ML20205N086

Safety Evaluation Re Licensee Response to GL 95-07, Pressure Locking & Thermal Binding of Safety-Related Power-Operated Gate Valves

ML20205N086
Person / Time
Site:	Monticello
Issue date:	04/12/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20205N085	List: ML20205N082 ML20205N086
References
GL-95-07, GL-95-7, NUDOCS 9904160147
Download: ML20205N086 (4)

Text

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION LICENSEE RESPONSE TO GENERIC LETTER 95-07. " PRESSURE LOCKING AND THERMAL BINDING OF SAFETY-RELATED POWER-OPERATED GATE VALVES" NIONTICELLO NUCLEAR GENERATING PLANT DOCKET NO. 50-263

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, 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.

2.0 REGULATORY REQUIREMENTS 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 9904160147 990412 PDR ADOCK 05000263 P PDR

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 GL (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 valve , 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 GL, provide the NRC a summary description of (1) the susceptibility evaluation used to determine that valves are or are not susceptible to pressure locking or therr, al 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)(i) because modificatior; may be necessary to bring facilities into compliance with the rules of the Commission referenced above.

in a letter of February 12,'1996, Northem States Power Company submitted its 180 day response to GL 95-07 for Monticello Nuclear Generating Plant. The NRC staff reviewed the licensee's submittal and requested additional information in a letter dated June 18,1996. In a letter dated July 17,1996, the licensee provided the additional information.

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 Northern States Power Company letters of February 12, ant.

July 17,1996, 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 parformed in response to GL 95-07 and found it complete and acceptable.

Normally open, safety-related power-operated gate valves which are closed for test or surveillance but must retum to the open position were evaluated within the scope of GL 95-07.

The staff finds 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 appropriGe, 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. The licensee's submittals discussed 9

. 3 problems. The staffs evaluation of the licensee's actions is discussed in the foilowing paragraphs:

a. The ficensee stated that the following valves have bean modified to eliminate the potential for pressure locking:

MO-1753 Core Spray Inboard injection MO-1754 Core Spray Inboard injection MO-2006 Residual Heat Removal (RHR) Torus Cooling / Spray MO-2007 RHR Torus Cooling / Spray MO-2014 A RHR Low Pressure Coolant injection (LPCI) Inboard MO-2015 B RHR LPCI Inboard MO-2036 High Pressure Coolant injection ( HPCI) Steam Supply MO-2067 HPCI Injection MO-2068 HPCI Injection MO-2106 Reactor Core isolation Cooling (RCIC) Injection MO-2107 RCIC Injection The steff finds that physical modification to valves susceptible to pressure locking is are appropriate corrective action to ensure operability of the valves andis thus acceptable.

b. The licensee stated that the following valves were susceptible to pressure locking when closed for testing and that procedures were revised to declare the affected system / train inoperable during the time that the valves are in the closed position.

MO-1751 Core Spray Outboard injection MO-1752 Core Spray Outboard injection MO-2034 HPCI Steam isolation inboard MO-2035 HPCI Steam isolation Outboard The licensee stated that these valves may be modifieo in the future to eliminate the potential for pressure locking and that the system / train would not be declared inoperable when a modified valve is in the closed position. The staff finds that physical modification or declaring the system / train inoperable when these valves are closed is acceptable corrective action for considering the potential for gate valves to undergo pressure locking during testing.

c. The licensee stated that the following valves were susceptible to thermal binding and that procedures were revised to cycle the valves during plant shutdown when the technical specifications reouire that the system be operable:

MO-2036 HPCI Steam Supply MO-2068 HPCIInjection MO-2107 RCIC Injection

a

, 4 The staff finds that the licensee'r, procedural changes to require cycling the valves provide assurance that thermal-binding conditions, are adequately identified and eliminated, and are thus acceptable.

d. The licensee stated that a!! 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. 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 addless 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 Monticello Nuclear Generating Plant that are susceptible to pressure locking or thermal binding. In addition, the NRC staff finds that the licensee has taken 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. NRR Date: April 12, 1999