The analysis of Thermal-Hydraulic Phenomena in the MYRRHA Heavy Liquid-Metal Pool Reactor in forced and natural circulation

The MYRRHA project, based at the Belgian Nuclear Research Centre (SCK CEN), is an innovative nuclear energy initiative focused on using Accelerator Driven System (ADS) technology to help manage nuclear waste. MYRRHA aims to provide a solution for long-lived nuclear waste transmutation and operate as a versatile research facility for nuclear fuels and materials. Key aspects of its design are the use of liquid metal lead-bismuth as primary coolant and of passive safety principles, like natural circulation, to ensure the core remains safely cooled, even in accidents where primary pumps might fail. One of the important challenges in the design of the MYRRHA reactor is managing the thermal behavior, especially in the upper part of the reactor (the “upper plenum”). When the reactor is running at low flow rates, e.g. in loss of flow (LOF) accidents, the temperature differences in this region can create thermal stratification where hot and cold liquid metal layers do not mix well. This stratification can hinder cooling and lead to material issues, impacting safety. To better understand these phenomena, researchers use the E-SCAPE experiment, a scaled model of the MYRRHA reactor. By simulating the thermalhydraulic behavior of MYRRHA at a smaller scale, they can study the core cooling in forced and natural circulation. They can also investigate how thermal stratification happens under various conditions, both in normal operation and in accidental scenarios. The experiments show that the core cooling can be guaranteed in LOF accidents. When the flow rate drops below 40% of its normal level, thermal stratification becomes important. The experiments also help to validate advanced numerical simulations that model reactor thermal-hydraulic behavior. These simulations will play an essential role in MYRRHA’s final design and safety assessments. While there are some small differences between the experimental results and the simulations, the overall findings are promising and provide valuable insights into improving the reactor’s design and safety features. In summary, this research is essential for ensuring the safe operation of MYRRHA, even under challenging conditions like LOF accidents. By better understanding natural circulation and thermal stratification, the project helps paving the way for safer and more efficient nuclear energy systems.