Neutronic and thermal hydraulic analysis of ALFRED during Unprotected transients considering the reactor staged approach

Generation IV nuclear reactors represent the next step in nuclear technology, designed to enhance safety, sustainability, efficiency, and cost-effectiveness compared to their predecessors. These advanced reactors aim to overcome the limitations of current nuclear power systems, offering long-term energy solutions while mitigating environmental impact and improving waste management. Among the various Generation IV designs, ALFRED (Advanced Lead Fast Reactor European Demonstrator) stands out as a promising project. ALFRED is a lead-cooled fast reactor (LFR), one of the six reactor types identified by the Generation IV International Forum (GIF) as having the potential to meet the ambitious goals set for future nuclear energy systems. The reactor leverages the benefits of liquid lead as a coolant, including excellent thermal properties, a high boiling point, and inherent safety features. These characteristics allow operation at high temperatures and low pressures, reducing accident risks and enhancing overall safety. ALFRED represents a collaborative European effort under the FALCON Consortium to develop a demonstrator for lead-cooled technology, focusing on economic competitiveness and strict safety standards. While LFR technology offers significant advantages, it also introduces challenges—such as the high freezing temperature of lead (327 ◦ C), which is particularly critical in accidental scenarios where decay heat must be removed while preventing coolant solidification. Additionally, lead’s opacity complicates refuelling operations, requiring further research and development (R&D) efforts. In this context, critical R&D gaps exist in the analysis of ALFRED under unprotected transient scenarios, where limited numerical investigations are available. The complex coupling between neutronics and thermal-hydraulics in such conditions necessitates high-fidelity modelling to capture key physical phenomena that remain insufficiently explored in the literature. To address these gaps, the present work conducts detailed numerical simulations, providing valuable insights into the reactor’s response under transient conditions. By leveraging advanced computational techniques, this study improves the understanding of ALFRED’s safety-relevant behaviour, supporting the design and the future experimental campaigns and refining modelling strategies. For this study, three unprotected transients were selected: Unprotected Loss of Flow (ULOF), Unprotected Loss of Heat Sink (ULOHS), and Unprotected Transient Over-Power (UTOP). This paper presents the results of these scenarios, analysing ALFRED’s behaviour under each condition. The transient simulations were performed using the RELAP5/Mod3.3 thermal–hydraulic system code, with a detailed description of ALFRED’s nodalization scheme, transient boundary conditions, and transient evolution.