05000423/LER-2014-003
| Millstone Power Station Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4232014003R00 - NRC Website | |
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1. EVENT DESCRIPTION:
On May 1, 2014 with Millstone Power Station Unit 3 (MPS3) in operating mode 1 and 100% reactor power, an evaluation determined the installation of an incorrect vendor supplied bearing introduced a non-conforming condition, which over time and under certain variable conditions, resulted in excessive friction affecting the ability of the turbine driven auxiliary feedwater (TDAFW) pump to operate properly. The root cause evaluation (RCE) noted that the frictional conditions were highly variable and time dependent. Factors such as environmental conditions, wear, corrosion, actual starting position, time between start, and other factors could result in different responses. There were numerous successful tests between 3R15 and the TDAFW pump trips in which the installation of the incorrect bearing did not result in an inoperable TDAFW pump. The installation of the incorrect bearing created an intermittent condition (i.e., pump trip events on November 4, 2013, December 18, 2013, and January 23, 2014).
NUREG 1022 Rev. 3 "Event Report Guidelines 10 CFR 50.72 and 50.73" provides guidance on assessing reportability. The NUREG introduces the concept of "firm evidence" in terms of assessing reportability. It states in part "...it should be assumed that the discrepancy occurred at the time of discovery unless there is firm evidence, based on a review of relevant information such as the equipment history and the cause of the failure, to indicate that the discrepancy existed previously".
For the purposes of reportability, the installation of the incorrect bearing during refueling outage 3R15 (spring of 2013) is firm evidence that a discrepant condition existed for periods of time exceeding the limiting conditions for operations (LCO) actions for Technical Specifications (TS) 3.7.1.2.
Plant TS 3.7.1.2 Action c, requires, if one auxiliary feedwater pump is inoperable in operating modes 1, 2, and 3, the pump must be returned to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, or the plant must be placed in at least hot standby within six hours and in hot shutdown within the following 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
For the pump trip events, the requirements of TS action statements (TSAS) were met with the exception of January 23, 2014 trip, for which the NRC granted enforcement discretion on January 26, 2014. This was reported in LER MPS3 2014-001-00.
Installation of the incorrect bearing was a non-conforming condition that introduced a degradation mechanism, which over time and under certain conditions could render the TDAFW pump inoperable (i.e., an intermittent failure). This does not alter the conclusion that this condition is reportable, but is considered in the assessment of safety consequences.
This condition is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by the plant's TS.
2. CAUSE:
The RCE determined the direct cause of the TDAFW pump over speed trips was the installation of an incorrect bearing in the turbine control valve linkage. The cause for the incorrect bearing being installed was that the vendor shipped the incorrect bearing with a certificate of conformance and with the same part number as the correct bearing. The bearing manufacturer issued a 10 CFR 21 report on this issue. (ADAMS accession # ML14079A353). This non-conforming bearing caused binding of the TDAFW pump steam supply control valve linkage, due to friction between the bearing and the cam plate.
3. ASSESSMENT OF SAFETY CONSEQUENCES:
The direct cause that led to the MPS3 TDAFW pump over speed trip events resulted in a reduction in nuclear safety defense in depth. The auxiliary feedwater system (AFW) consists of three pumps, the TDAFW pump and two motor-driven AFW pumps, which are designed to remove decay heat from the steam generators following a reactor trip. Failure of the TDAFW pump results in loss of one of the redundant sources. The TDAFW pump contributes 1.5% to the total core damage frequency and 7.1% to the total large early release frequency.
As stated above, the direct cause of the TDAFW pump over speed trips was the installation of the incorrect bearing. The degraded bearing caused binding of the TDAFW pump steam supply control valve linkage due to friction between the bearing and the cam plate. This friction is most impacfful nearest the linkage full open position on a pump automatic start. When steam was admitted to the control valve, the steam pressure placed forces on the control valve stem, increasing the friction at the bearing/cam plate interface. Sometimes this friction was sufficient to overcome the output force of the governor, which prevented the control valve from closing, resulting in an over speed trip. When the trip/throttle valve closed on over speed trip, the force on the control valve stem, and the friction at the bearing/cam plate interface, was removed, and the control valve shut which supports the ability to restart the TDAFW pump.
After the November 4, 2013 over speed trip, the TDAFW pump was started and successfully run by manually opening the trip/throttle valve, as required for a maintenance run. No work was performed between the trip and the maintenance run. After the December 18, 2013 over speed trip, the TDAFW pump was started and successfully run from a TS surveillance automatic start signal. No work was performed, except for isolating the 'D' steam supply line. Emergency Operating Procedures (EOPS) provide guidance on starting a tripped TDAFW pump by manually throttling open the steam supply valve. Therefore, it is expected that, if the TDAFW pump had tripped on a demand start on either November 4, 2013 or December 18, 2013, the operators would have successfully restarted the TDAFW pump based on the guidance in the existing EOPs.
After the January 23, 2014 trip, the control valve shut and then, as the TDAFW pump coasted down, the governor opened the control valve, as designed. If the TDAFW pump was then started in manual, the steam supply valve would control steam pressure to the control valve. The initial force on the control valve stem would be less, reducing the resistance at the bearing/cam plate interface and allowing the governor to take control as the speed increased. Therefore, it is expected that, if the TDAFW pump had tripped on a demand start, the operators would have successfully restarted the pump based on the existing guidance in the EOPs.
4. CORRECTIVE ACTION:
The incorrect bearing was replaced, the governor and associated linkage was groomed/adjusted and the TDAFW pump was tested satisfactorily. Additional corrective actions are being taken in accordance with the station's corrective action program.
5. PREVIOUS OCCURRENCES:
None 6. Energy Industry Identification System (EIIS) codes
- Auxiliary feedwater system — BA
- Turbine — TRB
- Pump — P
- Valve — V