05000302/LER-2001-002
| Crystal River Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3022001002R00 - NRC Website | |
On September 26, 2001, Florida Power Corporation's (FPC's) Crystal River Unit 3 (CR-3) was in MODE 1 (POWER OPERATION) at 100 percent RATED THERMAL POWER. While performing surveillance procedure SP-650, "ASME Code Safety Valve Test," on the 'A' Once Through Steam Generator (OTSG) [SB, SG], main steam safety valves (MSSVs) MSV-33 and MSV-42 [SB, RV] setpoints were found outside their required tolerance. The acceptance criteria specified in Improved Technical Specification (ITS) Table 3.7.1-1 is +1- 3 percent of the nominal setpoint. At 1549 on September 26, 2001, MSV-42 lifted at 5.1 percent above the nominal setpoint. At 2005 on September 26, 2001, MSV-33 lifted at 3.7 percent above the nominal setpoint. The MSSVs were declared inoperable and actions associated with Improved Technical Specification (ITS) 3.7.1, Condition A, became applicable. The remaining six tested 'A' OTSG MSSVs were within their required setpoint tolerance.
The MSSVs are required to be tested for relief setpoint accuracy in accordance with the Crystal River Unit 3 Inservice Testing Pump and Valve Program. This program requires that each MSSV be tested once every five years. Typically, 50 percent of the MSSVs are tested during each refueling outage. The 'A' OTSG MSSVs are normally tested during even numbered outages (e.g., R12) and the 'B' OTSG MSSVs are normally tested during the odd numbered outages (e.g., R13).
MSV-33 and MSV-42 were scheduled to be tested during R12, scheduled to begin on September 29, 2001.
ITS 3.7.1 states that the MSSVs shall be operable as specified in Table 3.7.1 -1 in MODES 1, 2 and 3. With one or more required MSSVs inoperable, THERMAL POWER shall be reduced to less than the reduced power limit of Table 3.7.1-1 within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> (Action A.1) and the nuclear overpower trip setpoint shall be reduced in accordance with Table 3.7.1-1 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> (Action A.2). Once declared inoperable, MSV-33 and MSV-42 were restored to an operable status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.
The existence of similar discrepancies in multiple relief valves is an indication that the discrepancies may well have developed over a period of time. Therefore, FPC concludes that the condition existed during plant operation and the event is reportable under 10CFR50.73(a)(2)(i)(B) as a condition prohibited by ITS.
While reviewing SP-650 test results over the last three years for evidence of similar occurrences, FPC identified two 'B' OTSG MSSV setpoints that were found outside their required tolerance in 1999. On September 28 and 29, 2001, FPC's CR-3 was in MODE 1 (POWER OPERATION) at 97.6 percent RATED THERMAL POWER. While performing SP-650 on the 'B' OTSG MSSVs, MSV-45 and MSV-48 setpoints were found outside their required tolerance. At 1500 on September 28, 1999, MSV-45 lifted at 3.3 percent below the nominal setpoint. At 0030 on September 29, 1999, MSV-48 lifted at 11.1 percent above the nominal setpoint. MSV-45 and MSV-48 were declared inoperable and actions associated with ITS 3.7.1, Condition A, became applicable. Once declared inoperable, MSV-45 and MSV-48 were restored to an operable status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. The remaining six tested 'B' OTSG MSSVs were within their required setpoint tolerance.
NR FORM 366A (1-2001) discrepancies may well have developed over a period of time. Therefore, FPC concludes that the condition existed during plant operation and the event should have been reported under 10CFR50.73(a)(2)(i)(B) as a condition prohibited by ITS.
SAFETY CONSEQUENCES
The MSSVs provide overpressure protection for the main steam piping and the OTSGs. The technical evaluation below concludes that, even with the out-of-tolerance setpoints of either MSV- 33 and MSV-42 or MSV-45 and MSV-48, the safety relief valves (as a group) were capable of protecting the Main Steam System [SB] and the OTSGs from overpressurization.
