Conference Agenda

The IAEA Coordinated Research Project (CRP) ‘Benchmark of Transition from Forced to Natural Circulation Experiment with Heavy Liquid Metal Loop (NACIE)-CRP-I31038’, is a benchmark based on experimental data provided by the Heavy Liquid Metal (HLM) loop NACIE-UP (NAtural CIrculation Experiment- UPgraded) located at the ENEA Brasimone Research Centre. The primary system of the NACIE-UP facility consists in a rectangular loop which allows performing experimental campaigns in the field of thermal-hydraulics, fluid-dynamics and heat transfer of HLM. The primary loop is composed of two vertical pipes, working as riser and downcomer, hydraulically connected by two horizontal pipes. The facility includes also an ancillary gas system and a pressurized water secondary side for the heat removal from the primary loop. The test section for the experiments consists of a 19 electrically heated Fuel Pin Simulator (FPS) arranged in 3 ranks with a triangular pitch. The pins are placed on a hexagonal lattice by a suitable wrapper, while the wire spacer is adopted. The main objective of the performed experimental campaign was to perform integral system and local thermal-hydraulic analysis. Moreover, some of the performed tests were characterized by non-uniform heating of the bundle. The benchmark is divided in an open phase with cases ADP10 and ADP06 and a ‘blind’ phase with an active sector in the FPS ADP07. The benchmark is divided into 5 Work Package: WP1-System Thermal Hydraulics, WP2-Computational Fluid Dynamics, WP3-Subchannel Analysis, WP4-Multiscale Analysis, WP5- Uncertainty Quantification. In the paper, the different experimental test cases with boundary conditions are presented.

Benchmarking codes and methodologies against experimental data increases credibility of tools for liquid metal reactor design. Decades of experience have been gained during past and ongoing EU projects at KIT investigating liquid metal thermo-hydraulics. (CRP) ‘Benchmark of Transition from Forced to Natural Circulation Experiment with Heavy Liquid Metal Loop (NACIE)-CRP-I31038’, is used for the verification of predictive capabilities of our modelling approach. The experiments test section consists of 19 electrically heated wire wrapped fuel pin simulator, arranged in 3 ranks with a triangular pitch. The benchmark offers in the open phase data for symmetric-heated forced and natural convection cases, which we analysed. These results showed very good local agreement and were published.

Appreciable effects of asymmetrical heating for forced and natural convection only become relevant in the new blind study presented here. The comparison of experimental data to all participants solutions will be presented in the NURETH21 in a separate paper. This work concentrates on the employed model, uncertainty quantification and comparison of our blind case results to the previous symmetric cases and experimental data.

The IAEA Coordinated Research Project (CRP) ‘Benchmark of Transition from Forced to Natural Circulation Experiment with Heavy Liquid Metal Loop’, provides an opportunity to validate and improve thermal-hydraulic analysis codes used for simulating heavy liquid metal systems. The benchmark consists of two open cases to be used for verification of computational models – one each with uniform and non-uniform symmetric heating. After this first phase of the CRP, a blind case with asymmetric heating will be used to validate the accuracy of the models.

Within the CRP, one work package is devoted to Computational Fluid Dynamics (CFD) benchmark for the Fuel Pin Simulator (FPS), which represents a prototypical wire-spaced fuel pin bundle. Unlike system and subchannel codes, CFD is capable of resolving a detailed three-dimensional model of the FPS providing better a representation of the friction and heat transport phenomena involved, albeit at higher computational costs. The CFD benchmark is limited to the two steady states for each case, before and at the end of the forced-to-natural circulation transient. The benchmark participants use different CFD codes, implementing different sets of physical and/or numerical models. The present paper reports the collective CFD results obtained by the participants and comparisons with the experimental data. The comparison, here, is focused on temperature predictions for 67 thermocouple locations inside the FPS. The first phase of the benchmark provides an insight into the efficacy of different modelling strategies considered, and highlights the need of further investigations to improve the modelling of liquid metal-cooled wire-spaced bundles.

Recently there has been a notable increase in interest in Small Modular Reactors (SMRs) and Micro Modular Reactors (MMRs) due to their potential to improve energy supply reliability and reduce carbon emissions in isolated power grids. MMRs use high-temperature heat pipes, which typically employ liquid metals such as potassium or sodium as the working fluid, to extract heat from the reactor core. The MISHA research project, funded by BMBF, seeks to expand expertise in the application of heat pipes as the primary heat transfer mechanism in MMRs. This project includes the construction and the testing of full-scale high-temperature heat pipes using a newly established modular Heat Pipe Tester (HPT). The HPT's flexible design allows for testing heat pipes of different sizes and under various conditions, providing a thorough evaluation of their performance and advancing knowledge of heat pipe efficiency in various settings. Moreover, the experimental results will be used for the further development and validation of the GRS nuclear safety code system ATHLET.

The first heat pipe with reduced length of 2 m has been assembled, filled with potassium, and sealed. While the main HPT is still under construction, initial tests have been conducted using a smaller version of the tester. The heat pipe was tested at temperatures reaching up to 850°C, with up to 4 kW of power supplied. The behavior of the heat pipe during startup, steady-state operation, and cool-down phases was monitored and analyzed. Results have been compared to findings from other experiments documented in the literature.

This paper outlines recent and ongoing collaboration efforts between Canadian Nuclear Laboratories (CNL) and Korean Atomic Energy Research Institute (KAERI) on nuclear reactor safety research and development (R&D). The collaboration was motivated by a memorandum of understanding (MOU) between KAERI and Atomic Energy of Canada Limited (AECL) to work together in an innovative nuclear R&D partnership, with a broad focus including the safe deployment of small modular reactors (SMRs). The strategic drivers for SMR development and deployment in Canada are described by the Government of Canada’s SMR Roadmap and Action Plan (2018, 2020), which recognize that Canada needs viable and clean energy sources for different applications, and these needs could be supported by various types of SMRs. Korea’s SMR R&D priorities are described based on national programs actively promoting the securing of core technologies for the development of next-generation nuclear reactors. KAERI’s R&D covers a wide spectrum of scientific, engineering, and technical activities, supported by the utilization of large research facilities including the Advanced Thermal- hydraulic Test Loop for Accident Simulation (ATLAS), which is an integral effect test facility. The discussion herein focuses on topics of thermal-hydraulics for the development and safe deployment of SMRs, including appropriately scaled integral and separate effects experiments and analysis. Topics include core flow distribution, instabilities of two-phase natural circulation, and passive safety systems including the participation of CNL in the ATLAS-4 project led by KAERI starting in 2025, as well as the associated activities, schedules and expected outcomes.