A knowledge-driven licensing-focused roadmap for HLM research in Europe through a PIRT for ALFRED and MYRRHA reactors

The Phenomena Identification and Ranking Table (PIRT) is a structured and systematic methodology used to elicit expert knowledge, identify relevant physical phenomena, and rank them according to their safety significance and level of understanding. Initially developed for light water reactors, the PIRT approach has been successfully applied also to advanced reactor concepts, novel fuels, and components. In the framework of the Horizon-Euratom -2021-NRT-01-02 Call, the European Commission co-financed the Advanced Nuclear Safety Evaluation of Liquid Metal Using Systems (ANSELMUS) project, aiming to advance heavy liquid metal (HLM)-cooled fast reactor technology. One of the main activities in ANSELMUS is a coordinated PIRT exercise for the MYRRHA and ALFRED designs, selected as reference European systems for the HLM technology development. The initiative aims to create a harmonised, knowledge-driven assessment of the phenomena governing plant behaviour under both design-basis and design extension conditions, with the ultimate goal of supporting the identification of knowledge gaps and guiding future R&D efforts. The methodology consists of selecting reference, safety-relevant, scenarios for the given systems and then defining figures of merit linked to key safety functions and barrier integrity. Phenomena relevant to the selected scenarios and for the figures of merit are then listed and ranked according to their importance and state of knowledge. The process has been conducted through a series of expert-panel meetings, ensuring broad multidisciplinary coverage across reactor physics, thermal-hydraulics, fuel behaviour, and materials science. Numerical simulations also played a pivotal role in the PIRT for identifying key phenomena importance, complementing expert judgement and enabling consensus on complex or uncertain phenomena with limited experimental knowledge. Reference calculations as well as sensitivity studies have been conducted with a variety of codes encompassing system-scale codes like RELAP5-3D and SIMMER III and IV to study global behaviours or multi-physics aspects, and detailed CFD models to study in-detailed component behaviour or strong asymmetric situations and complex 3D flow patterns. The paper will summarize the PIRT exercise performed in the framework of the ANSELMUS project, combining the preliminary outcomes of expert elicitation and targeted numerical analyses so to inform the subsequent creation of a knowledge-driven and licensing-oriented R&D roadmap. This will further support the coordinated development and validation of computational tools and experimental programmes, ultimately strengthening the safety case and licensing readiness of next-generation HLM-cooled fast reactors in Europe.