ML20211N045
| ML20211N045 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 09/02/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20211N037 | List: |
| References | |
| GL-95-07, NUDOCS 9909100035 | |
| Download: ML20211N045 (6) | |
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NUCLEAR REGULATORY COMMISSION wasHMOTON, D.C. asses eelH 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" UNION ELECTRIC COMPANY CALLAWAY PLANT. UNIT 1 DOCKET NO. 50-483
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 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 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 provisions of 10 CFR Part 50 (Appendix B, Criterion XVI), licensees are expected to act to ensure that safety-related power-operated gate valves that are susceptible to pressure locking or thermal binding are capable of performing their required safety functions.
On August 17,19g5, 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 9909100035 990902 PDR ADOCK 05000483 P
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. 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, the NRC in GL 95-07 requested that licensees, within 180 days of the date of issuance of the GL, 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 letters of February 9 and 15,1996, Union Electric Company (the licensee) submitted its 180-day response to GL 95-07 for Callaway Plant, Unit 1 (Callaway). The staff reviewed the
. licensee's submittals and requested additional information in a letter dated May 30,1996. In a letter of July 24,1996, the licensee provided the additionalinformation. In a letter of March 15, 1999, the NRC issued a second request for information and in letters of May 19 and July 28, 1999, the licensee provided the requested additional information.
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. The licensee's letters of February 9 and 15, and July 24,1996, and May 19 and July 28,1999, described the scope of valves that the licensee evaluated in response to GL 95-07. The staff has reviewed the scope of the licensee'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 Callaway is hot standby. Normally open, safety-
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related power-operated gate valves which are closed for test or surveillance but must retum to the open position vare 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 'or determining the scope of power-operated valves for GL 95-07 are consistent with the siaffs 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, snd take appropriate corrective actions (or justify longer schedules), to ensure that the
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' 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 Callaway discussed corrective actions to address potential pressure-locking and thermal-binding t
L problems at the plant. The staff's evaluation of the licensee's actions for Callaway is discussed in the following paragraphs:
a.
The licensee stated that it used the thrust-prediction methodology developed by Commonwealth Edison Company (Comed) to demonstrate that these valves are l
capable of opening during pressure-locking conditions:
BBHV8000A/B Pressurizer Power Operated Relief Valve (PORV) Block EJHV8811A/B RHR Pump Suction from Containment Sump ENHV0001 Containment Spray Pump Suction from Containment Sump ENHV0007 Containment Spray Pump Suction from Containment Sump The licensee also stated that air expansion pipes were also installed on the bonnet of ENHV0001, ENHV0001, 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 put>isc 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 e diagnostic equipment accuracy and methodology limitations are defined in a letter from l
Comed to the NRC dated May 29,1998. The staff 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, 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 Comed methodology could not be used to demonstrate that valves EJHV8811 A/B were capable of operating during pressure-locking' conditions l.
following operation of the RHR system in the shutdown cooling mode. As long-term corrective action, the licensee stated that it plans to install a locally operated bonnet vent valve on each valve and revise procedures to vent each valve's bonnet after the RHR system is isolated from the RCS and prior to entering Mode 3. In cases where 12 or more hours will elapse between securing shutdown cooling and entering Mode 3, the i
licensee will rely on valve leakage to vent the valve bonnet. The licensee stated that L
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4 EJHV8811 A is scheduled to be modified during Refueling Outage 11 in Spring 2001, and that EJHV8811B during Refueling Outage 12 in Fall 2002. As a short-term corrective action, the licensee leak-checked the valves during operation of the RHR in the shutdown cooling mode and verified that leakage did not pressurize the bonnets of the valves.
The staff finds that the licensee's short-term and long-term actions provide assurance that pressure locking conditions are adequately identified and eliminated, and are thus acceptable.
b.
