05000446/LER-2005-003
| 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 |
| 4462005003R00 - NRC Website | |
I. DESCRIPTION OF THE REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
10CFR50.73(a)(2)(i)(B); "Any operation or condition which was prohibited by the plant's Technical Specifications.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
On April 27, 2005, Comanche Peak Steam Electric Station (CPSES) Unit 2 was in Mode 2, conducting a plant startup after its eighth refueling outage. The Reactor Coolant System (RCS) [EIIS:(AB)] was at normal operating temperature and pressure (NOT/NOP).
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE
INOPERABLE AT THE START OF THE EVENT AND THAT CONTRIBUTED
TO THE EVENT
There were no inoperable structures, systems, or components that contributed directly to the event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND
APPROXIMATE TIMES
On April 4, 2005, the three (3) Pressurizer Safety Valves (PSVs) [EIIS: (AB)(PZR)(RV)] on Unit 2 were removed from the system and were sent off-site to NWS Technologies for surveillance testing in support of the eighth refueling outage on Comanche Peak Unit 2.
Testing was performed using a procedure designed to comply with ASME OM Code Appendix I and Westinghouse Owner's Group guidance, using saturated steam as the test medium.
During as-found surveillance testing on April 7, 2005, the lift pressure of PSV 2-8010A was found to be 2451 psig, approximately 1.4% below the Technical Specification (TS) 3.4.10 TS Limiting Condition for Operation (LCO) setpoint of >/=2460 psig and psig (2485 psig +1-1%). On April 11, 2005, the as-found lift pressure of PSV 2-8010B was found to be 2452 psig, which is approximately 1.3% below the TS LCO setpoint range. PSV 2-8010C was found with an acceptable lift pressure. Both of the unsatisfactory PSVs were reworked by the vendor and the as-left lift pressures were verified to be within TS limits.
The PSVs were subsequently returned to the plant by the vendor and reinstalled in the system. Upon review of the vendor testing results, CPSES IST Engineering identified the two (2) as-found surveillance test failures and the failures were entered into the station Corrective Action Program.
On April 18, 2005, the PSVs were declared operable by the Operations Shift Manager, based upon a review of the completed surveillance test packages.
On April 27, 2005, review of the as-found surveillance testing failures resulted in the determination that this condition was reportable pursuant to 10 CFR 50.73(a)(2)(i)(B); "Any operation or condition which was prohibited by the plant's Technical Specifications.
Additionally, on May 15, 2005 during the root cause analysis investigation for this event, it was discovered that a similar sequence of events existed during the previous (seventh) Unit 2 refueling outage for PSVs 2-8010B and 2-8010C in October of 2003. At that time, PSVs 2-8010B and 2-8010C failed their as-found surveillance tests with results of 2.8% and 1.6% below the acceptable setpoint range respectively. This condition was not identified in the corrective action database and therefore was not determined to be a reportable condition. This past condition has since been entered into the station corrective action program with a reportability discovery date of May 19, 2005. Since the underlying situation and material conditions of this October 2003 condition have been evaluated as identical in both root cause and consequences to the April 2005 event, this Licensee Event Report (LER) is intended by TXU Generation Company LP (TXU Power) to encompass both reportable events.
E. THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM
FAILURE, OR PROCEDURAL OR PERSONNEL ERROR
The PSVs were being tested to satisfy the requirements of the CPSES Inservice Testing Plan and to satisfy Technical Specification (TS) surveillance requirements. The unsatisfactory as-found lift pressures were discovered as the result of this test.
II. COMPONENT OR SYSTEM FAILURES
A. FAILURE MODE, MECHANISM, AND EFFECT OF EACH FAILED
COMPONENT
Failure to meet the as-found lift pressure requirement of 2485 psig, +/-1%, during as found surveillance testing.
B. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE
Inability of the PSVs to repetitively perform within the restrictive tolerance required by the Technical Specification (TS) acceptance criteria (+1-1%). Per past discussions with the valve vendor, deviations within this range are within the design requirements of the valve and do not indicate a material problem with the valves.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AliTECTED BY
FAILURE OF COMPONENTS WITH MULTIPLE FUNCTIONS
Not applicable - No failures of components with multiple functions have been identified.
