05000263/LER-2011-003
| Monticello Nuclear Generating Plant | |
| Event date: | 02-11-2011 |
|---|---|
| Report date: | 03-30-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2632011003R00 - NRC Website | |
Operations started Reactor Building (NG) 1027 Supply Fan V-AH-4A (FAN), on February 11, 2011, at 0120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br />. Secondary Containment (SCT) differential pressure became more negative. Operators adjusted Reactor Building exhaust fan flow to compensate. Operations then identified that V-AH-4A , did not have adequate flow. V-AH-4A was secured and Reactor Building 1027 Supply Fan V-AH-4I3 was started, restoring normal differential pressure.
V-D-61 (Reactor Building Outboard Isolation Damper) (DMP) was discovered frozen by ice in the closed position, with its actuator shaft bent and the linkage between the shaft and crank arm disconnected. The corresponding inboard damper, V-D-62 (DMP), was found blocked partially open, again due to icing. Investigation revealed that moisture released from a recently repaired steam leak on the V-AH-4A preheat steam coil condensed and, combined with low temperatures, resulted in the dampers freezing.
Since a SCT penetration flow path with two isolation valves inoperable condition existed, Limiting Condition for Operation (LCO) 3.6.4.2 was declared not met. Technical Specification Condition 3.6.4.2.B was entered for one or more penetration flow paths with two isolation valves inoperable.
The ice was removed and V-D-62 was verified closed at about 0354 hours0.0041 days <br />0.0983 hours <br />5.853175e-4 weeks <br />1.34697e-4 months <br />, isolating the affected penetration flow path by use of at least one closed and de-activated automatic valve and satisfying the required action of Condition 3.6.4.2.B. Repairs were completed, the system retested, and LCO 3.6.4.2 was met on February 11, 2011 at about 2001 hours0.0232 days <br />0.556 hours <br />0.00331 weeks <br />7.613805e-4 months <br />.
On March 11, 2011, with Secondary Containment not required, the site tested V-D-61 with the actuator disconnected (event condition), V-AH-4A operating, and without ice holding the damper closed. Under these test conditions, the damper was subjected to a higher differential pressure than would be seen during Standby Gas Treatment System operations. The test demonstrated that the damper would not open under these conditions, thereby validating that the safety function of the damper was maintained throughout the event.
EVENT ANALYSIS
The event is reportable to the NRC under 10 CFR 50.73(a)(2)(v)(C and D) — Event or Condition that could have Prevented Fulfillment of a Safety Function. The site reported the event to the NRC under 10 CFR 50.72 (b)(3)(v)(C and D) on February 11, 2011.
This event is considered a Safety System Functional Failure due to entering an unplanned SCT TS LCO not met condition. However, subsequent testing and an engineering evaluation have demonstrated that the Secondary Containment safety function was maintained, as at least one of the two dampers would have remained in the closed position.
There were no nuclear, radiological or industrial safety significant consequences related to this event.
The Monticello risk assessment group reviewed the event for risk impact. The position of the dampers for Secondary Containment has no direct or indirect impact on the frequency of core damage (CDF).
For the period of time the dampers were inoperable, the conservative assumption was made that all core damage events caused a large early release event. The incremental large early release probability was calculated to be 2.07E-9, well below the risk significance threshold of 1E-7. Therefore, the conclusion is that the safety significance in terms of reactor safety and radiological release to the environment from this event is considered to be very low.
CAUSE
This was caused by a combination of condensation from a steam leak from a heating coil and low outdoor ambient air temperatures, which resulted in the dampers freezing in position. The steam leak from the heating coil was caused by an incorrect installation which had been repaired on February 10, 2011.
CORRECTIVE ACTION
The following actions were taken or are planned and will be tracked in the Monticello Corrective Action Program:
- The ice was removed from the ventilation dampers and V-D-62 was closed.
- The actuator for V-D-61 was repaired.
- V-D-61 and V-D-62 were tested satisfactory for proper operation.
- V-D-61 was tested in the conditions it was found in (closed with the actuator disconnected) except there was no ice holding the damper closed. The test proved that a differential pressure would not cause the damper to open and validated the safety function of the damper was maintained throughout the event.
- The steam coil proper installation information will be entered into maintenance documents.
PREVIOUS SIMILAR EVENTS
There have been no similar events in the last three years. However, the site has had recent events related to secondary containment. On June 3, August 5, and November 4, 2010 the site had events which momentarily degraded secondary containment having to do with interlock doors.
(LER 2010-002, 2010-003, and 2010-004)