Thermal mixing and stratification in the liquid metal-cooled reactor pool experiment E-SCAPE

Natural circulation in the primary system is the chosen option in terms of decay heat removal (DHR) for the liquid–metal cooled, pool-type research reactor MYRRHA, keeping temperatures in the core within the safety limits and maintaining structural integrity of the coolant boundaries. Experiments in E-SCAPE, the 1/6-scale 3-D model of the primary system of MYRRHA, demonstrate that in DHR conditions, a buoyancy-driven natural circulation flow establishes and is capable to remove the core power with temperatures that are within acceptable limits. The effects of the core power, the system pressure losses and the initial conditions on the flow and the temperatures are analyzed. The experiments show that, in natural circulation thermal stratification is present in the upper plenum. From forced circulation experiments in the same facility, one can conclude that good thermal mixing is obtained for forced flow rates above 40% of the nominal flow rate. Below this value, buoyancy becomes important and thermal stratification is observed in forced flow conditions as well. Results demonstrate that the Richardson number in the upper plenum Ri_UP can be used as an indicator for thermal stratification. For values below unity, good thermal mixing is observed. If Ri_UP is higher than 1, thermal stratification occurs.