 INES 2 |
SMALL LOCA DUE TO REACTOR COOLANT RELIEF VALVE FAILURE |
Emergency coolant injection was required after an instrumented reactor coolant pressure relief valve opened due to failure of its air supply line. When the valve opened, discharging coolant to the bleed condenser, the reactor automatically began to setback from full power on high bleed condenser level. After a minute it tripped at 75% reactor power on low coolant pressure. The coolant flow from the valve filled the bleed condenser, which went solid. At about 6 minutes the turbine was manually tripped when it was noted that it had only unloaded to 60%. A rapid rise in heat transport pressure resulted, causing the bleed condenser relief valve to open to the boiler room sump. This pressure spike likely caused the relief line to break at one of the bleed condenser RVs, resulting in a small LOCA and a rise in boiler room pressure. At about 9 minutes the Emergency Coolant Injection System (ECI) automatically actuated. Containment isolation of the unit was manually initiated since the automatic setpoint is not reached for pipe breaks of this small magnitude. At 90 minutes operators entered the unit end manually closed the failed pressure relief valve, stopping the leakage through the broken relief line. About 150 Mg of water was pumped into the unit from the ECI storage tank, resulting in sumps inside the unit being filled with downgraded heavy water. At 4 hours the unit was in a stable configuration, with the coolant system depressurized in the normal shutdown cooling mode, with ECI isolated.
No abnormal readings from air or waterbourne missions were reported. Following environmental sampling an analysis of coolant chemistry indicated that no fuel failures resulted from this event.
Pickering unit 4 was shut down until implications of the failure are evaluated. Units 1 and 3 were already in the shutdown state.
The rating is INES level 2 on degradation of defence-in-depth, an incident with significant failure in safety provisions. Table II indicates level 2/3 for the high/expected initiator "loss of reactor coolant pressure control due to failure of a relief valve", combined with adequate safety function. Level 2 was chosen as safety function redundancy remained to cope with further failures.
Event date: Sat, 10-12-1994
