De analyse van thermohydraulische fenomenen in de zwaar-metaalgekoelde pooltype reactor MYRRHA in geforceerde en natuurlijke circulatie

Various innovative nuclear systems and advanced nuclear fuel cycle options are currently being proposed to enhance the sustainability of nuclear energy, particularly in terms of resource utilization and waste management. To support the advancement of fast neutron systems, the MYRRHA project was initiated at SCK CEN, the Belgian Nuclear Research Centre. MYRRHA is a versatile irradiation facility aimed at both fuel and material research, as well as demonstrating the viability of Accelerator Driven System (ADS) technology for the transmutation of long-lived nuclear waste. It features a subcritical nuclear reactor powered by a high-power linear particle accelerator. At the interface between the reactor and accelerator, a spallation target generates neutrons through spallation, which are then multiplied in the reactor core. MYRRHA employs Lead-Bismuth Eutectic (LBE), a heavy liquid metal, as both the primary reactor coolant and the spallation target. One of the core principles of MYRRHA's design is the use of passive systems to ensure safety functions. Natural circulation in the primary system has been selected as the method for decay heat removal (DHR), keeping core temperatures within safety limits while maintaining the structural integrity of the coolant boundaries. The primary, secondary, and tertiary cooling systems of MYRRHA serve as the main DHR systems in the event of an accident. Understanding the thermal-hydraulic phenomena in the upper and lower plena of a reactor pool is crucial for safety analysis, structural integrity assessment, and design optimization. Thermal stratification is an important phenomenon in natural-circulation decay heat removal. In a Lead Fast Reactor (LFR), for example, stratified liquid metal layers can form in the upper plenum during a loss of flow (LOF) transient. Research indicates that these stratified layers may hinder the development of natural circulation during LOF accidents, thereby reducing the effectiveness of passive emergency cooling systems. From a structural perspective, thermal stratification could induce thermal stresses and fatigue in the reactor vessel and its internal components, potentially compromising reactor safety. This PhD research addresses the following questions through experimental analysis using an integral thermal-hydraulic reactor scale model, E-SCAPE (European SCAled Pool Experiment, scale 1:6): - Can core cooling in the MYRRHA reactor be ensured under both normal and accidental (LOF) conditions, including passive decay heat removal? - What parameters affect the balance between thermal mixing and stratification in the upper plenum of the MYRRHA reactor? Additionally, experimental data will be generated to validate thermal-hydraulic codes for their application in a pool-type reactor configuration with LBE as coolant. E-SCAPE is designed using a multilevel scaling approach based on nondimensional similarity analysis, supported by simplified analytical modeling of the MYRRHA reactor prototype and the scaled facility. The scaling method for E-SCAPE focuses on preserving the overall system behavior and replicating key thermal-hydraulic phenomena during decay heat removal conditions. In natural circulation, steady-state integral system behavior is matched by ensuring similarity in Richardson, Euler, and Heat Source numbers. In forced circulation, Richardson similarity is not necessary. The scaling approach places particular emphasis on stratification phenomena in the upper plenum and the mixing effects caused by jets entering the plena. Experimental data from steady-state isothermal operation provide insights into pressure losses in the main vessel and external circuits. The relationship between pressure loss in the core and the Above Core Structure (ACS), and the level difference between the upper and lower plena is confirmed. The hot-plug experiments, which use temperature as a flow tracer, provide valuable insights into the flow patterns in forced-circulation mode without the need for direct velocity measurements. These experiments reveal the behavior of pump jets entering the lower plenum, showing a distinct difference between high and low flow rate cases in terms of penetration depth and radial distribution. In the low flow rate scenario, recirculation is evident in the lower plenum. Furthermore, the LBE exiting the core moves upwards through the ACS before distributing radially in the uppermost fluid layers. From these layers the fluid then flows downward in the upper plenum toward the primary heat exchanger inlet windows. From steady-state forced-circulation experiments with core heating, it can be concluded that E-SCAPE achieves good thermal mixing for flow rates above 40% of the nominal flow rate at 7% of the nominal power (i.e. decay heat power level