ML20199F478

Safety Evaluation Concluding That Bg&E Performed Appropriate Evaluations of Operational Configurations of safety-related power-operated Gate Valves to Identify Valves Susceptible to Pressure Locking.Concludes GL 95-07 Actions Were Addressed

ML20199F478
Person / Time
Site:	Calvert Cliffs
Issue date:	01/08/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20199F418	List: ML20199F410 ML20199F478
References
GL-95-07, GL-95-7, NUDOCS 9901210324
Download: ML20199F478 (5)

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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" CALVERT CLIFFS NUCLEAR POWER PLANT. UNIT NOS.1 AND 2 DOCKET NOS. 50-317 AND 50-318 i

1.0 INTRODUCTION

Pressure locking and thermal binding represent potential common-cause failure mechanisms 3 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 requires 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. Otherindividual 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.

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i 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 those 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 (orjustify 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 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 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 dated February 13,1996, Baltimore Gas and Elecinc Company (BGE) submitted its 180-day response to GL 95-07 for Calvert Cliffs Nuclear Power Plant, Units 1 and 2. The NRC staff reviewed the licensee's submittal and requested additional information in a letter dated June 18,1996. In a letter dated July 25,1996, the licensee provided the additional information. The NRC staff performed two inspections to review specific aspects of information summarized in the licensee's responses to GL 95-07. These inspections are documented in NRC Inspection Reports 50-317,318/97-02 and 50-317,318/98-09. In a letter dated December 2,1998, 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 eva!uate 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 BGE letters dated February 13, and August 7,1996, and December 2,1998, 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-G' .ad found it complete and acceptable. The licensee did not include the shutdown cooling hea exchanger low-pressure safety injection supply isolation valves, '

1/2MOV-658, in the scope of GL 95-07 because these valves are used during plant conditions below hot standby. This is acceptable because the safe shutdown design basis for Calvert Cliffs Nuclear Power Plant, Units 1 and 2,is hot standby. 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 (orjustify 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 submittats discussed 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 instrument air containment isolation valves,1MOV-651 and 1MOV-652, are not susceptible to pressure locking because a relief path is installed between each valve's bonnet and its upstream pipe. 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 it used a thrust-prediction methodology developed by Commonwealth Edison Company (Comed) to demonstrate that the following valves could open under pressure-locking conditions:

1/2MOV-504 Charging Pump Suction From Refueling Water Tank 1/2MOV-514 Boric Acid Pump Discharge 1/2MOV-4144 Emergency Sump Recirculation 1/2MOV-4145 Emergency Sump Recirculation 1/2MOV-6903 Containment Hydrogen Purge 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 meeting 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). NRC considers the 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.

c. The licensee stated that instrument air containment isolation valves,2MOV-651 and 2MOV-652, would be modified during the Unit 2 refueling outage scheduled for the spring of 1999, to eliminate the potential for pressure locking. Until these valves are

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modified, the licensee is relying on specific operating experience for these valves to demonstrate operability. The staff finds that the licensee's analysis provides reasonable assurance of operability of these va!ves until the planned modifications to 1 prevent pressure locking are completed as scheduled. The licensee's commitment to l modify these valves in the long term is an acceptable final resolution.

d. The licensee stated that containment hydrogen purge valves,1/2MOV-6900 and 1/2MOV-6901, were not be susceptible to pressure locking. These valves are shut during certain accident scenarios and are not re-opened until 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> later into the event. The licensee stated that these valves are not exposed to thermal induced pressure-locking conditions during the 48-hour period and that seat leakage during the 48-hour period would prevent the valves from pressure locking. The staff finds the licensee's analysis acceptable to eliminate the potential for pressure locking.

e. A modified industry double disk gate valve thrust equation was used to calculate the thrust required to open instrument air containment isolation valves,1/2MOV-2080, during pressure locking conditions. The results of the calculation demonstrated that the margin between calculated pressure locking thrust and actuator capability exceeds 40 percent.

Pressure locking tests sponsored by the NRC were conducted by Idaho National Engineering and Environmental Laboratory on a double disk gate valve. The results of this testing are documented in NUREG/CR-6611,"Results of Pressure Locking and Thermal Binding Tests of Gate Valves." Test data demonstrated that the modified industry gate valve thrust equation trended with the pressure locking test results but generally underestimated the thrust required to open a pressure-locked valve. The staff finds that the modified industry gate valve thrust equation with the margin indicated in the Calvert Cliffs calculation provides reasonable assurance that valves susceptible to pressure locking are capable of performing theirintended safety-related function. Until more definitive industry criteria are developed, the staff concludes that the licensee's action to address pressure locking of gate valves 1/2MOV-2080 is acceptable,

f. 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. The screening criteria used by the licensea uppear 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 i acceptable.

' Operating conditions for pressurizer power operated relief block valves,1/2MOV-403 and 1/2MOV-405, exceed these temperature thresholds. These valves are position seated which reduces the potential for thermal binding. The operating conditions for 1/2MOV-651 and 1/2MOV-652 also exceed these temperature thresholds. The licensee stated that testing performed during thermal binding conditions demonstrated that the valves were not susceptible to thermal binding. This testing involved the

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measurement of unseating forces with diagnostic test equipment to verify that the unseating forces did not significantly increase. The staff finds that position seating and testing are acceptable methods for demonstrating that 1/2MOV-403,1/2MOV-405, 1/2MOV-651 and 1/2MOV-652 are not susceptible to thermal binding. ,

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 Calvert Cliffs Nuclear Power Plant, Units 1 and 2, that are susceptible to pressure locking or thennal binding. In addition, the NRC staff finds that the g licensee has taken, or is scheduled to take, appropriate corrective actions to ensure that these valves are capable of performing theirintended safety functions. Therefore, the staff l

concludes that the licensee has adequately addressed the requested actions discussed in GL 1 95-07.

1 PrincipalContributor. S.Tingen Date: January 8,1999 i

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