05000281/LER-2001-001
| Document Number | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2812001001R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT In November 2000, a team was formed to review possible causes for Unit 2 leaking Pressurizer Safety Relief Valves (PSV) [IEEE-AB-PZR-RV]. The valves were observed to have minor leakage following start-up from the fall refueling outage. After evaluating several possible causes, it was determined that excessive PSV nozzle loading could contribute to PSV leakage. Snubber [IEEE-AB-SNB] 2-RC-HSS-116 on the "B" PSV discharge piping was identified as being a critical support for "B" PSV nozzle loading. On February 9, 2001, with Unit 2 at 100% reactor power, inspection of snubber 2-RC-HSS- 116 verified the snubber to be fully extended. The snubber was declared inoperable and on February 9, 2001 at 1049 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.991445e-4 months <br />, a 72-hour limiting condition of operation (LCO) was was brought to an intermediate shutdown condition at reduced reactor coolant system (RCS) pressure to alleviate industrial safety concerns associated with snubber replacement. The snubber was replaced with minor modification and was declared operable on February 10, 2001 at 1647 hours0.0191 days <br />0.458 hours <br />0.00272 weeks <br />6.266835e-4 months <br />. At this time, the Technical Specification action statement was exited.
A Root Cause Team was assembled to determine the cause of this event. The team determined that this snubber had been changed from a Lynair 11/2-inch diameter snubber with 6-inch stroke to a Miller 11/2-inch diameter snubber with 5-inch stroke in 1985.
However, the extension hardware necessary to accommodate the shorter stroke had not been installed. This resulted in an inadequate stroke to accommodate full thermal movement of the piping. Although this snubber had been inspected and functionally tested since installation, the procedures for removal, installation and inspection did not direct verification of cold piston setting (CPS). Therefore, the improper installation went undetected until February 9, 2001.
Technical Specifications 3.20.A requires all snubbers, required to protect the RCS, to be operable during reactor operation. Technical Specification 3.20.B requires that any snubber found inoperable must be repaired or replaced within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. As a result of the as found condition and ensuing investigation, snubber 2-RC-HSS-116 was considered inoperable for a time exceeding 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Therefore, this event is reportable pursuant to 10CFR50.73(a)(2)(i)(B) for a condition prohibited by Technical Specifications.
FACILITY NAME (1) SURRY POWER STATION, UNIT 2
DOCKET
05000 - 281 LER NUMBER (6) _ 2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS With the snubber in the fully extended condition, calculations of stress levels in the piping indicated that piping components (other than the snubber) would maintain structural integrity in all dynamic loading conditions including seismic, PSV, and Power Operated Relief Valve discharge conditions. Although the overextended snubber could contribute to PSV leakage due to excessive nozzle loading, measured leakage was within Technical Specification allowable limits. As such, there were no significant safety consequences or implications. Therefore, the health and safety of the public were not affected by this event.
3.0 CAUSE In April 1984, LER 84-006 reported the failure of a number of hydraulic and mechanical snubbers at Surry Unit 1 to pass the inservice periodic functional testing required by Technical Specifications. As corrective action, all Lynair-cylinder Grinnell hydraulic snubbers were replaced with Miller-cylinder Grinnell hydraulic snubbers or rebuilt with new acceptable seals. 11/2-inch diameter Lynair snubbers have a 6-inch stroke, while 11/2-inch diameter Miller snubbers have a 5-inch stroke. Replacements of the snubbers began in 1984. Engineering Work Requests 84-90 and 84-90A were written to change snubber models and required the new 11/2-inch snubbers to be installed with elongated rod eyes, or spacer plates, to compensate for the change in length. In 1985, snubber 2-RC-HSS- 116, a Lynair 11/2-inch diameter snubber with 6-inch stroke, was replaced with a Miller 11/2- inch diameter snubber with a 5-inch stroke. During snubber replacement, the new snubber was installed without the necessary extension hardware resulting in an inadequate stroke to accommodate full thermal movement. Review of the work order used to replace the original snubber determined it did not provide appropriate direction for changing from a 6-inch stroke to a 5-inch stroke snubber.
Further investigation found that the initial installation guidance did not check initial CPS nor did the station snubber program contain necessary inspection elements to periodically verify snubber CPS. Therefore, the improper snubber CPS went undetected until February 9, 2001.
4.0 IMMEDIATE CORRECTIVE ACTION(S) A station corrective action system report was submitted for the inoperable snubber and on February 9, 2001, at 1049 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.991445e-4 months <br />, a 72-hour LCO was started IAW Technical Specifications 3.20.B to repair or replace the snubber. On February 10, 2001, Unit 2 was brought to intermediate shutdown and depressurized to facilitate snubber replacement.
The replacement snubber was installed with 3 inches of spacer plates to place the snubber within the range of required settings to allow for thermal expansion.
FACILITY NAME (1) SURRY POWER STATION, UNIT 2
DOCKET
05000 - 281 LER NUMBER (6) � SEQUENTIAL I REVISION 5.0 ADDITIONAL CORRECTIVE ACTIONS During 1984 and 1985, certain 11/2-inch diameter size snubbers were replaced with snubbers having a different stroke. Engineering has compared snubber CPSs recorded for all 11/2-inch diameter snubbers in both Units 1 and 2 to the required allowable ranges.
Seven other snubbers were identified to be outside the allowable range for cold piston settings. A review of a sample of different sized snubbers was also conducted to validate the extent of the condition. During this review, three 5-inch stroke snubbers with various diameters were also found to be outside the allowable range for CPS. However, thermal loads combined with design dynamic loads are within vendor allowable loads and affected piping and support stresses are within code allowable values. Therefore, these snubbers are operable. Evaluations are continuing to verify the full extent of the condition. Field verification will be conducted and necessary corrective actions implemented as unit operation and exposure levels permit.
6.0 ACTIONS TO PREVENT RECURRENCE The station snubber program and applicable maintenance procedures will be revised to include criteria for verification of snubber cold piston settings. The current design change process in use today has the necessary controls to prevent similar problems from occurring.
7.0 SIMILAR EVENTS None.
8.0 MANUFACTURER/MODEL NUMBER ITT Grinnell Figure 200 Size 11/2-inch x 5-inch stroke/Miller cylinder 9.0 ADDITIONAL INFORMATION Unit 1 was operating at 100% at the time of this event.