ML20209H766
| ML20209H766 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 07/15/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20209H755 | List: |
| References | |
| NUDOCS 9907210064 | |
| Download: ML20209H766 (5) | |
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W aru g 4 UNITED STATE 8 4 s" j- NUCLEAR REGULATORY COMMISSION l
& WASHINGTON, D.C. 20066 4 001
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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" ,
COMANCHE PEAK STEAM ELECTRIC STATION (CPSES). UNITS 1 AND 2 DOCKET NOS. 50-445 AND 50-446 . :
1.0 INTRODUCTION
I Pressure locking and thermal binding represent potential common-cause failure mechanisms l 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 s 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 j made to open the valve.
Pressure locking or thermal binding occurs as a result of the valve design characteristics j (wedge and valve body configuration, flexibility, and material thermal coefficients) when the }
valve is slibjected to specific pressures and temperatures during various modes of plant operation. Operating experience indicates that these situations were not always considered in f many plants as part of the design basis for valves, j
-. 2.0 . REGULATORY REQUIREMENTS Part 50 of Title 10 of the Code of Federal Reaulations (10 CFR) (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.
Enclosure 9907210064 990715 PDR ADOCK 05000445 '
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On August 17,1995, the NRC issued Generic Letter (GL) 95-07, Pressure Locking and Thermal Binding of Safety-Re!ated Power-Operated Gc.te Valves," to request that r 3ees take certain actions to ensure ihose safety-related power-operated gate valves th. ., e 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 conyigura' ions 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, , {
GL 95-07 requested that licsnsees, within 180 days of the date of issuance of the generic letter, provide, to the NRC staff, a summary descript;on of (1) the susceptibility evaluation used to ,
determine that valves are or are not susceptible to p essure locking or thermal binding, (2) the i 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 pressurs locking or thermal binding. The NRC issued GL 95-07 as a " compliance backfic 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 previously discussed.
in a letter of February 13,1996, TU Electric (:he licensee), submitted its 180-day response to GL 95-07 for CPSES, 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 of July 18,1996, the licensee provided the additionalinformation. The licensee supplemented its 180-day response to GL 95-07 in letters dated March 14,1997, and May 25 and June 14,1999. During the period of January 24 through March 6,1999, the NRC staff performed an inspection to review specific aspects of the licensee's operability evaluation of valves susceptible to pressure locking. This inspection is documented in NRC Inspection Report 50-445,446/99-03.
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 Aeir plants to identify valves that are susceptible to pressure locking or thermal binding. The TU Electric letters of February 13 and June 18,1996, March 14,1997, and May 25 and June 14,1999, 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. The licensee did not include the residual heat removal (RHR) pump suction valves from the reactor coolant system (RCS) hot legs, 1/2-8701 A/B and 1/2-8702A/B, 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 CPSES is Hot Standby. 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 criteria for determining the scope of power-operated valves for GL 95-07 are consistent with the NRC staff's 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 discussed corrective actions to address potential pressure-locking and thermal-binding problems. The NRC staff's evaluation of the licensee's actions is discussed in the following paragraphs:
- a. The licensee stated that it used a pressure locking thrust-prediction methodolcgy developed by Commonwealth Edison Company (Comed) to demonstrate that the following valves could open under pressure-locking conditions:
1/2-HV4776 Containment Spray Pump Discharge 1/2-HV4777 Containment Spray Pump Discharge 1/2-8801A/B Charging Pump Discharge to RCS Cold Legs 1/2-8000A/B Pressurizer Power-Operated Relief Valve Block 1/2-8802A/B Safety injection Pump Discharge to RCS Hot Legs 1/2-8804A/B RHR to Charging Pump Suction 1/2 8807A/B RHR to Safety injection Pump Suction 1/2-8812A/B RHR Pump Suction From Refueling Water Storage Tank 1/2-8840 RHR Pump Discharge to RCS Hot Leg '
The licensee stated that the margin between actuator capability and calculated pressure-locking thrust for valves 1-8801 A and 1-8804A/B was less than that I' recommended by Comed. As corrective action, the actuators will be modified (1-8804A/B) and the torque switch will be adjusted (1 8801 A) to increase actuator capability to obtain the desired margin. These corrective actions are scheduled to be implemented during the fall 1999 Unit 1 refueling outage.
The licensee stated that the containment spray pump containment sump suction valves, 1/2-HV4782 and 1/2-HV4783, and the RHR pump containment sump suction valves, 1/2-8811 A/B, are susceptible to pressure locking. Relief valves are installed in the bonnet of each valve to limit the pressure inside each valve's bonnet. The Comed pressure locking thrust prediction methodology was used to calculate the thrust required to open these valves during pressure-locking conditions at pressures less than the relief ,
valve setpoint. The results of the calculation demonstrated that the valves would '
operate during pressure-locking conditions. The margin between actuator capability and !
calculated pressure-locking thrust for valves 1-8811 A/B, was less than that recommended by Comed. As corrective action, the relief valves will be modified to lower the setpoint to obtain the desired margin. These corrective actions are scheduled to be implemented during the fall 1999 Unit 1 refueling outage..
The licensee stated that the RHR crosstie isolation valves,1/2-8716A/B, are susceptible !
to pressure-locking. These valves are required to open when aligning the RHR system to the RCS hot legs. As corrective action, the licensee plans to install relief valves in the l bonnet of each valve to limit the pressur9 inside each valve's bonnet. The Comed pressure locking thrust prediction methodology will be used to calculate the thrust required to open these valves during pressure-locking conditions at pressures less than
'4. .h l.
x the relief valve setpoint. These corrective actions are scheduled to be implemented during the spring 2001 Unit 1 refueling outage and the fall 2000 Unit 2 refueling outage.
As temporary corrective action, the licensee performed an analysis that concluded that the safety injection to RCS hot leg flow path valves,1/2-8802A/B, were evaluated for pressure locking and capable of operating during pressure-locking conditions. The NRC staff finds the licensee's pressure-locking analysis acceptable until the modifications are implemented. -
On April 9,1997, the NRC staff held a public meeting to discuss the technical adequacy of the Comed pres:eure-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 ComEo to the NRC dated May 29,1998 (Accespon Number 9806040184). The NRC considers the use of the Comed pressure-locking thrust prediction methodology acceptable for long-term corrective action provided these margins, diagnostic equipment accuracy J
requirements, and methodology limitations are incorporated into the pressure-locking calculations. The Comed pressure locking thrust prediction methodology is acceptable for short-term corrective action provided that there is a positive margin between calculated pressure-locking thrust and actuator capability. 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'.he requirements of 10 CFR Part 50, Appendix B, Quality Assurance Criteria for Nuclear
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1 Power Plants, and therefore, controls are required to be in place to ensure that any I industry pressure locking thrust prediction methodology requirements and revisions are i properly implemented. Under this condition, the NRC 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. The licensee stated that all flexible and solid wedge gate valves in the scope of GL 95 07were 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 pressurizer power-operated relief block valves,1/2-8000A/B, exceed these temperature thresholds.
The valve actuators for 1/2 8000A/B are equipped with a spring compensators and are position seated, which reduces the potential for 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 NRC staff concludes that the licensee's actions to i 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 pc wrmed appropriate i evaluations of the operational configurations of safety-related power-operatsd gate valves to identify valves at CPSES, Units 1 and 2, that are susceptible to pressure locking or thermal l m l
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l binding. In addition, the NRC 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. Therefore, the staff concludes that the licensee has adequately addressed the requested actions discussed in GL 95-07.
Principal Contributor: S. Tingen Date: July 15,1999 l
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