Transient behaviors of heavy water reflector system during postulated initiating events (PIEs)

This study investigates the transient thermal-hydraulic behaviors of a heavy water reflector system during various postulated initiating events (PIEs). The following events are considered in the analysis: (1) loss of heavy water system (HWS) flow, (2) heavy water leakage, (3) loss of secondary cooling flow, and (4) dilution of heavy water. The sequence of each event is described, and thermal-hydraulic parameters such as the heavy water temperature, system pressure, flow rate, and heavy water level are monitored. To regulate the HWS, the heavy water temperature at the outlet of the heavy water vessel, the heavy water level in the expansion tank, and the flow rate in the HWS are used as the reactor regulating system (RRS) setbacks. In the event of loss of HWS flow, the standby pump kicks off to recover it, and none of the RRS setback signals are triggered. In the event of heavy water leakage, a low-level RRS setback signal trips the reactor, and the temperature rise in the HWS is not significant. The total amount of heavy water leakage with a break at the outlet of the heat exchanger is approximately 550 L. In the event of loss of secondary cooling flow, a high-temperature RRS setback signal trips the reactor. In the event of heavy water dilution, a low-level RRS setback signal trips the reactor. In the case of a pipe rupture at the highest point in the HWS inside the pool, the RRS setback signal is triggered at 83.9 s from the initiation. At the instant of the reactor trip, the reactor power increases to 150% of full power because of the degraded purity of the heavy water--95.7%. For the event of heavy water dilution, a reactor protection system (RPS) setback such as the reactor power difference between a neutron detector and a gamma detector should be used to trip the reactor.