Flow pattern transition instabilities in a natural circulation cooling facility

Our research analyzed results from a two-phase natural circulation test facility designed to study the performance of reactor cavity cooling systems, and determined the presence of a flow transition instability. Reactor cavity cooling systems are passive safety system's designed to remove decay heat from high temperature generation IV reactors. Water-based designs achieve this function by boiling-off a set water inventory held in a storage tank. During normal operation under accident conditions, the two-phase natural circulation facility undergoes a transition from stratified to intermittent flow regimes as the water level in the storage tank is lowered due to inventory evaporation. The transition is accompanied with an oscillatory flow rate due to the differing pressure drops between the flow regimes. The instability ceases after fully transiting into the intermittent flow regime. The presence of the transition was determined with a steady-state drift flux model and wire-mesh sensor data.