Fluid retention in a tank with a horizontal positively buoyant jet

In the boron dilution process, the volume control tank in the Chemical and Volume Control System of nuclear power plants has a retention effect on the demineralized water which is injected into the tank as a horizontal positively buoyant jet. The fluid retention hinders the boron concentration in the reactor coolant system from reaching the target value, eventually destabilizing the reactor power. Minimizing retention is consequently essential. However, there are few studies on fluid retention.The current study focuses on studying the fluid retention in the tank with a horizontal positively buoyant jet. An experimentally validated numerical model was used to simulate the flow and mixing inside the tank. A retention coefficient was introduced to compare fluid retention quantitatively. The effects of the operating conditions and tank configuration (inlet flow rate, concentration difference, and nozzle diameter) on fluid retention were studied in the non-stratified and stratified patterns. The results show that the effect of these parameters on fluid retention is negligible in the non-stratified pattern. However, in the stratified pattern, the increase of the inlet flow rate or the decrease of the concentration difference/nozzle diameter leads to the shorter delay stage and weaker fluid retention in the decline stage. Furthermore, the effect of the stratification Richardson number (RiSF) on the fluid retention of the tank with fixed geometric parameters is analyzed. The retention coefficient at the characteristic time always increases as the RiSF increases. Meanwhile, the coefficient increases very slowly in the non-stratified pattern. In the stratified pattern, it rises rapidly first and then slowly. Thus, a smaller RiSF is not necessary to reduce fluid retention when the mixing pattern is already the non-stratified pattern in practice. A criterion for pattern transition between stratified and non-stratified is pro-posed for practical purposes.