ML17354A720
| ML17354A720 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 11/12/1997 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML17354A717 | List: |
| References | |
| GL-95-07, GL-95-7, NUDOCS 9711250293 | |
| Download: ML17354A720 (7) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2055&0001 EAR T
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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.
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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.
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.
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 that safety-related power-operated gate valves that are susceptible to pressure Enclosure 97ii250293 97iii2 PDR ADQCK 05000250 PDR.
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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 (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 configuration.
In addition, GL 95-07 requested that licensees, within 180 days of the date of issuance of the generic letter, provide to the Nuclear Regulatory Commission (NRC) a summary
'escription 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)(l) because modification may be necessary to bring facilities into compliance with,the rules of the Commission referenced above.
ln a letter. of February 9, 1996, Florida Power and Light Company submitted its 180-day response to GL 95-07 for Turkey Point Units 3 and 4. The NRC staff reviewed the licensee's submittal and requested additional information in a letter dated June 26, 1996.
In a letter of July 30, 1996, the licensee provided the additional information.
NRC Inspection Report 50-250, 251/97-08, dated, September 5, 1997, documented the results of an inspection of the information summarized in the licensee's responses to GL 95-07.
The licensee responded to the inspection report. findings in a letter dated September 18, 1997.
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0 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 Florida Power and Light Company's letters of February 9 and July 30, 1996, and September 18, 1997, 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.
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r c'veA in 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 configuration.
NRC Inspection Report 50-250, 251/97-08 and the licensee's submittals discussed proposed 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 the following valves were modified to eliminate the potential for pressure locking:
MOV 3/4-350, Emergency Boration MOV 3/4-750/751, Residual Heat Removal Suction From the Reactor Coolant System (RCS)
MOV 3/4-843 A/B, Boron Injection to RCS Cold Leg MOV 3/4-860 A/B, Containment Sump Isolation MOV 3/4-861 A/B, Containment Sump Isolation MOV 3/4-869, Safety Injection (Sl) Hot Leg Injection Isolation MOV 3/4-872, Alternate Low Head Sl to Cold Leg 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 acceptable.
b.
The licensee stated that the low head Sl isolation valves, MOV 3/4-744 A/B, will be modified to eliminate the potential for pressure locking during the Unit 3 refueling outage, Cycle 17, scheduled for Fall 1998 and the Unit 4 refueling outage, Cycle 17, scheduled for Fall 1997.
Until these valves are modified, a methodology developed by the Commonwealth Edison Company (ComEd) to predict the thrust required to overcome pressure locking was used to demonstrate operability. The staff finds that these licensee actions provide reasonable assurance that the valves are acceptable until the planned modifications to prevent pressure locking are completed as scheduled.
See paragraph 3.2.e for more discussion of the ComEd pressure-locking methodology.
The licensee's modification of these valves will be an, acceptable final resolution.
c.
The licensee stated that procedures were in place to cycle the following valves following evolutions that could potentially create a pressure-locking condition:
MOV 3/4-863 A/B, Sl/CS Recirculation Phase Suction MOV 3/4-880 A/B, CS Pump Discharge The staff finds that the licensee's procedural controls to cycle the valves provide assurance that pressure-locking conditions are adequately identified and eliminated, and are acceptable.
d.
The licensee stated that procedures were in place to cycle the following valves following evolutions that could potentially create a thermal-binding condition:
MOV 3/4-744 A/B, Low Head Safety Injection Isolation MOV 3/4-535/536, Pressurizer PORV Block MOV 3/4-863 A/B, Sl/CS Recirculation Phase Suction MOV 3/4-872, Alternate Low Head Safety Injection Isolation The staff finds that the licensee's procedural controls to cycle the valves as corrective action provide assurance that thermal-binding conditions are eliminated, and are acceptable.
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valves, MOV 3/4-535/536, could open under pressure-locking conditions.
Under certain conditions, 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.
On April 9, 1997, a public meeting was held to discuss the ComEd pressure-locking thrust prediction methodology presented in licensee 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.
For those valves identified in this safety evaluation that use the ComEd pressure-locking thrust prediction methodology for interim and long-term corrective action, the licensee should ensure that the margin between calculated pressure-locking thrust and actuator capability meets or exceeds these minimum requirements.
At the meeting, ComEd indicated that its methodology is undergoing review and may be revised or enhanced.
The licensee is responsible for ensuring that the thrust values calculated to overcome pressure locking for.these valves remain valid following implementation of any revisions or enhancements to the ComEd methodology.
The staff is reviewing the ComEd methodology and will issue a safety evaluation for the methodology.
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.'he 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 valves are acceptable.
4.0 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 Turkey Point Units 3 and 4 that are susceptible to pressure locking or thermal 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, NRR Date:
November 12, 1997
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