Small LOCA Analysis in Support of the Automatic PHTS Pump Trip Design for Point Lepreau NGS

Abstract

Operation of the Point Lepreau NGS PHTS pumps under cavitation conditions will result in vibration of the pumps. These conditions are predicted for postulated events such as lora of coolant accidents. Consequential vibration of the main heat transport system piping may be caused by fluid flow instabilities or by transmission of pump vibrations through connected piping. Although some vibration is acceptable in emergency situations, sustained vibration could jeopardize the integrity of the heat transport system boundary. NBEPC is installing an automatic pump trip which will preclude continuous pump operation under cold cavitation conditions. This will ensure piping integrity and avoid an unnecessary burden on the operator to trip the pump a in a timely manner following a loss of coolant accident. Safety analyses of LOCAs with Class IV power available had previously considered the continued operation of the PHTS pumps. It was therefore necessary to confirm the adequacy of fuel cooling in the absence of forced circulation. A systematic review of the accident analysis matrix was performed (1). This identified specific events which required detailed study. These were large LOCA, steam main failure end small LOCA. However, extensive re-analysis was performed for small LOCA. since channel stagnation could occur following pump rundown. The small LOCA analysis reported in Reference 3 demonstrated: 1. Heat removal by steady single phase density driven flow in the intact loop and the unbroken pass of the broken loop; 2. Heat removal by intermittent flow in the broken pass for breaks above 10.8 cm2 (0.5% RIH) immediately following pump trip. For smaller breaks, steady density driven flow develops in the short term and intermittent flow occurs in the longer term. Bidirectional thermosyphoning establishes for some break sizes. For most breaks, reverse flow establishes in the broken para after a few days; 3. A pressurization period can follow pump trip in the intact loop, due to the thermal imbalance during the transition to density driven flow. Peak pressures will remain well below the LRV opening setpoint of 10.34 MPa(a) provided PHTS pump gland seal injection is isolated following pump trip. In summary, the small LOCA analysis has shown that the proposed pump trip design would not produce a negative influence on the heat removal capability following ECCS initiation. This same conclusion applies equally to the other accident scenarios analyzed.