ML20211N016
| ML20211N016 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 09/02/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20211N012 | List: |
| References | |
| GL-95-07, NUDOCS 9909100025 | |
| Download: ML20211N016 (5) | |
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t UNITED STATES NUCLEAR REGULATORY COMMISSION U
f WASHINGTON, D.C. 3088H001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION GENERIC LETTER 95-07. " PRESSURE LOCKING AND THERMAL BINDING OF SAFETY-RELATED POWER-OPERATED GATE VALVES" WOLF CREEK NUCLEAR OPERATING CORPORATlq1 WOLF CRFFK GENERATING STATION. UNIT 1 DOCKET NO. 50-482
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 4
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 s
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 tha design basis for valves.
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-2.0 REGULATORY REQUIREMENTS Appendix A, General Design Criteria 1 and 4 of 10 CFR Part 50 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
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provisions of 10 CFR Part 50 (Appendix B, Criterion XVI), licensees are expected to act to j
ensure that safety-related power-operated gate valves that are susceptible to pressure locking or thermal binding re capable of performing their required safety functions.
. On August 17,1995, the NRC issued Generic Letter (GL) 95-07, " Pressure Locking and Thamal Binding of Safety-Related Power-Operated Gate Valves," to request that licensees taL 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 9909100025 990902 PDR ADOCK 05000482
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2-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 configurations. In addition, the NRC in 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 f cilities into compliance with the rules of the Commission referenced above, in a letter of February 14,1996, Wolf Creek Nuclear Operating Corporation (the licensee) submitted its 180-day response to GL 95-07 for Wolf Creek Generating Station, Unit 1 (WCGS). The staff reviewed the licensee's submittal and requested additional information in a letter dated May 30,1996. In a letter of July 3,1996, the licensee provided the additional informatiort in a letter of April 1,1999, the NRC issued a second request for information and in a letter of August 3,1999, the licensee provided the requested additional information. -
3.0 STAFF EVALUATION
- 3.1. Scope of Licensee's Review i
GL 95C 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 3
locking or thermal binding. The licensee's letters of February 14 and July 3,1996, and August 3,1999, described the scope of valves that the licensee evaluated in response to GL 95-07. The staff has reviewed the scope of the licensos's susceptibility evaluation performed in response to the GL and found it complete and acceptable.
The residual heat removal (RHR) pump suction valves from the reactor coolant system (RCS) hot legs, BBPV8702A/B and EJHV8701 A/B, were not included 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 WCGS is hot standby. 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 except in the instances when the system / train is declared inoperable in accordance with the technical specifications for the plant. 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-Operated Valve Testing and Surveillance."
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. 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 configurations. The licensee's submittals for WCGS discussed corrective actions to address potential pressure-locking and thermal-binding problems at the plant. The staffs evaluation of the licensee's actions for WCGS is discussed in the following paragraphs; a.
The licensee stated that it used a thrust-prediction methodology developed by Commonwealth Edison Company (Comed) to demonstrate that the following valves are 4
capable of opening during pressure-locking conditions:
BBHV8000A/B Pressurizer Power Operated Relief Valve (PORV) Block BNHV8812A/B RHR Pump Suction from Refueling Water Storage Tank EJHV8804A/B RHR to Safety igiection and Charging Pump Suction EJHV8811 A/B RHR Pump Suction from Containment Sump EJHV8840 RHR Pump Discharge to RCS Hot Leg EMHV8801A/B Charging Pump Discharge to RCS Cold Leg EMHV8802A/B Safety injection Pump Discharge to RCS Hot Leg EMHV8803A/B Charging Pump Discharge to RCS Cold leg -
ENHV0001 Containment Spray Pump Suction from Containment Sump ENHV0006 Containment Spray Pump Discharge ENHV0007 Containment Spray Pump Suction from Containment Sump ENHV0012 Containment Spray Pump Discharge ENHV0015 Spray Additive Eductor Isolation ENHV0016 Spray Additive Eductor isolation The licensee also stated that air expansion pipes were also installed on the bonnet of ENHV0001, ENHV0007, and EJHV8811 A/B to provide a volume of air to limit the increase of pressure in the bonnet of each valve during thermal induced 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 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. 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 Fuel Reprocessing Plants." Therefore,
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4 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.
The licensee stated that the valve actuator force was greater than the required thrust for valves BBHV8000A/B, BNHV88128, EJHV8811NB, EJHV8804A/B, EJHV8840, EMHV8801NB, EMHV8802NB, EMHV8803B, ENHV0001, ENHV0006, ENHV0012, ENHV0015, and ENHV0016; however, the margin between the actuator capability and the required thrust was less than that required to meet the Comed methodology pressure-locking acceptance criteria. The licensee stated that the short-term corrective action was the demonstration that there was margin for the valve actuators to provide the required thrust for these valves and that the long-term corrective action would be modifications / facility changes to (1) eliminate the potential for pressure locking; or (2) increase the margin to meet the Comed pressure-locking acceptance criteria. The long-term corrective actions are scheduled to be completed by the end of the Spring 2002 refueling outage.
The staff finds that the licensee'c short-term and long-term actions provide assurance that pressure locking conditions are adequately identified and eliminated, and are thus acceptable.
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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. Operating conditions for the pressurizer PORV block valves, BBHV8000NB, and RHR pump minimum flow recirculation valves, EJFCV0610 and EJFCV0611, exceed these temperature thresholds. The valve actuators for BBHV8000NB are equipped with a spring compensator and are position seated which reduces the potential for thermal binding. As a corrective action, the valve actuators for EJFC0610 and EJFC0611 were modified to close using the limit switches with minimal seating thrust to reduce the potential for the valves to thermally bind.
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 va'ves are acceptable.
4.0 CONCLUSION
On the basis of this evaluation, the staff finds that the licensee has performed appropriate evaluations of the op3 rational configurations of safety-related power-operated gate valves to idenkfy valves at WCGS that are susceptible to pressure locking or thermal binding. In addition, the staff finds that the licensee has taken, or is scheduled to take, appropriate corrective actions to ensure that these valves are capable of performing their intended safety functions. The licensee has committed to take certain future actions to be completed by the i
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L I i Spring 2002 refueling outage for 18 valves, as discussed in Section 3.2.s above. This is acceptable.
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. Therefore, the staff concludes that the licensee has adequately addressed for WCGS the requested actions discussed in GL 95-07.
PrincipalContributor: S.Tingen Date:
September 2, 1999 i
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