The design pressure for the Main Steam System is 1050 psig. Enhanced Design Basis Document Tab 6/10, "Main Steam System," states the total MSSV capacity to be such that steam pressure will not exceed 110 percent of system design pressure (1155 psig) for the limiting overpressurization event. The limiting event that challenges the MSSV capacity is the Turbine Generator trip that occurs at 112 percent rated thermal power. The total required relieving capacity for this event is 11,103,336 Ibm/hr or 5,551,668 Ibm/hr per steam generator. The full open relieving capacity for each of the safety valves is as follows:
A OTSG Capacity Set Pressure B OTSG Capacity Set Pressure (Ibm/hr) (psig) (Ibm/hr) (psig) MSV-33 845,759 1050 MSV-35 845,759 1050 MSV-34 845,759 1050 MSV-36 845,759 1050 MSV-37 845,759 1070 MSV-39 845,759 1070 MSV-38 845,759 1070 MSV-41 845,759 1070 MSV-42 845,759 1090 MSV-44 845,759 1090 MSV-43 845,759 1090 MSV-45 845,759 1090 MSV-40 583,574 1100 MSV-47 845,759 1100 MSV-46 845,759 1100 MSV-48 583,574 1100 Total A = 6,503,887 Total B = 6,503,887 In 2001, the as-found setpoints for MSV-33 and MSV-42 were 1089.72 and 1145.88 psig, respectively. Both valves would have lifted before reaching the 1155 psig limit. Per vendor documentation, the minimum capacity of these valves when lifted at their as-found setpoints (high) is assumed to be 70 percent of full rated flow. The remaining six 'A' OTSG MSSVs passed the as- found setpoint test. Reproducing the above table for the 'A' OTSG, with 70 percent flow for both MSV-33 and MSV-42:
The total required capacity is 5,551,668 Ibm/hr. The total available capacity is 5,996,432 Ibm/hr.
The available relieving capacity exceeds the required relieving capacity. Therefore, the 'A' OTSG MSSV's, were collectively capable of providing overpressure protection with the as-found setpoints.
In 1999, the as-found setpoints for MSV-45 and MSV-48 were 1053.88, and 1222.5, respectively.
MSV-45 would have lifted before reaching 1155 psig limit. MSV-48 would not have lifted before reaching the 1155 psig limit. The minimum capacity of MSV-45 when lifted at its as-found setpoint (low) is assumed to be 100 percent of full rated flow. The remaining six "B" OTSG MSSVs passed the as-found setpoint test. � Reproducing the above table for the 'B' OTSG, with 0 percent flow for MSV-48:
MSV-35 845,759 MSV-36 845,759 MSV-39 845,759 MSV-41 845,759 MSV-44 845,759 MSV-45 845,759 MSV-47 845,759 MSV-48 0 Total A = 5,920,313 The total required capacity is 5,551,668 Ibm/hr. The total available capacity is 5,920,313 Ibm/hr.
The available relieving capacity exceeds the required relieving capacity. Therefore, the 'B' OTSG MSSV's, were collectively capable of providing overpressure protection with the as-found setpoints.
Based on the above discussion, FPC concludes that the inoperability of either MSV-33 and MSV-42 or MSV-45 and MSV-48 did not represent a reduction in the public health and safety. This event does not meet the definition of a Safety System Functional Failure.
The cause for the tested MSSV as-found setpoints being out of tolerance is indeterminate. The most probable cause is disc/seat bonding. Disc/seat bonding is defined as the physical surface bonding of the disc and seating surfaces. The fusion of the martensitic stainless steel A422 disc to the austenitic stainless steel A316 seat occurs by an oxide film that forms and results in a bonding mechanism. Research has shown that the disc material has essentially transferred to the seat on a microscopic level. The disc/seat bonding will frequently lead to a high initial first lift as the bond is broken.
