ML20206G747

Safety Evaluation Accepting Corrective Actions Taken by SNC to Ensure That Valves Perform Intended Safety Functions & Concluding That SNC Adequately Addressed Requested Actions in GL 95-07

ML20206G747
Person / Time
Site:	Farley
Issue date:	05/04/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20206G736	List: ML20206G734 ML20206G747
References
GL-95-07, GL-95-7, NUDOCS 9905100134
Download: ML20206G747 (4)

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~* -[ NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20688-0001

' SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION

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LICENSEE RESPONSE TO GENERIC LETTER 95-07. " PRESSURE LOCKING AND THERMAL BINDING OF SAFETY-RELATED POWER-OPERATED GATE VALVES" JOSEPH M. FARLEY NUCLEAR PLANT. UNITS 1 AND 2 DOCKET NOS. 50-348 AND 50-364 1.0 ' INTRODUCTION .

l Pressure locking and thermal binding represent potential common-cause failure mechanisms

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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 1 occurs in flexible-wedge and double-disk gate valves when fluid becomes pressurized inside i the valve bonnet and 'he actuator is not capable of overcoming the additional thrust .

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' 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 l valve is subjected to specific pressures and temperatures during various modes of plant i 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 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 Valve: ' 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 I L9905100134'990504

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I 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 l 4

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 surnmary 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)(i) because modification may be necessary to bring facilities into

, compliance with the rules of the Commission referenced above.

In a letter of February 12,1996, Southern Nuclear Operating Company, Inc. (SNC) submitted l

its 180-day response to GL 95-07 for Joseph M. Farley Nuclear Plant, Units 1 and 2 (Farley).

The NRC staff reviewed SNC's submittal and requested additional information in a letter dated July 1,1996. In a letter of July 30,1996, SNC provided the additionalinformation. In addition, in a letter dated April 13,1999, you submitted an amended page to your February 12,1996, GL 95-07 response to correct an error.

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. SNC letters of February 12 and July 30,1996, 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 performed in response to GL 95-07 and found it complete and acceptable.

The licensing basis for Farley, Units 1 and 2, is Hot Standby; therefore, valves that are operated during conditions below Hot Standby are not in the scope of GL 95-07. 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 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 configuration. SNC's submittals discussed 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:

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a. SNC stated that it used a thrust-prediction methodology developed by Commonwealth Edison Company (Comed) to demonstrate that the following valves could open under pressure-locking conditions:

1(2)-8000A/B Pressurizer Power Operated Relief Valve Block 1(2)-8801 A/B Boron Injection Tank Isolation 1(2)-8803A/B Boron Injection Tank isolation 1(2)-8884 Charging Pump to Reactor Coolant System Hot Leg 1(2)-8885 Charging Pump to Reactor Coolant System Cold Leg 1(2)-8886 Charging Pump to Reactor Coolant System Hot Leg 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 minutes of the public meet;ng 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. These margins along with diagnostic equipment accuracy and methodology limitations are defined in a letter from Comed to the NRC dated May 29,1998 (Accession Number 9806040184). The NRC considers tt,e use of the Comed pressure locking methodology acceptable provided these margins, diagnostic equipment accuracy requirements and methodology limitations are incorporated into the 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 industry 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 locking are capable of performing their intended safety-related function.

b. SNC stated that procedures require that the piping between the residual heat removal pump containment sump suction valve,2-MOV8811 A, and the containment sump be filled with water to a level that maintains approximately 10 feet of filled piping between 2-MOV8811 A and the containment sump to insulate the valve from the hot, post-accident sump fluid. The staff finds that SNC's procedural change to fill the piping between 2-MOV8811 A and the containment sump provide assurance that pressure locking conditions are eliminated and is acceptable corrective action.

c. SNC stated that the residual heat removal to reactor coolant system hot leg injection valves,1(2)-MOV8889, are susceptible to pressure locking but are not opened until approximately 11 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br /> after the initiating event. These valves are not exposed to thermal induced pressure-locking conditions during the 11-hour period, and seat leakage during the 11-hour period will prevent the valve from pressure locking. Also, there are 2 reactor coolant system hot leg injection flow paths (1(2)-

8884 and 1(2)-8888) from the charging pumps that were evaluated for pressure locking and are capable of operating during pressure locking conditions. The staff finds SNC's pressure locking analysis acceptable.

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d. SNC 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, SNC assumed that thermal binding would not occur below specific temperature thresholds. The screening criteria used by SNC appears 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 SNC'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 SNC has performed appropriate .  !

evaluations of the operational configurations of safety-related power-operated gate valves to l identify valves at Farley, Units 1 and 2, that are susceptible to pressure locking or thermal  !

binding. In addition, the NRC staff finds that SNC has taken appropriate corrective actions to ensure that these valves are capable of performing their intended safety functions. Therefore, the staff concludes that SNC has adequately addressed the requested actions discussed in GL 95-07.

Principal Contributor: S. Tingen, NRR l Date: May 4, 1999 1

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