immediately after SCRAM). Below the 40% flow rate, buoyancy effects become significant, leading to thermal stratification in the upper plenum and, through conduction, in the annular region between the internal diaphragm and the vessel. The hot LBE exiting the core flows vertically upward in the ACS before spreading radially in the upper plenum. The core bypass heating has only a minimal impact on temperatures, confined to the core outlet. There are no temperature differences across the horizontal planes in the upper plenum, and the temperatures in the lower part of the lower plenum are uniform. From steady-state natural-circulation experiments, it can be concluded that a buoyancy-driven natural-circulation flow regime is established, which is capable of removing the decay heat with acceptable temperatures. The natural-circulation flow rates increase with rising power and decrease with increasing pressure losses in the system, while the core outlet temperature shows the opposite trend. Thermal stratification is present in all cases in the upper plenum and, through conduction, in the annular region between the diaphragm and the vessel. From the core outlet, the hot LBE flows vertically upwards in the ACS before spreading radially in the upper plenum. Due to core bypass heating, the core outlet temperature increases slightly in the core region but more significantly in the core bypass region. There are small temperature differences in each horizontal plane in the upper plenum, and temperatures in the lower part of the lower plenum remain uniform. The initial conditions have no effect on the final flow and temperature fields. More generally, the Richardson number in the upper plenum serves as an indicator for thermal stratification in this region. When the Richardson number ( ) is below unity, good thermal mixing is observed, while values above one lead to thermal stratification. In E-SCAPE, flow rates above 40% of the nominal flow do not induce thermal stratification, whereas stratification is expected for all flow rates below 20% of the nominal flow. Following the scaling approach used for E-SCAPE, these findings can be extrapolated to MYRRHA. The Péclet number in E-SCAPE is smaller than that expected in MYRRHA. Comparing the E-SCAPE temperature profiles in stratified cases with those obtained from the water-cooled MYRRHABELLE facility, it becomes evident that the Péclet number influences the temperature gradient in the upper plenum. Simplified analytical models of the reactor prototype and the scaled facility show that E-SCAPE offers a configuration that accurately represents MYRRHA, enabling the study of key physical phenomena observed during loss of flow transients. In all transient LOF experiments, a smooth transition from forced to natural circulation is observed, with stable lower flow rates being established. Decay heat is effectively removed from the core, as the maximum core temperatures recorded during the LBE transients remain within an acceptable range. The final steady state in natural circulation reveals significant thermal stratification of LBE in the upper plenum. During the initial phase, which is dominated by mass inertia, the LBE free surface levels in the lower and upper plenums quickly equalize. In both circuits, the LBE flow rate reaches a minimum value before increasing again, driven by buoyancy forces, and ultimately stabilizes over a longer time span, influenced by the heat capacity of the LBE system. A notable observation is the increase in temperature at the inlet of the core bypass region, which may indicate a flow reversal in this area during the initial phase of the transient. The impact of system pressure losses and heating power on flow rates and temperatures has also been analyzed. With the extensive instrumentation, including 320 thermocouples installed in the main vessel, the experiments are designed for direct comparison with numerical simulations. Given that the MYRRHA reactor design has evolved since the commissioning of the E-SCAPE facility, validating simulation tools under representative conditions is critical for the safety assessment and licensing process. Multiple STH (System Thermal-Hydraulic), CFD (Computational Fluid Dynamics), and coupled STH/CFD models of E-SCAPE have been developed for scaling validation, design, pre-test and post-test analyses. These simulations have been conducted at SCK CEN and in collaboration with European partners. The results of the simulations have been compared with experimental data, with the overall findings showing good qualitative agreement, confirming that the phenomena driving the flow are accurately represented in both the numerical models and the experimental facility. However, there are some quantitative discrepancies between experimental and numerical results. Potential sources of these discrepancies have been identified, offering pathways for further refinement and development.