ML15218A154
| ML15218A154 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 11/06/1997 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML15218A153 | List: |
| References | |
| GL-95-07, GL-95-7, NUDOCS 9711180315 | |
| Download: ML15218A154 (6) | |
Text
FP A UNITED STATES o
NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RESPONSE TO GENERIC LETTER 95-07 "PRESSURE LOCKING AND THERMAL BINDING OF SAFETY-RELATED POWER-OPERATED GATE VALVES" OCONEE NUCLEAR STATION, UNITS 1. 2. AND 3 DOCKET NOS. 50-269, 50-270, AND 50-287
1.0 INTRODUCTION
Pressure locking and thermal binding represent potential common-cause failure mechanisms that can render redundant safety systems incapable of performing their safety functions. The identification of susceptible valves and the determination of when the phenomena might occur require a thorough knowledge of components, systems, and plant operations. Pressure locking occurs in flexible-wedge and double-disk gate valves when fluid becomes pressurized inside the valve bonnet and the actuator is not capable of overcoming the additional thrust requirements resulting from the differential pressure created across both valve disks by the pressurized fluid in the valve bonnet. Thermal binding is generally associated with a wedge gate valve that is closed while the system is hot and then is allowed to cool before an attempt is made to open the valve.
Pressure locking or thermal binding occurs as a result of the valve design characteristics (wedge and valve body configuration, flexiblity, and material thermal coefficients) when the valve is subjected to specific pressures and temperatures during various modes of plant operation. Operating experience indicates that these situations were not always considered in many plants as part of the design basis for valves.
2.0 REGULATORY REQUIREMENTS Title 10 of the Code of Federal Regulations (10 CFR) Part 50 (Appendix A, General Design Criteria 1 and 4) and plant licensing safety analyses require or commit (or both) that licensees design and test safety-related components and systems to provide adequate assurance that those systems can perform their safety functions. Other individual criteria in Appendix A to 10 CFR Part 50 apply to specific systems. In accordance with those regulations and licensing commitments, and under the additional provisions of 10 CFR Part 50 (Appendix B, Criterion XVI), licensees are expected to act to ensure that safety-related power-operated gate valves susceptible to pressure locking or thermal binding are capable of performing their required safety functions.
Enclosure 9711180315 971106 PDR ADOCK 05000269 P
-2 On August 17, 1995, the NRC issued Generic Letter (GL) 95-07, "Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves," to request that licensees take certain actions to ensure that safety-related power-operated gate valves that are susceptible to pressure locking or thermal binding are capable of performing their safety functions within the current -licensing bases of the facility. GL 95-07 requested that each licensee, within 180 days of the date of issuance of the generic letter (1) evaluate the operational configurations of safety-related power-operated gate valves in its plant to identify valves that are susceptible to pressure locking or thermal binding, and (2) perform further analyses and take needed corrective actions (or justify longer schedules) to ensure that the susceptible valves, identified in (1) above, are capable of performing their intended safety functions under all modes of plant operation, including test configuration. In addition, GL 95-07 requested that licensees, within 180 days of the date of issuance of the generic letter, provide to the NRC a summary description of (1) the susceptibility evaluation used to determine that valves are or are not susceptible to pressure locking or thermal binding, (2) the results of the susceptibility evaluation, including a listing of the susceptible valves identified, and (3) the corrective actions, or other dispositioning, for the valves identified as susceptible to pressure locking or thermal binding. The NRC issued GL 95-07 as a "compliance backfit" pursuant to 10 CFR 50.109(a)(4)(1) because modification may be necessary to bring facilities into compliance with the rules of the Commission referenced above.
By letter dated February 13, 1996, Duke Energy Corporation (the licensee) submitted the 180-day response to GL 95-07 for the Oconee Nuclear Station Units 1, 2, and 3. The NRC staff reviewed the submittal and requested additional information in letters dated June 13, 1996, and June 23, 1997. By letters dated July 18, 1996, and August 21 and October 9, 1997, the licensee provided the additional information. The NRC staff has reviewed the licensee's submittals as discussed in this safety evaluation.
