of investigating complex thermal-hydraulics phenomena. Within this project, issues like the performance of passive heat removal systems and Design Extension Conditions (DEC) scenarios were investigated. The simulation of such events is of high importance to ensure relevant understanding of key thermal-hydraulic phenomena, to perform adequate safety analysis, to assess the efficiency of the adopted accident management procedures and to optimize operator training.

ETHARINUS project is highly relevant for the improvement and validation of thermal-hydraulic safety codes and their use, to maintain a high level of competence and expertise in the field of system thermal hydraulics. It is also a way to gather the actors within the area (safety authorities, operators, experimental facilities operators, university, R&D institutes, etc.) to develop knowledge and common culture about key safety issues, for the operating fleet and for innovative designs.

The objective of this paper is to describe the opportunities of the thermal-hydraulic research facilities employed for the activities, to briefly outline the tests programme, and finally to highlight the key safety issues and safety relevance of these tests programme. Such programmes are conducted in an international context, to share the costs and the expertise, and to promote quicker and deeper international consensus on safety issues. Recommendations are finally proposed regarding how to address the loss of critical research infrastructure (i.e. facilities, capabilities and expertise).

For several years, the OECD Nuclear Energy Agency (NEA) has conducted extensive experimental research through joint projects with broad participation from member countries. These collaborations enable shared costs and expertise, accelerating the global consensus on critical nuclear safety issues.

This paper presents key achievements from recent NEA joint projects on thermal-hydraulics. It outlines the capabilities of the research facilities involved, the critical safety issues addressed, the relevance of the test programs and related analytical activities. These projects aim to investigate phenomena where safety knowledge is insufficient, providing qualified data to develop and validate thermal-hydraulics computer codes used for nuclear safety assessment.

A major product is the experimental data itself, a priority for NEA members. Such data are then used for comparative code assessment to identify strengths and weaknesses of the codes.

Additionally, the paper highlights the importance of international cooperation in preserving unique experimental infrastructure, addressing the challenges of the closure of unique research facilities, and fostering the preservation of expertise while advancing new knowledge. A companion paper will offer insights into the new NEA Framework for Advanced Thermal-hydraulic Experiments for Nuclear Analysis and safety application (ATHENA).

The main outcomes of the following projects will be discussed:

• Advanced Thermal-hydraulic Test Loop for Accident Simulation (ATLAS)
• Experimental Thermal Hydraulics for Analysis, Research, and Innovations in Nuclear Safety (ETHARINUS)
• THAI Experiments on Mitigation Measures and Source Term Issues for Enhanced Severe Accident Management (THEMIS)
• PANDA Experiments Addressing Complex Safety Challenges for Current Water Reactors and Small Modular Reactor Projects