ML20209H828

Safety Evaluation Accepting GL 95-07, Pressure Locking & Thermal Binding of Safety-Related Power-Operated Gate Valves, for Cooper Nuclear Station

ML20209H828
Person / Time
Site:	Cooper
Issue date:	07/15/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20209H824	List: ML20209H819 ML20209H828
References
GL-95-07, GL-95-7, NUDOCS 9907210077
Download: ML20209H828 (4)

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!' WASHINGTON, D.C. 20656 0001 l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION l LICENSEE RESPONSE TO GENERIC LETTER 95-07. " PRESSURE LOCKING l

[ AND THERMAL BINDING OF SAFETY-RELATED POWER-OPERATED GATE VALVES" COOPER NUCLEAR STATION DOCKET NO. 50-298 *

1.0 INTRODUCTION

i Pressure locking and thermal binding represent potential common-cause failure mechanisms that can render redundant safety systems incapable of performin thir safety functions. The identification of susceptible valves and the determination of wher & phenomena might occur require a thorough knowledge of components, systems, and plars :perations. 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 L gate valve tnat is closed while the system is hot and then is allowed to cool before an attempt is l made to.open the valve.

i i Pressure locking or thermal binding occurs as a result of the valve design characteristics

l. (wedge and valve body configuration, flexibility, 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.

l-2.0 REGULATORY REQUIREMENTS >

l Part 50 of Title 10 of the Code of Federal Reaulations (10 CFR) (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.

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 those safety-related power-operated gate valves that are Enclosure r.

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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 sue::sptible valves, identified in (1) above, are capable of performing their intended safety functions under all modes of plant operation, including test configurations. 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 60.109(a)(4)(i) because modification may be necessary to bring facilities into compliance with the rules of the Commission referenced above.

In a letter of February 13,1996, Nebraska Public Power District submitted its 180-day response to GL 95-07 for the Cooper Nuclear Station. The NRC staff reviewed the licensee's submittal and requested additional information in a letter dated May 28,1996. In a letter of June 27, 1996, the licensee provided the additional information. In a letter dated March 15,1999, the NRC staff requested additionalinformation. The licensee provided the additional information in a letter dated June 14,1999.

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 Nebraska Public Power District letters of February 13 and June 27,1996, and June 14,1999, described the scope of valves evaluated in response to GL 95-07. Normally open, safety-related power-operated gate valves that are closed for test or surveillance but must return to the open position were evaluated within the scope of 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. The criteria for determining the scope of power-operated valves for GL 95-07 are consistent with the staff's acceptance of the scope of motor-operated valves associated with GL 89-10, " Safety-Related Motor-Operated Valve Testing and Surveillance."

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 configurations. The licensee's submittals discussed corrective actions to address potential pressure-locking and thermal-binding problems. The staff's evaluation of the licensee's actions is d;scussed in the following paragraphs:

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a. The licensee stated that the following valves were modified to eliminate the potential for pressure locking:

CS-MOV-MO12A/B Core Spray injection Valves HPCI-MOV MO19 High Pressure Coolant injection (HPCI) Injection RCIC-MOV-MO21 Reactor Core Isolation Cooling System injection 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 residual heat low pressure coolant injection valves, RHR-MOV-MO25A/B, are equipped with body drains and that procedures were revised to align the body of each valve to vent to the reactor side piping to eliminate the potential for the valves to pressure lock. The staff finds that the licensee's ,

procedural changes eliminate the potential for pressure 'ocking conditions, and are ,

thus acceptable.

c. A flexible wedge pressure locking thrust prediction methodology was used to calculate the thrust required to open the following valves during pressure locking .

conditions: -

HPCI-MO14 Steam Admission to HPCI Turbine .

HPCI-MO58 HPCI Suppression Pool Suction  :

RHR-MO39A/B Suppression Pool Cooling Loop Isolation The results of the licensee's flexible wedge pressure locking thrust prediction

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methodology demonstrates that there is a positive margin between calculated pressure locking thrust and actuator capability. However, the margin between actuator capability and calculated pressure locking thrust was less than that required for long-term corrective action. As long-term corrective action, the licensee stated that the actuators will be modified to increase actuator capability to obtain the desired margin or the valves will be modified to eliminate the potential for pressure locking. The corrective action is scheduled to be implemented during the 2001 refueling outage. The staff finds that the licensee's pressure locking thrust prediction methodology provides reasonable assurance that flexible wedge gate g

valves susceptible to pressure locking are capable of performing their intended safety-related function, and is an acceptable short-term corrective action.

Pressure locking tests sponsored by the NRC were conducted by Idaho National Engineering and Environmental Laboratory. The results of this testing are documented in NUREG/CR-6611,Results of Pressure Locking and Thermal Binding Tests of Gate Valves." NUREG/CR-6611 test results demonstrate that the licensee's pressure locking thrust prediction methodology conservatively estimates the thrust required to open a pressure locked flexible wedge gate vahre. The staff finds that the licensee's long-term corrective action to modify the actuators to increase actuator capability and use of the pressure locking thrust prediction methodology provides reasonable assurance that flexible wedge gate valves susceptible to pressure locking are capable of performing their intended safety-

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I related function. The staff considers that calculations used to demonstrate that valves can overcome pressure locking are required to meet the requirements of 10 CFR Part 50, Appendix B, Quality Assurance Criteria for Nuclear Power Plants, and therefore, controls are required to be in place to ensure that pressure locking thrust prediction methodology requirements and revisions are properly implemented. Until more definitive industry criteria are developed, the staff concludes that the licensee's action to address pressure locking of these flexible wedge gate valves is acceptable.

d. 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. Operating conditions for the LPCI inject:on valves, RHR M039A/B, exceed these temperature thresholds.

The licensee stated that instructions were issued to cycle the valves following evolutions that could potentially create a thermal binding condition. The staff finds that (1) the licensee's procedures to cycle the valves as corrective action provide assurance that thermal binding conditions are identified, (2) the screening criteria used by the licensee appear to provide a reasonable approach to identify those valves that might be susceptible to thermal binding, and (3) until more definitive industry criteria are developed, the licensee's actions to address thermal binding of gate valves are acceptable.

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 Cooper Nuclear Station 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: July 15,1999 l-