 INES 2 |
PRESSURIZER DRAIN VALVE FAILURE CAUSING HEAT TRANSPORT SYSTEM LEAKS AND FORCED OUTAGE |
At 16:26, with the unit at full power, a heat transport (HT) leak due to a passing pressurizer drain valve was detected, allowoing hot D2O to flow directly into the HT collection system. The 0.5 kg/s leak filled the collection system with hot D2O and steam which backed up collection lines to other parts of the HT system, including the feed pumps. A total of 10m3 of D2O was spilled into several areas of the reactor building.
A controlled shutdown of the unit was initiated at 18:20 with cooldown and depressurization achieved by 00:30 the next morning. A radiological alert was declared at 19:00 with airborne tritium concentrations in the reactor building at about 800 MPCa. Following manual containment isolation reactor building pressure rose to 2.28 kPa. With the authorization of the advisory group the containment pressure was vented through driers. External release of tritium for the day following the event was estimated at less than 1% of the Derived Operational Limit.
The leakage of a large amount of hot D2O to the HT collection system is a common mode failure because of the effect on many important systems which tie into the collection system. Damage to a number of HT pump seals and external leakage resulted.
Justification of Rating:
Rating based on Degradation of Defence in Depth (no significant off-site or on-site impact). Initiator frequency - expected. Reactor coolant system leakage that would not prevent a controlled reactor shutdown and cooldown.
Safety function availability - Within Operational Limits and Conditions (see below)
Basic rating: INES level 1. Anomaly beyond the normal operating regime.
INES users' manual Table II indicates a choice of level 1 or 2.
Level 1 was chosen as safety function was closer to "full" than just adequate.
Uprating: the rating was raised to INES level 2 due to the following additional factors.
- Common cause failure - effects on process systems, and possibly safety functions, due to migration of steam and water to different area of the reactor building via the collection system.
- procedural inadequacies - procedures and conception did not cover failure mode of hot water return from the collection system, which resulted in a non-conservative decision to continue operating at full power with excessive coolant leakage and complicating effects on process and safety systems.
Safety function availability:
1. Reactivity control - Within Operational Limits and Conditions
No apparent effect, reactor was shut down in a controlled fashion with no action by shutdown systems required. However, degradation of control/shutdown capability may have occurred due to migration of steam and water to different areas of the reactor building via the collection system.
2. Containing Radioactive Material - Within Operational Limits and Conditions
- No apparent effect on containment, external radioactivity releases were well within operating limits.
- The degradation of the primary heat transport pressure boundary due to the leakage through the pressurizer drain valve and the pump seals is a process system failure.
3. Cooling radioactive material - Within Operational Limits and Conditions
- Safety function for fuel cooling was apparently available but not required. Possible degradation due to migration of steam and water in reactor building.
-Process systems for fuel cooling were impaired as follows:
- only one of two shutdown cooling pumps available
- excessive seal leakage on available shutdown cooling pump
- damage to both PHT feed pumps seals (external leaks), P2 shutdown, P1 started. Main HT pumps available but would require shutdown if pressure control lost due to failure of both pressurizing pumps.
Event date: Mon, 04-09-1995
