ML20206L424

Safety Evaluation Supporting Corrective Actions to Ensure That Valves Are Capable of Performing Intended Safety Functions & OPPD Adequately Addressed Requested Actions Discussed in GL 95-07

ML20206L424
Person / Time
Site:	Fort Calhoun
Issue date:	05/10/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20206L421	List: ML20206L415 ML20206L424
References
GL-95-07, GL-95-7, NUDOCS 9905140011
Download: ML20206L424 (4)

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UNITED STATES j

• NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. RoseH001 )

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION GENERIC LETTER 90 07. " PRESSURE LOCKING  !

AND THERMAL BINDING OF SAFETY-RELATED POWER-OPERATED GATE VALVES" OMAHA PUBLIC POWER DISTRICT I FORT CALHOUN STATION. UNIT 1 DOCKET NO. 50-285 1.0 jfiTRODUCTION Pressure locking and thermal binding represent potential common-cause failure mechanisms l that can rendor redundant safety systems incapable of performing their safety functions. The l 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 the valve bonnet and the actuator is not capable of overcoming the additional thrust requirements resulting frcm 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 ir hot and then is allowed to cool before an attempt is made to open the valve.

Pressure locking or thermal b!nding 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 Part 50 of Title 10 of the Code of Federal Regulations (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 ossurance that those systems j 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 tM additional provisions of 10 CFR Part 50 (Appendix B, Criterion XVI), licensees I are expected So act to ensure that safety-related power-operated gate valves susceptible to pressure kx ang 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 susceptible to pressure Icting or thermal binding are capable of performing their safety functions within the current licensing basis of the facility. GL 95-07 requested that each I licensee, within 180 days of the date of issuance of the generic letter (1) evaluate the  !

9905140011 990510 PDR ADOCK 05000285 P PDR 1

. .2-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 ar.d 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 iest configurations. In addition, GL;95-07 requested that licensees, within 180 days of the date of issuance of the generic letter, prr,; vide 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, [ncluding 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 13,1996, Omaha Public Power District (OPPD) submitted its 180-day response to GL 95-07 for Fort Calhoun Station, Unit 1. The NRC staff reviewed OPPD's submittal and requested additional information in a letter dated June 27,1996. In a letter of July 30,1996, OPPD provided the additionalinformation. In letters of February 27 and November 9,1998, the licensee revised its February 13 and July 30,1996, submittals.

3.0 STAFF EVALUATION 3.1 Scope of : icensee's Review GL 95-07 requested that licensees evaluate the operational configurations of safety-related power-opersted gate valves in their plants to identify valves that are susceptible to pressure locking or thermal binding. The OPPD letters of February 13 and July 30,1996, and February 27 and November 9,1998, described the scope of valves evaluated in response to GL 95-07. During a telephone conversation on December 1,1998, OPPD clarified its basis for determining that feedwater header valves, HCV-1384, HCV-1385 and HCV-1386, are not in the

, scope of GL 95-07. These valves are not in the scope of GL 95-07 because this attemate flow path is nct classified as safety-related. The information provided by OPPD is subject to confirmation during future NRC inspections.

The NRC staff has reviewed the ccope of OPPD'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 staffs acceptance of the scope of motor-operated valves associated with GL 89-10, " Safety-Related Motor-Opere'.ed Valve Testing and Surveillance."

3.2 Corrective Actions GL 95-07 requested that licensees, within 180 days, perform further analyses as appropriate, j 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

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

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

HCV-268 Boric Acid Pump to Charging Pump Suction HCV-347/348 Shutdown Cooling Suction isolation 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. OPPD 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. The operating conditions for the following valves exceeded the temperature thresholds:

HCV-150/151 Pressuriter Power Operated Relief Valve Block HCV-258/265 Boric Acid Storage Tank Outlet Isolation HCV-268 Boric Acid Pump to Charging Pump Suction '

HCV-308 High Pressure Safety injection / Charging Cross tie LCV-218-3 Charging Pump Suction Header and Boric Acid Supply OPPD stated that the heat trace was removed from HCV-258/265/268 and LCV-218-3 to eliminate the source of heat; therefore, the valves are not clot.ed when hot and that procedures were modified to cycle HCV-308 followirig evolutions that could potentially create a thermal binding condition.

During a telephone conversation on December 1,1998, OPPD clarified its basis for assuring that valves HCV-150/151 would operate during thermal binding conditions.

OPPD stated that testing performed at a test facility confirmed that HCV-150/151 are susceptible to thermal binding. This testing involved the measurement of unseating forces with diagnostic test equipment and identified the unseating forces required to open the valves during thermal binding conditions. OPPD stated that calculations dsmonstrated that the valves are capat;le of opening during thermal binding and degraded voltage conditions. The information provided by the licensee t is subject to confirmation during future NRC inspections.

!l The screening criteria used by OPPD appear to provide a reasonable approach to identify those valves that might be susceptible t 1 therrnH binding. Until more definitive industry criteria are developed, the staff concludes that OPPD's actions to address thermal binding of gate valves are acceptable.

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4.0 CONCLUSJQH .

On the basis of this evaluation, the NRC staff finds thot OPPD has performed appropriate evaluations of the operational configurations of safety related power-operated gate valves to ,

I identify valves at the Fort Calhoun Station, Unit 1, thist are susceptible to pressure locking or I thermal binding. In addition, the NRC staff finds that OPPD has taken appropriate corrective actions to ensure that these valves are capable of performing their intended safety functions.

l Thsrefore, the staff concludes that OPPD has adequately addressed the requested actions l . discussed in GL 95-07.

Principal Contributor: S. Tingen 1 l

Date: May 10, 1999 l

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