3.0 STAFF EVALUATION 3.1 Scope of Licensee's Review GL 95-07 requested that licensees evaluate the operational configurations of safety-related power-operated gate valves in their plants to identify valves that are susceptible to pressure locking or thermal binding. The licensee's letters of February 13 and July 18, 1996, and August 21 and October 9, 1997, described the scope of valves evaluated in response to GL 95-07. The NRC staff has reviewed the scope of the licensee's susceptibility evaluation performed in response to GL 95-07 and found it complete and acceptable.
3.2 Corrective Actions GL 95-07 requested that licensees, within 180 days, perform further analyses as appropriate, and take appropriate corrective actions (or justify longer schedules), to ensure that the susceptible valves identified are capable of performing their intended safety function under all modes of plant operation, including test configuration.
-3 The licensee's submittals discussed its proposed corrective actions to address potential pressure-locking and thermal-binding problems. The staffs evaluation of the licensee's actions is discussed in the following paragraphs.
- a.
The licensee stated that the following valves were evaluated for pressure locking and modified or replaced with a globe or ball valve to eliminate that potential:
UNIT VALVE PURPOSE 2,3 CCW-269 SSF Aux Service Water Discharge 2,3 FDW-347 SSF Aux Service Water to B OTSG 2,3 HP-409 HPI Pump Discharge Crossconnect 1,2,3 HP-410 HPI Pump Discharge Crossconnect 1,2,3 LP-1 LPI Return Block Valves from RCS 1,2,3 LP-2 LPI Return Block Valves from RCS 3
LP-17 LPI RB Isolation Valves 2,3 LP-18 LPI RB Isolation Valves UNJI VALVE PURPOSE 1,3 LP-1 03 Post LOCA Boron Dilution to the Emergency Sump 1,3 LP-1 04 Post LOCA Boron Dilution to the Emergency Sump 1
LP-105 Post LOCA Boron Dilution to the Emergency Sump 3,
LPSW-566 RBCU Inlet Isolation Valve The staff finds that physical modification to valves susceptible to pressure locking is an appropriate corrective action to ensure operability of the valves and is, thus, acceptable.
- b.
The licensee stated that the following valves Will be modified or repiaced with a ball or globe valve to eliminate the potential for pressure locking during the Unit 1 refueling outage, Cycle 17, scheduled for fall 1997, Unit 2 refueling outage, Cycle 16, scheduled for spring 1998, or Unit 3 refueling outage, Cycle 17, scheduled for summer 1998:
U.NI VALVE PURPOSE 1
CCW-269 SSF Aux Service Water Discharge 1,2,3 CCW-287 SSF Aux Service Water Discharge I
FDW-347 SSF Aux Service Water to B OTSG 1,2 LP-17 LPI RBI Isolation Valve 1
LP-18 LPI RBI Isolation Valve 2
LP-103 Post LOCA Boron Dilution to the Emergency Sump 2
LP-1 04 Post LOCA Boron Dilution to the Emergency Sump 1,2,3 SF-82 SF Pool to RC Makeup Pump Suction Block Valves 1,2,3 SF-97 RC Return from Letdown Line
-4 Until these valves are modified, a methodology developed by the Commonwealth Edison Company (ComEd) to predict the thrust required to overcome pressure locking was used by the licensee to demonstrate operability for valves 1,2,3 CCW-287; 1 FDW-347; 1,2 LP-1 7; 1 LP-1 8, and 1,2,3 SF-82. A double disk gate valve pressure locking prediction methodology was used to demonstrate operability for valves 1,2,3 SF-97.
Valves 2 LP-103 and 2 LP-104 were declared inoperable until modifications are implemented.
The staff finds that these actions by the licensee provide reasonable assurance that the valves are acceptable until the planned modifications to prevent pressure locking are completed as scheduled. See paragraph 3.2.d for more discussion of the ComEd pressure-locking methodology. The licensee's modification of these valves will be an acceptable final resolution.
- c.
The licensee stated that procedures were modified to cycle the following valves following evolutions that could potentially create a thermal binding condition:
UNIT VALVE PURPOSE 1,2,3 LP-15 LPI Header Isolation Valve to HPI 1,2,3 LP-16 LPI Header Isolation Valve to HPI 1,2 LP-17 LPI RB Isolation Valves 1
LP-18 LPI RBI Isolation Valve The licensee stated that 2 LP-18 was replaced with a globe valve and that 1 LP-18 would be replaced with a globe valve during a future Unit 1 refueling outage to eliminate any potential for thermal binding. The staff finds that the licensee's procedural changes to require cycling the valves as corrective actions provide assurance that thermal binding conditions are eliminated, and are, thus, acceptable.
