05000263/LER-2009-003
| Docket Number | |
| Event date: | 05-12-2009 |
|---|---|
| Report date: | 07-13-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2632009003R00 - NRC Website | |
Event Description
On 05/12/2009 at 21:30, the Monticello Nuclear Generating Plant was in Mode 1 increasing power from 30 to 50% following a refueling outage. During the increase, the "B" steam flow indication read low compared to the other Main Steam [SB] Line (MSL) flow indications. l&C Maintenance investigated and determined that although the indicator [Fl] was within the acceptable calibration range, it was slightly lower than expected. The instrument was adjusted however, the adjustment did not correct the unexpectedly low indication Operations entered an action statement pertaining to Technical Specification 3.3.6.1 due to the apparent failure of 'EY Main Steam Line Flow Isolation Sensor. The action statement was related to the loss of safety function due to the 'B' MSL flow isolation indication reading approximately 25% lower than the other three flow isolation indicators and showing signs of continuing to degrade.
A partially open equalizer valve can cause the differential pressure across all of the indications on a common line to degrade, causing abnormally low indication. A maintenance technician, as instructed, attempted to manually tighten all of the manifold equalizing valves [ISV] on the `BI MSL flow indications to determine whether a mispositioned valve may have caused leakage. Although none of the equalizing valves felt open, the technician noted that one of the manifold equalizer valves was warm to the touch. Further investigation 15 minutes later revealed that the manifold equalizer valve had heated significantly. An infrared temperature camera revealed that the manifold body was approximately 470°F.
From this information, Operations duty crew concluded a manifold equalizing valve was leaking by. The Shift Supervisor directed isolation of the manifold immediately using the manifold isolation valves [ISV]. "B" MSL flow isolation and steam flow indication returned to normal following the isolation and on 05/12/09 at 22:13 the action statement was exited. At 01:36 on 05/13/09 an eight-hour report was made to the NRC Operations Center per 10 CFR 50.72(b)(3)(v)(C and D).
Event Analysis
The event was reportable under 10 CFR 50.72(b)(3)(v)(C and D) - An event or condition that would have prevented the fulfillment of a safety function i.e.: loss of Group I "B" MSL high flow isolation signal (Control Rad release and Accident Mitigation). Therefore a Licensee Event Report is required for this event under 10 CFR 50.73(a)(2)(v)(C and D) "Any event or condition that could have prevented the fulfillment of a safety function.
The event is considered a safety system functional failure since the event would have had the potential to prevent the fulfillment of the safety functions of Control of Radiological Release and Mitigation of an Accident.
Safety Significance
The Probabilistic Risk Assessment group review of the nuclear safety significance associated with failure of the "B" Main Steam Line (MSL) high flow Group I isolation signal found the significance to be very low. The significance is bounded by the probability of a MSL break on the "B" line for the period of time from the start of reactor pressurization to the time when the instrument was isolated. Several other considerations would reduce the risk of a fuel failure event significantly further. Some of these considerations include:
o The high steam flow group I isolation would be backed up by a reactor water level low level signal and a steam chase high temperature isolation signal for steam breaks outside the primary containment. Steam breaks inside the drywell would result in a direct scram from high drywell pressure. Under some conditions, the high steam flow isolation would occur from high flow signals generated in the remaining three steam lines via the equalizing steam header and out the break on "B" steam line.
e Safety systems were available following the startup from outage, and would have been capable of performing their function of providing makeup water to the reactor vessel and removing decay heat from the containment.
Cause
The root cause of the failure of the manifold equalizing valve is a program deficiency involving the failure to adequately maintain lubricant on the threaded components over the life of the valve. Contributing causes include valve designs that expose the threads to the process fluid and the subsequent loss of lubrication after repeated valve operations.
Corrective Action The investigation was performed under AR01181868 and the following corrective actions have been taken or are planned: Immediate corrective actions were to isolate the indicator and replace the manifold equalizing valve. Planned actions are to replace all three-way Fetterolf safety related and/or risk significant instrumentation manifold equalizing valves and four ATWS three-way manifolds and two manifolds associated with risk significant instruments.
Until the manifolds are replaced and since the valves cannot be lubricated an interim action to revise applicable surveillance procedures to require the technician to ensure the number of turns for closing of the valve matches the as number of turns for opening the valve will be performed. This will help to ensure no galling has taken place and to prevent the flow of fluid between instruments.
Failed Component Identification Instrument Valve Manifold, Fetterolf IVM Three-way manifold
Previous Similar Events
CAP #035989 (December, 2004) - Documented the removal of a level transmitter from service due to galling on the equalizer valve. The seizing of the valve threads prevented l&C Specialists from restoring the transmitter from service. The system was repaired and returned to service. The instrument manifold was an Imperial Eastman model.
CAP #01180972 (May, 2009) - The investigation revealed galled threads preventing the equalizing valve for a level transmitter from fully closing which drained the reference leg. The failed valve was replaced. The instrument manifold was an Imperial Eastman model.