05000412/LER-2008-002
| Docket Numbersequential Rev | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4122008002R00 - NRC Website | |
Energy Industry Identification System (EMS) codes are identified in the text as [XX].
DESCRIPTION OF EVENT
Beaver Valley Power Station (BVPS) Unit No. 2 was operating at approximately 25 percent power on September 21, 2008 during a plant downpower to perform maintenance on the Main Feedwater System [SJ].
At 1038 on September 21, 2008, a crew of three utility non-licensed maintenance/radiation protection personnel entered BVPS Unit No. 2 Containment [NH] through the personnel air lock [AL] to inspect and perform minor maintenance on the containment sump pump.
Approximately 71 minutes later, the crew exited containment after completing their work.
At approximately 0900 on September 25, 2008 with the Unit returned to full power, it was discovered that the air lock one-inch outside equalizing valve (2PHS-111) was open during the performance of a routine Technical Specification surveillance to demonstrate the integrity of the BVPS Unit No. 2 personnel air lock door 0-rings. This valve is used to equalize pressure across the personnel air lock door during personnel entry/exit since the BVPS Unit No. 2 containment is normally maintained at a slight vacuum with respect to outside atmosphere during power operation. This equalizing valve's normal system arrangement position is closed when not being used. With this equalizing valve open, one of the two containment boundaries provided at the personnel air lock interface was not intact. 2PHS-111 was closed upon discovery. Subsequent review concluded that this valve was last operated during the containment entry on September 21, 2008.
There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event.
CAUSE OF EVENT
The procedure for airlock door operation is not a "step by step" level of use procedure.
Manipulation and restoration of the containment airlock equalization valves affects Containment Integrity and has Technical Specification implications, therefore, should be controlled by a step by step procedure. 20M-47.4.B, "Personnel Air Lock Operations" was previously an "In-Field Reference" which is not adequate to ensure a successful performance of this task.
ANALYSIS OF EVENT
With 2PHS-111 inadvertently left open after use, the outer personnel air lock door was inoperable per BVPS Unit 2 Technical Specification 3.6.2. Required Action A.2 requires that the operable door (i.e., the inner door) be locked closed in the affected air lock within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Although the containment personnel air lock door-opening operating mechanism is routinely de-energized following use, which would effectively prohibit inadvertent use, there were no administrative controls implemented associated with 'locking' the operable air lock door between September 21 and when it was identified on September 25. Therefore, the inner operable air lock door was not locked within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> as required by Technical Specification Required Action A.2. Not meeting A.2 causes entry into Technical Specification Required Action D (Required Action and associated Completion Time not met) which requires the plant to enter into Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Since BVPS Unit No. 2 did not enter Mode 3 within 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> (24 plus 6) after inadvertently leaving 2PHS-111 open when exiting containment on September 21, 2008, this was a condition prohibited by plant Technical Specifications and is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B).
The inner door on the personnel air lock remained closed during the time that 2PHS-111 was inadvertently left open. Thus, there was no breach in containment at the personnel air lock interface during this time period.
The failure to close the Personnel Airlock outer door inside manual equalizing valve is not modeled in the Unit 2 Probabilistic Risk Analysis (PRA) since it met the screening criteria assumed in the Individual Plant Examination (IPE), i.e., normally closed manual valve.
The current PRA model does not consider the passive failure of normally closed containment isolation valves transferring open, since their probability is low compared to active valve failures. Therefore, a qualitative risk assessment of this event was performed.
The impact on risk for the condition described in the Licensee Event Report (LER) is considered to be very low for the following reasons:
1) The inadvertent failure to close the Personnel Airlock outer door inside manual equalizing valve does not prevent any systems from mitigating core damage.
2) The inadvertent failure to close the Personnel Airlock outer door inside manual equalizing yalve does not increase the frequency of any initiating events modeled in the PRA.
3) The inner air lock door was closed and intact (inside equalizing valve closed) as evidenced by the lack of any adverse loss of containment vacuum.
4) The PRA model defines a large early release based on WCAP-16378, which states that "Based on NUREG-1493, the minimum containment penetration size that can result in a large release is determined by a 100 percent containment volume per day leak rate at design pressure." This definition leads to a minimum containment penetration nominal size of two-inches in diameter. Since 2PHS-111 is a one-inch valve, its preexisting condition of being open can not result in a large early release even with the consequential failure of the inner airlock door following core damage.
Based on a qualitative risk assessment, the inadvertent opening of 2PHS-111 does not increase any initiating event frequencies or impact any systems that mitigate accidents leading to core damage. Additionally, since it is a one-inch valve, it will not contribute to the large early release of radionuclides following an accident. Therefore, this condition does not contribute to the core damage frequency or large early release frequency at Unit 2, and is considered to be of very low risk significance.
Based upon the above, the safety significance of the event condition was very low.
There was no loss of safety function for the containment during this event pursuant to 10CFR 50.73(a)(2)(v) since the containment integrity remained intact.
CORRECTIVE ACTIONS
1�Beaver Valley Power Station Unit No. 2 Operating Procedure 20M-47.4.B, "Personnel Air Lock Operations" was revised to be a "Step by Step" Level of Use procedure with performer and independent verifier sign offs for the four airlock pressure equalizing valves.
2. Beaver Valley Power Station Unit No. 1 Operating Procedure 10M-47.4.B, "Normal Personnel Air Lock Operations" was revised to be a "Step by Step" Level of Use procedure with performer and independent verifier sign offs for the airlock pressure equalizing valves necessary for containment integrity.
3. The site containment entry administrative controls procedure will be revised to be consistent with the above procedures becoming "Step by Step" procedures along with other enhancements for ensuring correct air lock door operation.
Completion of the above and other corrective actions are being tracked through the BVPS corrective action program.
PREVIOUS SIMILAR EVENTS
A review found no prior BVPS Unit No. 1 or BVPS Unit No. 2 Licensee Event Report within the last three years for an event involving a containment airlock.
CR 08-46883