05000334/LER-2010-001
| Docket Numbersequential Rev | |
| Event date: | 08-26-2010 |
|---|---|
| Report date: | 10-21-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 3342010001R00 - NRC Website | |
There were no structures, components, or systems that were inoperable at the start of the event that contributed to the event. Energy Industry Identification System (EllS) codes are identified in the text using the format [XX].
DESCRIPTION OF EVENT
With Beaver Valley Power Station (BVPS) Unit No. 1 at 100 percent power on 08/26/2010, the "A" High-Head Safety Injection (HHSI) Pump [BQ] was being returned to service following scheduled maintenance. {Note: One High-Head Safety Injection pump is also used at BVPS Unit 1 for routine continuous Reactor Coolant System charging [CB] during power operation, and this pump is commonly referred to as a Charging Pump.} At this time, the "B" HHSI pump was operating with the "C" HHSI pump in standby pursuant to meeting BVPS Unit 1 Technical Specification 3.5.2 which requires two HHSI pumps to be operable.
Following filling of the drained "A" HHSI (charging) pump/piping, ultrasonic tests (UT) were performed on the in-service Unit 1 HHSI pump common suction pipe headers (6 inch and 8 inch). Preliminary indications at 1345 hours0.0156 days <br />0.374 hours <br />0.00222 weeks <br />5.117725e-4 months <br /> showed the 6 inch header was full of water, but an air void existed within the 8 inch suction header. After initial void identification, the 8 inch line was vented multiple times and Engineering was notified to assess the size of the void.
The UT process of locating potential voids only identifies one dimension (i.e., depth of bubble). HHSI void acceptance criteria is defined in cubic feet and is unique to each specific piping location (in this case, over 30 specific locations for the HHSI piping, with individual void size limits for each pipe grouping). Thus, the UT process can not directly assess the acceptability of an identified void.
Engineering evaluation of the void data concluded that the measured void exceeded the allowable charging suction piping void size acceptance criteria as defined in the void-measuring test procedure. The control room was notified of this conclusion at 1649 hours0.0191 days <br />0.458 hours <br />0.00273 weeks <br />6.274445e-4 months <br /> and both trains of HHSI were immediately declared inoperable, and Technical Specification (TS) LCO 3.0.3 was entered pursuant to Technical Specification 3.5.2 Condition C for less than one full train of Emergency Core Cooling System (ECCS).
Additional venting was initiated and it was subsequently verified that both suction piping headers were full of water. Technical Specification 3.5.2 and LCO 3.0.3 were exited at 1718 hours0.0199 days <br />0.477 hours <br />0.00284 weeks <br />6.53699e-4 months <br />.
An investigation determined that the procedure being used to return the "A" HHSI pump to service introduced the void by allowing air to enter the operating charging pump suction header by opening the clearance isolation valve without removing the air on the cleared side of the valve. The drained HHSI pump was filled with pressurized water from the operating charging suction header by opening the clearance valve, which forced any air in the cleared pump/pipes to migrate upwards into the operating charging suction header.
This pump recovery procedure was used multiple times in past years without recognizing any adverse conditions because the existing high point vent was used to vent off the air prior to taking UT measurements. A recent procedure change resulted in the order of venting and UT testing being altered during a HHSI pump recovery, and this was the first time that the-UT was performed before venting of-the common suction header, which allowed the void to be discovered this time.
Applicable ECCS pump recovery procedure has beenplaced on administrative hold pending completion of corrective actions.
CAUSE OF-EVENT The cause of this event is the charging pump suction piping layout at BVPS Unit 1 is conducive to allowing a void to migrate to the charging pump common suction header during a fill and vent procedure. There is no proper high point vent within the suction side clearance boundary for each of the three HHSI/charging pumps which does not allow fill and vent without void migration to the suction header. Currently the only available high point vent is not sufficient in this case because the vent is on a common header to all three HHSI pumps outside the clearance boundary. {One charging pump operates continuously when at power.} A pre-conditioned mindset existed for the individuals that performed the previous operations procedure, reviews of Emergency Core Cooling System procedures for potential gas voiding issues that this procedure was acceptable based on successful past performances with no anomalies noted.
A similar design/configuration anomaly does not exist for the BVPS Unit 2 HHSI/charging system suction header.