- d.
The licensee stated that, for long-term corrective action, the thrust prediction methodology developed by CornEd was used to demonstrate that the pressurizer power-operated relief valve (PORV) block valves, 1,2,3 RC-4, could open under pressure-locking conditions. Under certain conditions, the staff finds that the ComEd methodology provides a technically sound basis for assuring that valves susceptible to pressure locking are capable of performing their intended safety-related function.
On April 9, 1997, a public meeting was held to discuss the ComEd pressure-locking thrust prediction methodology presented in licensee submittals responding to GL 95-07.
The minutes of the public meeting were issued on April 25, 1997. At the public meeting, ComEd recommended that, when using its methodology, minimum margins should be applied between calculated pressure-locking thrust and actuator capability.
For those valves identified in this safety evaluation that use the ComEd pressure-locking thrust prediction methodology for interim and long-term corrective action, the licensee should ensure that the margin between calculated pressure-locking thrust and actuator capability meets or exceeds these minimum requirements.
-5 At the meeting, ComEd indicated that its methodology is undergoing review and may be revised or enhanced. The licensee is responsible for ensuring that the thrust values calculated to overcome pressure locking for these valves remain valid following implementation of any revisions or enhancements to the ComEd methodology. The staff is reviewing the ComEd methodology and ill issue a safety evaluation for the methodology.
- e.
The licensee stated that all flexible and solid wedge gate valves in the scope of GL 95-07 were evaluated for thermal binding. When evaluating whether valves were susceptible to thermal binding, the licensee assumed that thermal binding would not occur below specific temperature thresholds. These assumptions were based on industry experience. The licensee did not consider that gate valves in systems with a normal operating temperature less than approximately 200OF were susceptible to thermal binding. Further, the licensee did not consider that flexible wedge gate valves that are shut and that experience a cooldown differential temperature of less than 1000F, and solid wedge gate valves that are shut and that experience a cooldown differential temperature of less than 50OF prior to opening, were susceptible to thermal binding.
Operating conditions for the pressurizer PORV block valves, 1,2, 3 RC-4, exceed these temperature thresholds. These valves are equipped with a spring compensating actuator and are position seated, which reduces the potential for thermal binding.
The screening criteria used by the licensee appear to provide a reasonable approach to identify those valves that might be susceptible to thermal binding. Until more definitive industry criteria are developed, the staff concludes that the licensee's actions to address thermal binding of gate valves are acceptable.
- f.
A modified industry gate valve thrust equation was used to calculate the thrust required to open the reactor coolant return valves from the letdown line, 1,2,3 HP-428, during pressure locking conditions. The results of the calculation demonstrated that the margin between calculated pressure locking thrust and actuator capability is very large.
Pressure locking tests sponsored by the NRC were conducted by Idaho National Engineering and Environmental Laboratory on a double disk gate valve. The results of this testing are in the NRC's Public Document Room (Accession No. 9708060215). Test data demonstrated that the modified industry gate valve thrust equation trended with the pressure locking test results but generally underestimated the thrust required to open a pressure locked valve.
The staff finds that the modified industry gate valve thrust equation provides reasonable assurance that valves susceptible to pressure locking are capable of performing their intended safety-related function provided that the margin between calculated pressure locking thrust and actuator capability is very large. Until more definitive industry criteria are developed, the staff concludes that the licensee's action to address pressure locking of valves 1,2,3 HP-428 is acceptable.
-6
4.0 CONCLUSION
On the basis of this evaluation, the NRC staff finds that the licensee has performed appropriate evaluations of the operational configurations of safety-related power-operated gate valves to identify valves at the Oconee Nuclear Station Units 1, 2, and 3 that are susceptible to pressure locking or thermal binding. In addition, the NRC staff finds that the licensee has taken, or is scheduled to take, appropriate corrective actions to ensure that these valves are capable of performing their intended safety functions. Therefore, the staff concludes that the licensee has adequately addressed the requested actions discussed in GL 95-07.
Principal Contributor S. Tingen Date:
November 6, 1997