ML20210J927

SER Finds That Licensee Performed Appropriate Evaluations of Operational Configurations of safety-related power-operated Gate Valves to Identify Valves at Plant,Susceptible to Pressure Locking or Thermal Binding

ML20210J927
Person / Time
Site:	Hatch
Issue date:	08/02/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20210J907	List: ML20210J902 ML20210J927
References
GL-95-07, NUDOCS 9908050144
Download: ML20210J927 (4)

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g E WASHING TON, D. C. 20:i55 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" EDWIN l. HATCH. UNITS 1 AND 2 DOCKET NUMBERS 50-321 AND 50-366

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 press 6rized 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 Title 10 of the Code of Federal Reaulations (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 coinponents 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 Enclosure 9908050144 490802 PDR ADOCK 05000321 P PDR

i 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 of thermal binding, (2) the results of the cusceptibility evaluation, including a listing of the susceptible valves identified, and (3) the corrective actions, or other disoositioning, for the valves identified as susceptible to pressure locking or thermal binding. T 2 NRC issued GL 95-07 as a " compliance backfit" pursuant to 10 CFR 50.109(a)(4)(i) becs e modification may be necessary to bring facilities into compliance with the rules of me Commission referenced above.

In a letter of February 12,1996, Southern Nuclear Operating Company, Inc., submitted its 180-day response to GL 95-07 for Edwin I. Hatch Nuclear Plant, Units 1 and 2. The NRC staff reviewed the licensee's submittal and requested additional information in a letter dated May 23,1996. In a letter dated June 21,1996, the licensee provided the additional information.

In letters dated July 6,1998, and July 9,1999, the licensee supplemented its 180-day response to GL 95-07.

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 Southern Nuclear Operating Company, Inc., letters dated February 12, and June 21,1996, July 6,1998, and July 9,1999, 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.

I Normally open, safety-related power-operated gate valves which are closed for test or surveillance but must return 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 vat es 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 configuration. The licensee's submittals discussed L

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J proposed corrective actions to address potential pressure-locking and thermal-binding problems. The staff's evaluation of the licensee's actions is discussed in the following  !

paragraphs:

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

l' 1/2E11-F015A/B Lcw Pressure Coolant Injection (LPCI) 1/2E21-F005A/B Core Spray injection j 1/2E41 F006 High Pressure Coolant Injection (HPCI) l 1/2E51-F013 Reactor Coie Isolation Cooling injection 2P41-F115A/B Essential LPCI Inverter Room Cooler inlet The staff finds that physical modification to valves susceptible to pressure locking is an appropriato corrective action to ensure operability of the valves and is thus acceptable.

b. The licensee stated that it used a pressure locking thrust prediction methodology developed by Commonwealth Edison Company (Comed) to demonstrate that the residual heat removal torus spray outboard isolation valves, 1/2E11-F028A/B, could open under pressure-locking conditions.

On April 9,1997, the staff held a public meeting to discuss the technical adequacy of the Comed pressure locking thrust prediction methodology and its generic use by licensees in their submittals responding to GL 95-07. The i minutes of the public meeting were issued on April 25,1997. At the public l meeting, Comed recommended that, when using its methodology, minimum  ;

margins should be apphed between calculated pressure-locking thrus'. and i actuator capability. These margins along with diagnostic equipment accuracy and methodology limitations sie defined in a letter from Comed to the NRC dated May 29,1998 (Accession Number 9806040184). NRC considers the use of the Comed pressure locking thrust prediction methodology to be acceptable as a long-term corrective action provided these margins, diagnostic equipmen' l accuracy requirements and methodology limitations are incorporated into the l pressure-locking calculations. Comed indicated that its methodology may be revised. The staff considers that calculations that are 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 any icdustry pressure locking thrust prediction methodology requirements and revisions are properly implemented. Under this condition, the staff finds that the Comed methodology provides a technically sound basis for assuring that valves susceptible to pressure lockir,g are capable of performing their intended safety-related function and that the licensee's action to address pressure locking of these flexible wedge gate valves is acceptable.

c. 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 L

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! would not occur below specific temperature thresholds. The HPCI turbine steam

( admission valves,1/2E41-F001, exceeded these temperature limitations;

! however, operational history demonstrated that the valves are not susceptible to 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.

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 l

identify valves at the Edwin I. Hatch Nuclear Plant, Units 1 and 2. that are susceptible to pressure locking or thermal binding. In addition, the NRC staff finds that the licensee has taken l 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 j addressed the reque .ed actions discussed in GL 95-07.

< Principal Contributor: S. Tingen Date: August 2,1999 l

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