ANALYSIS OF EVENT
Both trains of HHSI required to be operable pursuant to BVPS Unit 1 Technical Specification 3.5.2 were not fully capable of performing to design basis bounding requirements. With both trains of HHSI being declared inoperable due to a gas void in the common suction header leading to both HHSI pumps, this event/condition is reportable as an event that could have prevented the fulfillment of a safety function which adversely affects the ability to safely shutdown the plant { 10CFR50.73(a)(2)(v)(A) }, the ability to mitigate the consequences of an accident { 10CFR50.73(a)(2)(v)(D) }, and reportable as a single condition that caused two independent trains to be inoperable { 10CFR50.73(a)(2)(vii) }.
Notwithstanding the above, an Engineering evaluation was performed which showed that both HHSI trains could have performed their safety function during a postulated design basis accident, using a best-estimate assessment of plant parameters for a void of the size measured in this event located in the common charging header. In addition, since this same fill and vent process has been used in the past, a best-estimate evaluation of a postulated void from a bounding scenario where the entire void volume from a cleared and drained HHSI pump/piping migrates into the common charging suction header also showed acceptable HHSI train capability. Therefore, this event is not reportable pursuant to 10CFR50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety.
This- event was previously reported as an event-that could have prevented the fulfillment of a safety function pursuant to 10 CFR 50.72(b)(3)(v)(A)/(b)(3)(v)(D) at 2222 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.45471e-4 months <br /> on 08/26/2010 (Event Notification 46208). However, the initial reportability of this event in EN 46208 for 10 CFR 50.72(b)(3)(ii)(B) is retracted, with the same basis as provided above for 10 CFR 50.73(a)(2)(ii)(B).
This event is also reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) as an event/condition prohibited by plant Technical Specifications. BVPS Unit 1 Technical Specification 3.5.2, "ECCS-Operating", Required Action C.1 requires the entry into LCO 3.0.3 immediately when less than 100 percent of ECCS flow equivalent to a single operable ECCS train is available.
TS LCO 3.0.3 requires action to be initiated within one hour to place the plant in a Mode or other condition in which the condition of operation is not applicable. The HHSI System was made inoperable longer-than one hour when the HHSI fill and vent procedure introduced an unacceptable air void into the common HHSI suction pipe header during this event and likely during past performances. Therefore, BVPS Unit No. 1 was in a condition prohibited by plant Technical Specifications whenever the plant was in this unacceptable HHSI condition for longer than one hour.
The plant risk associated with the BVPS Unit 1 HHSI pump suction header void event that occurred on 08/26/2010 and with prior drained HHSI pump recovery events is considered to be very low. This is based upon an Engineering technical assessment, which concluded that. the HHSI pump capability would not have been significantly challenged by the measured void using best-estimate considerations. Therefore, the conditional probability of the HHSI pumps failing due to the void is negligible, and the increase in risk is insignificant. Therefore, the safety significance of the HHSI pump suction header void event was very low.
CORRECTIVE ACTIONS
1. A solution will be developed to allow filling/venting in order to return a charging pump to service while at power in a manner that does not challenge the operability of the system.
This may involve a design change to add a new vent valve between the suction isolation valve in the piping for each of the three charging pumps. Upon implementation of the solution, the fill and vent operations procedure will be revised to incorporate this solution.
2. Operations procedures at both BVPS Unit 1 and Unit 2 that fill and vent the ECCS systems will be reviewed, with a focus on prevention of air void migration into operating parts of systems. These procedure reviews will look at elevation differences, the order of UT and venting sequence, and the manipulation of valves outside of the clearance boundary.
3. A plant operating experience report has issued on this event (OE 32074).
4. The site procedure on procedure cross-discipline review guidance will be revised to ensure fill and vent procedures on ECCS systems include a review of applicable isometric drawings (or physical plant walk down) to ensure the proposed procedure modification will not adversely migrate or introduce a gas void.
Completion of the above and other corrective actions are being tracked through the BVPS corrective action program.
PREVIOUS SIMILAR EVENTS
Gas voids have been found several times in both BVPS Units' HHSI systems within the last five years, pursuant to surveillances performed for normal post-maintenance testing and to address both industry and regulatory initiatives on gas voids. However, no voids previously identified were above the applicable gas void size limit such that the HHSI system(s) needed to be declared inoperable.
CR 10-81835/10-81969