05000263/LER-2011-004
| Monticello Nuclear Generating Plant | |
| Event date: | 06-08-2011 |
|---|---|
| Report date: | 08-08-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2632011004R00 - NRC Website | |
EVENT DESCRIPTION
The Reactor Building Ventilation system provides outside air to all levels and equipment rooms of the Reactor Building. Air pressure in the secondary containment (SCT) is maintained at a slight negative pressure by operating exhaust fans at a higher flow than the supply fans. The negative pressure, together with the integrity of the SCT minimizes exfiltration from the SCT.
The refueling floor ventilation is provided by redundant air handling units, V-AH-4A and V-AH-4B, which supply outside air. Only one supply fan is operated at a time to prevent positive pressure in the Reactor Building. During normal operation, the SCT differential pressure is controlled by throttling the variable inlet vane settings on the Reactor Building Exhaust fans V-EF-24A and V-E F-24B.
On 6/8/2011 at 0800, V-AH-4A was removed from service and V-AH-4B was placed in service. The SCT differential pressure became less negative; changing from approximately 0.6 inches water column (WC) vacuum to 0.17 inches WC vacuum. This did not meet Technical Specification surveillance requirement SR 3.6.4.1.1 for the SCT differential pressure which requires SCT pressure to be greater than or equal to 0.25 inches WC vacuum. Limiting Condition for Operation 3.6.4.1 was declared not met and the applicable action statement was entered. V-AH-4B was immediately removed from service and V-AH-4A was returned to service in order to meet SR 3.6.4.1.1.
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 because Limiting Condition for Operation (LCO) 3.6.4.1 was declared not met . The station reported the event to the NRC under 10 CFR 50.72 (b)(3)(v)(C and D) on June 8, 2011. However, the Reactor Building ventilation system, including the intake fans, will trip on a SCT isolation and would not have affected the integrity of the SCT envelope or the ability of the Standby Gas Treatment system to perform its safety function of drawing a 0.25 inches WC vacuum. Therefore, this event is not considered a Safety System Functional failure for the purposes of Reactor Oversight Process performance indicator reporting per the guidance in NEI 99-02.
SAFETY SIGNIFICANCE
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 failure to maintain SCT vacuum has no direct or indirect impact on the frequency of core damage (CDF). No systems supporting critical safety functions, including support systems, were impacted due to the loss of SCT, and initiating event frequencies were not impacted. Large Early Release Frequency is not significantly impacted since CDF is not affected, and the duration of the low SCT vacuum was very small. Based on the above, the safety significance was minor.
CAUSE
Although V-AH-4A and B are equally sized air handling units, significant differences in flow rates existed between them due to a seized reheat coil bypass damper on V-AH-4A. This resulted in significant swings in the SCT differential pressure during fan swapping before system dampers were repositioned to compensate. Following the previous fan swap that removed V-AH-4B from service and placed V-AH-4A into service on June 5, 2011, the Reactor Building exhaust fan variable inlet vane settings had been throttled closed from 80% to 55% to lower Reactor Building differential pressure to compensate for the lower air throughput of V-AH-4A. It was not recognized that when V-AH-4B was returned to operation for post maintenance testing on June 8, 2011, that the incoming Reactor Building supply air flow would increase and the previously throttled variable inlet vane setting would prevent the Reactor Building exhaust fans from maintaining the Reactor Building greater than 0.25 inches WC vacuum. Although the seizing of the reheat coil bypass damper had been identified in January 2011, the effect was masked somewhat during fan swaps during cold weather because the reheat face dampers were free to modulate as heating needs required. During warm weather the reheat face dampers are closed and no longer modulate flow.
CORRECTIVE ACTIONS
1. The bound reheat coil bypass damper on V-AH-4A will be reworked.
2. A procedure change has been generated to add precautions to the operating procedure for transferring refuel floor fans.
PREVIOUS SIMILAR EVENTS
On February 11, 2011 secondary containment was declared inoperable due to ice buildup on a SCT isolation damper (LER 2011-003).