Performance of CANDU heat transport pumps under two-phase flow conditions

Heat Transport (HT) Pumps in a CANDU (CANadian Deuterium Uranium) reactor circulate heavy water coolant at high temperatures and pressures to remove fuel decay heat from the reactor core. Under a Loss-of-Coolant-Accident (LOCA), HT pumps are required to maintain their operational capability and structural integrity. Licensing requirements stipulate that HT pumps continue to operate under specific conditions, such as two-phase flow, for a specific duration (17 minutes). During this time period, decay heat has to be removed continuously from the reactor core before safe reactor shutdown is initiated. This paper presents a summary of the simulated two-phase now tests performed at the HT Pump supplier's test plant (Sulzer Bingham Pumps, Inc.) on a full-scale CANDU 6 HT pump-motor set. Results demonstrate that the HT pumps maintained their performance and structural integrity under the worst two-phase now conditions attainable on the test loop for the specific duration of 17 minutes. The pump hydrodynamic bearing, lubricated by the pumped fluid, did not suffer any notable damage. Despite the lack of external seal injection during two-phase now conditions, the mechanical seals showed no signs of damage.