The licensee stated that the following valves were not susceptible to pressure locking because the valves are shut during certain accident events and are not re-opened until 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> later into the event:
EJHV8716A/B RHR to Safety injection Hot Leg Recirculation EMHV8802A/B Safety injection to RCS Hot Leg injection EJHV8840.
RHR to RCS Hot Leg injection The licensee stated that the above valves are not exposed to thermal-induced pressure-locking conditions during the 13-hour period into the event and that seat leakage during this period would prevent the valves from being pressure locked. The staff finds that the licensee's analysis is an acceptable means to eliminate the potential for pressure locking of these valves.
c.
The licensee stated that the RHR to charging pump suction valves, EJHV8804A/B, are also susceptible to pressure locking. As a corrective action, the licensee modified procedures to cycle the valves following evolutions that could potentially create a pressure-locking condition. The staff finds that the modified procedures are acceptable because they provide assurance that pressure-locking conditions for these valves are adequately identified and eliminated.
d.
The licensee stated that the boron injection tank (BIT) inlet valves, EMHV8803A/B, are susceptible to pressure locking and that the valves' motor actuators may operate at locked-rotor conditions for a short period of time (approximately a second) following a loss of offsite power concurrent with' emergency core cooling system (ECCS) automatic initiation. The licensee stated that its analysis determined that the motor actuators will operate during this time period at locked rotor conditions. The Comed thrust-prediction methodology was then used to demonstrate that the valves are capable of opening during the pressure-locking conditions that exist for this time period.
The licensee also stated that the BIT outlet valves, EMHV8801 A/B, are susceptible to pressure locking and will operate for up to 1.5 seconds at locked-rotor conditions following a loss of offsite power concurrent with ECCS automatic initiation. The BIT inlet valves must open to allow the centrifugal charging pump (CCP) to pressurize the bonnets and upstream disks of the outlet valves. It takes a maximum of 1.5 seconds for inlet valves EMHV8803A/B to crack open and a charging pump to develop full
' discharge pressure and equalize pressure across the upstream disk of the outlet valves. Pressure-locking conditions do not exist at the outlet valves once the pressure
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across the upstream disk of the outlet valves is equalized. The Comed methodology was, therefore, not applied to the outlet valves.
The staff accepts operation of ac-powered motor actuators for short periods at locked-rotor conditions (approximately 1 second) because testing performed by ldaho National Engineering and Environmental Laboratory (NUREG/CR-6478) demonstrates that the capability of the actuator does not significantly degrade over that short time period.
Use of the Comed methodology is acceptable and is discussed in Section 3.2.a above.
e.
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, BBHV8000A/B, and RHR pump minimum flow recirculation valves, EJFCV0610 and EJFCV0611, exceed these temperature thresholds. The valve actuators for BBHV8000A/B are equipped with a spring compensator and are position seated which reduces the potential for thermal binding. The valve actuators for EJFCV0610 and EJFCV0611 are position seated to stop the motor approximately %
inch before the disks contact the seats which reduces the potential for thermal binding.
The screening criteria used by the licensee 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 acceptable.
4.0 CONCLUSION
On the basis of this evaluation, the staff finds that the licensee has performed appropriate evaluations of the operational configurations of safety-related power-operated gate valves to identify valves at Callaway 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 Fall 2002 refueling outage for two valves, as discussed in Section 3.2.a above. This is acceptable.
Therefore, the staff concludes that the licensee has adequately addressed the requested actions discussed in GL 95-07 for the Callaway Plant, Unit 1.
Principal Contributor: S. Tingen Date: September 2,1999
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pressure locking conditions, by the requirements in Appendix B to 10 CFR Part 50 because the methodology is being used to show that safety-related valves will meet their safety functions during accidents. This is also addressed in Section 3.2.a of the enclosed safety evaluation.
This completes the staff's efforts on TAC No. M93443.
Sincerely, ORIG. SIGNED BY Jack N. Donohew, Senior Project Manager, Section 2 Project Directorate IV & Decommissioning Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket No. 50-483
Enclosure:
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