A suggested industry fix for disc/seat bonding is to install Inconel X-750 discs (among others). This material is harder than the A422 disc and is less likely to transfer material to the seat. Another suggested fix is a pre-oxidation treatment of the disc/seat prior to re-assembly. The industry has no definite conclusion at this time that these recommendations will resolve the setpoint issue.
Owners groups and vendors are still conducting studies.
A contributing factor for the tested MSSV as-found setpoints being out of tolerance may be aging.
Aging is defined as valves that remain in service for extended periods of time with little or no preventive maintenance. The aging process is not just time dependent, but also the result of the sum of the total events the valve has experienced between maintenance activities. Aging is general wear of the components that make up the valve. There is no one component that causes setpoint drift more than another. It is the summation of slightly worn components throughout the valve. The industry identifies aging as a leading cause of setpoint drift. Industry experts conclude the best way to prevent aging is through a preventive maintenance program (valve rebuild). A contradiction to this recommendation is that disc/seat bonding is highly dependent upon maintenance practices that suggests valves should only be worked as necessary. Currently, MSSV's are only rebuilt at CR-3 if seat leakage is evident prior to a refueling outage.
Missed Reporting Opportunity In 1999 The cause for not reporting the 1999 MSSV setpoints found to be outside their required tolerance is attributed to the lack of familiarity with the examples of reportability provided in NUREG-1022, "Event Reporting Guidelines — 10CFR50.72 and 10CFR50.73." Operations and Licensing personnel reviewed occurrences of MSSV setpoints being found outside their required tolerance as individual deficiencies. Reviewing personnel did not recognize that similar discrepancies in multiple valves is an indication that the discrepancies may well have arisen over a period of time. As such, the condition existed during plant operation and the event is reportable under 10CFR50.73(a)(2)(i)(B) as a condition prohibited by ITS.
operable status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of identifying the associated setpoints to be outside their required tolerance.
FPC decided not to remove MSV-33 or MSV-42 for inspection, based on the following:
MSSV setpoint failures are a generic industry problem. Aging, disc/seat bonding, and temperature variations contribute to the difficulties of identifying a root cause for setpoint failures.
No apparent external mechanical or structural deficiencies were noted.
An open and inspect would likely not reveal a root cause. Both valves were subsequently retested within tolerance to meet the as-left requirements. No seat leakage was evident before or after the testing.
MSV-45 (Dresser, Model 3707 RA) and MSV-48 (Dresser, Model 3777 QA) were restored to an operable status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of identifying the associated setpoints to be outside their required tolerance in 1999.
A copy of this LER will be provided to appropriate Operations and Licensing personnel as a means of demonstrating applicability of the NUREG-1022 example concerning multiple relief valve failures to MSSV setpoints being found not to meet required tolerance.
FPC engineering personnel will continue to monitor industry progress associated with the disc/seat bonding issues/solutions and the benefit of increased preventive maintenance of the MSSVs.
PREVIOUS SIMILAR EVENTS
While reviewing SP-650 test results over the last three years for evidence of similar occurrences, FPC identified two B' OTSG MSSV setpoints that were outside their required tolerance in 1999.
That event was not reported to the NRC. Therefore, FPC has augmented this LER to contain information necessary to satisfy the missed reporting opportunity.
ATTACHMENTS
Attachment 1 - Abbreviations, Definitions, and Acronyms Attachment 2 - List of Commitments ) NRC FORM 366A (1-2001 The following table identifies those actions committed to by Florida Power Corporation in this document. Any other actions discussed in the submittal represent intended or planned actions by Florida Power Corporation. They are described to the NRC for the NRC's information and are not regulatory commitments. Please notify the Supervisor, Licensing & Regulatory Programs of any questions regarding this document or any associated regulatory commitments.
RESPONSE
SECTION
COMMITMENT DUE DATE
No regulatory commitments are being made in this submittal.