05000346/LER-2011-001
| Davis-Besse Nuclear Power Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3462011001R00 - NRC Website | |
Energy Industry Identification System (EllS) codes are identified in the text as [XX].
DESCRIPTION OF EVENT:
System Description The Reactor Coolant System (RCS) [AB] at the Davis-Besse Nuclear Power Station (DBNPS) has two identical safety valves [AB-RV] (RC13A and RC13B) each located on a flanged nozzle on the Pressurizer [AB-PZR] top head.
The Pressurizer Safety Valves (PSVs) were manufactured by Crosby Valve & Gage Company, Model Number HB-86-BP Type E series valves designed for nuclear service and certified under Section III of the ASME code for application in nuclear power systems. The valves are designed to be self-actuating, spring loaded, with balancing bellows and a balancing piston.
The function of the PSVs is to ensure the RCS pressure does not exceed the Technical Specification (TS) 2.1.2 safety limit of 2750 pounds per square inch gauge (psig).
Technical Specification(s):
Technical Specification (TS) 3.4.10, "Pressurizer Safety Valves", Limiting Condition for Operation (LCO) 3.4.10 requires two pressurizer safety valves be OPERABLE with lift settings less than or equal to 2525 psig in Modes 1, 2 and 3. With one PSV inoperable, TS 3.4.10 Action A requires the valve be restored to operable status within 15 minutes. If this required action cannot be met, or if two PSVs are inoperable, TS 3.4.10 Action B requires the plant to be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
Event Description:
The Pressurizer Code Safety Valves RC13A and RC13B were replaced during the mid cycle outage of Cycle 16 in April 2009.
On February 28, 2010, the DBNPS shutdown for the Sixteenth Refueling Outage activities. As part of this outage, the PSVs were removed, and sent to an off site vendor for testing and refurbishment at the conclusion of the outage. This testing was performed on August 6, 2010 (event date).
On December 13, 2010, with the plant operating in Mode 1 at 100 percent power, following receipt of final paperwork from the vendor test facility, it was identified that both valves had As- Found lift settings above the limits specified in TS 3.4.10. Because both valves had As-Found setpoints above the TS allowed value (2531 psig for RC13A and 2535 psig for RC13B), a past operability evaluation was performed. The past operability evaluation, completed on January 12, 2011 (discovery date), determined these valves were inoperable while they were installed in the plant.
CAUSE OF EVENT:
The cause of the as-found test setpoint of the PSVs found to be greater than the TS allowable value of 2525 psig was determined to be setpoint drift.
CAUSE OF EVENT (continued):
A contributing cause of the as-found test setpoint of the PSVs found to be greater than the TS allowable value of less than or equal to 2525 psig is the Pressurizer Code Safety Valve as-left allowable range (+/- 1 percent) does not provide a balanced margin to accommodate setpoint drift given the skewed +1 percent/-3 percent as-found acceptance requirements.
ANALYSIS OF EVENT:
While both valves had as-found setpoints that exceeded the TS allowed value, the highest out of tolerance setpoint was only 10 psig higher than the required value. The transients described by Updated Safety Analysis Report (USAR) that can produce an RCS overpressurization were reviewed with respect to the out-of-tolerance valve setpoints. The review indicated that all applicable transients were analyzed: (1) without modeling the valves, (2) with a bounding setpoint tolerance or, (3) would have resulted in an acceptable peak pressure when the out-of- tolerance setpoints are considered. Therefore this issue had very low safety significance and did not prevent the PSVs from fulfilling their safety function.
Reportability Discussion:
NUREG-1022, Event Reporting Guidelines, states that discrepancies found in TS surveillance tests are normally assumed to occur at the time of the test unless there is firm evidence, based on a review of relative information, to indicate the discrepancy occurred earlier. The NUREG provides an example that multiple safety valve testing failures is an indication that the discrepancies may well have arisen over a period of time and did not occur just at the time of discovery. Evaluation of the PSV test history and potential failure modes for the PSV did not identify any information that would allow a conclusion that the valves were operable while the plant was operating in Mode 1, 2 or 3 as required by TS LCO 3.4.10. Therefore, this condition (two pressurizer safety valves exceeding the TS allowed setpoint) is reportable as a Licensee Event Report (LER) per 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by the plant's TS based on the above guidance from NUREG-1022.
CORRECTIVE ACTIONS:
During the Sixteenth Refueling Outage concluding June 2010, two PSVs were installed in place of the removed valves. These valves were also manufactured by Crosby Valve & Gage Company, Model Number HB-86-BP Type E series valves. These valves had As-Left set pressures of 2504 psig (RC13A) and 2496 psig (RC13B).
Procedures that govern the testing of the PSVs will be revised to establish a desired As-Left setpoint range of +0 percent to -1 percent (2500 psig to 2475 psig) of set pressure (2500 psig) with the As-Left setpoint adjusted to as close to 2475 psig as practicable. The two PSVs tested in August 2010 will be sent to a vendor facility to have the As-Left setpoint adjusted to as close to 2475 psig as practicable.
PREVIOUS SIMILAR EVENTS:
Two previous events were found to be associated with high PSV setpoints at the DBNPS.
In 1996, a voluntary LER was submitted to the NRC which documented an occurrence where one of two Pressurizer Code Safety Valves removed during the Tenth Refueling Outage in Spring of 1996 had an initial lift setpoint of 3.08 percent greater than the setpoint value of 2525.
The apparent cause of the valve test failure was determined to be setpoint drift. The two safety valves were refurbished and their setpoints adjusted to within the allowable limit.
In 1994, a voluntary LER was submitted to the NRC which documented an occurrence where one of two Pressurizer Code Safety Valves removed during the Ninth Refueling Outage had an initial lift setpoint 2.44 percent greater than the setpoint value of 2525 psig. The apparent cause of the valve test failure was determined to be setpoint drift. The two safety valves were refurbished and their setpoints adjusted to within the allowable limit.