D. FAILED COMPONENT INFORMATION
Nomenclature Manufacturer Model Number Pressurizer Safety Valve Crosby 6M6
III. ANALYSIS OF THE EVENT
A. SAFETY SYSTEM RESPONSES THAT OCCURRED
Not applicable - No safety system responses occurred as a result of this event.
B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY
The PSVs were initially set to within Technical Specification limits in October 2003, and were considered operable until they were determined to have been inoperable on April 27, 2005.
Additionally, the previous unreported occurrence that is also included in this report covered a time period of April 2002 through October 2003. Although no specific duration of inoperability can be determined, this condition may well have arisen over a period of time during the previous operating cycle(s) and it is likely that the PSVs were inoperable for greater than the TS Limiting Condition for Operation (LCO) Action Statement completion time.
However, in both situations in which the PSV lift pressures were out of the TS LCO specified range, the PSVs were still capable of fulfilling their safety function.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
The PSVs operate to prevent the RCS from being pressurized above its Safety Limit of 2735 psig. Each PSV is designed to relieve 420,000 pounds per hour of saturated steam at the valve set pressure.
The adequacy of the PSVs to perform the overpressure protection function is demonstrated in the accident analyses by assuming that the valves do not fully open until the pressure has increased to 3% over the nominal set pressure. The potential adverse effect associated with premature PSV opening at pressures as low as 5% below the nominal set pressure (potentially resulting in interaction with the Pressurizer PORVs) has also been considered. The CPSES Unit 2 as-found PSV set pressures were within the analyzed range of +3/-5% about the nominal set pressure, therefore, the assumptions of the accident analyses remain valid. In addition, the actual relief capacities of the PSVs were not affected, and the PSVs would have fulfilled their overpressure protection function with the PSVs opening slightly lower than the assumed set pressure.
In conclusion, although these Unit 2 PSVs did not meet the setpoint criteria required by the CPSES Technical Specifications, the capability of these PSVs to fulfill their safety function in these instances was not affected. During the time period these PSVs were in service with the potential for set pressure drift, there were no plant events which challenged the PSVs.
Based on the foregoing, it is concluded that the health and safety of the public was unaffected by this condition and this event has been evaluated to not meet the definition of a safety system functional failure per 10CFR50.73(a)(2)(v).
IV. CAUSE OF THE EVENT
Based on an analysis of this event, performed by CPSES Engineering Programs personnel having experience with these valves, the root cause was determined to be the close tolerance required by the Technical Specification (TS) acceptance criteria (+/-1% of setpoint) and the inability of the valves to repetitively perform within that tolerance during as-found surveillance testing.
A specific material condition cause of the setpoint drift could not be determined. The testing results were within the 3% acceptance range of ASME OM Code Appendix I. Per past discussions with the valve vendor, deviations within this range are within the design requirements of the valve and do not indicate a material problem with the valves. This conclusion is further supported by the fact that the valves demonstrated satisfactory as-left test results after adjustment.
V. CORRECTIVE ACTIONS
In both cases, routine maintenance was performed by the vendor on both PSVs before retesting to restore them to the required lift setpoints. On October 24, 2003 for the prior instance and April 18, 2005 in the later, the required surveillances were completed with all three PSVs being as-left tested satisfactorily to state-of-the-art requirements.
The long term corrective action is for TXU Power to investigate revising the safety analysis to support a License Amendment for relaxation of the +/-1% TS Limiting Condition for Operation (LCO) requirement for the PSV as-found lift setpoint tolerance.
VI. PREVIOUS SIMILAR EVENTS
Three (3) previous similar events have been reported for CPSES pursuant to 10CFR50.73(a)(2)(i)(B) related to lift setpoint drift: LER 446/94-018 on Unit 2, LER 445/96-008 on Unit 1, and LER 445/02 004 on Unit 1.