ML20199G837
| ML20199G837 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 01/22/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20199G829 | List: |
| References | |
| GL-95-07, GL-95-7, NUDOCS 9802040352 | |
| Download: ML20199G837 (5) | |
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UNITED STATES 3
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NUCLEAR REGULATORY COMMISSION g
WASH'NGTON, D.C. 30846 4 001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION LICENSEE RESPONSE 10 GENERIC LETTER 95 07. " PRESSURE LOCKING AND THERMAL BINDING OF SAFETY-PElATED POWER-OPERATED GATE VALVES" THREE MILE ISLAND NUCLEAR STATION. UNIT 1 DOCKET NLHBER 50-289 l
1.0 INTRODUCTION
l Pressure locking and thermal binding represent potential common cause failure i
mechanisms which can render redundant safety systems incapable of performing l
their safety functions.
The identification of susceptible valves and the i
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, fle.Ubility, 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 as part of the design basis for valves in many plants.
2.0 REGULATORY REQUIREMENTS
-10 CFR Part 50 (Appendix A. General Design Criteria 1 and 4) and plant licensing safety analyses require and/or commit that licensees design and test Enclosure
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2-a ftty-related components and systems to provide adequate essurance that those 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 30 CFR Part 50 (Appendix B, Critarion XVI), licensees are expected to take actions 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 " t')
request that licensees take certain actions to ensure that safety-related pcNar-operated gate valves thit are susceptible to pressure locking or the,a i binding are capable of performing their safety functions within the current licensing basis of the facility. GL 95-07 requested that each licensee, within 180 days of the date of issuance, (1) evaluate the operatinaal configurations of safety-related power-operated gate valves in its plant to l
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) are capable of performing their intended safety functions under all modes I'
of plant operation, including test configuration.
In addition, GL 95-07 requested that licensees, within 180 days of the datG of issuance, provide to the NRC a summary description of (1) the susceptibility evaluation used to determine that va'ives are or are not susceptible to pressore locking or thermal bindirij, (2) the results of the suseptibility evaluation, including a listing of the susceptible valves identified, and (3) tu corrective actions, or other dispositioning, for the valves identified as sLsceptible to pressure lock:ng or thermal binding. The NRC issued GL 95-07 as a " compliance backfit" pursuant to 10 CFR 50.109(a)(4)(1) because modificatien may be necessary to bring facilities into compliance with the rules of the Comaission referenced above.
By letter dated February 13, 1996, GPU Nuclear Corporation submitted its 180-day response to GL 95-07 for the Th.ee Mile Island Nuclear Station, Unit 1 (TMI-1). The NRC staff reviewed the licensee's February 13, 1996, submittal, and mjuested additional information in a letter dated August 22, 1996.
By a letter dated September 25, 1996, the licensee provided 0.t11tional Wormation in response to the NRC staff's requi.st.
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3-3.0 STAFF EVALUATION 3.1 Scope of Licensee's Review GL 95-07 requested that licensees evaluate the operational configur6tions ot safety-related power-operated gate valves in their plants to identify valves that are susceptible to pressure locking or thermal binding.
GPU Nuclear Corporation's letters dated February 13 and September 25, 1996, described the scope of valves evaluated in respense~tc 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 to be complete and acceptable.
g=q THI-1 Technical Specifications only recuire that the emergency core cooling system be operable for criticality: therefore, the aperation of emergency core cooling system valves for modes of operation other than criticality are not in the scope of GL 95-07. The pressurizer power operated relief valve (PCRV) block valve, RC-V2, is not in the scope of GL 95 07 because TMI-1 Technical Specifications allow the unit to operate with the pressurizer PORV block valve shut and electrically isolated.
The licensee did not include the decay heat removal pump suction valves, DH-V1 and DH-V2, in the scope of GL 95-07 because these valves are used during plant conditions below hot shutdown.
Chapter 6B, section 2.3.2.1, of the Updated Final Safety Analysis Report states at the safe shutdown condition for THI-1 is defined as the hot shutdown conc e. ion. This criterion for determining the scop' of power-operatri valves for GL 95-07 is consistent with the staff's acceptance of the scope of motor-operated valves associated with GL 89-10.
" Safety-Related Motor-0perated Valve Testing and Surveillance."
l 3.2 Corrective Actions Taken Dy Liceri.,ee GL 95-07 requested that each licensee, within 180 days, perform further analyses as appropriate, and take appropriate corrective actions (or justify longer schedules), to ensure that the susceptible valves ioentified are capable of performing their intended safety function under all modes of plant operation including test configuration.
4 The licensee's submittals of February 13 and September 25, 1996, discuss the licensee's proposed corrective actions to address potential pressure locking and thermal bindino problems. The stcfi's evaluation of the licensee's ections is div ssed in the following paragraphs, The licensee stated that the low pressure injection valves, DH-V4A/B were a.
susceptible to pressure locking and modified to eliminate the potential for pressure locking. The staff finds that pnysical 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 the low oressure injection valves, DH-V4A/P, are susceptible to thermal binding and that procedures would be modified prior to the startup following the Fall 1997 refueling outage to cycle the vahes following evolutions that could potentially create a thermal binding condition.
In a telephone call dated January 14, 1998, the NRC confirmed with your staff that the proceduras had been modified as per your commitment. The staff finds that the licensee's procedural changes to require cycling the valves provide assurance that thermal binding conditions are adequately identified and that the valves are capable of performing their intended asfety functions. The licensee's actions are thus acceptable.
c.
During a telephone conversation on June 24, 1997, the licensee clarified to the staff its basis for assuring that the sodium hydroxide tank suction valves, BS-V2A/B, are capable of opening during a pressure locking event. A modified industry gate valve thrust equation was used to calculate the thrust required to open these double disk gate valves during pressure locking conditions. The results of the ulculation demonstrated that the margin between calculated pressure locking thrust and actuator capability is very large. The information provided by the licensee is subject to confirmation during future NRC inspections.
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 in the Public Document Roor 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 m
5-industry gate valve thrust equation provides reasonable assurance that valves susceptible to pressure locking are capable of performing their intended safety related function provided that the margin between calculated pressure locking thrust and actuator capability is very large. Until more c'efinitive industry criteria are developed the staff concludes that the licensee's action to address pressure locking of gate valves BS-V2A/B is acceptable, d.
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 temoerature threshoids. The licensee assumed that thermal binding is nut a concern for system temperatures below 221*F for flexible wedga gate valves and below 166*F for solid wedge gate valves. Above these temperatures, the differential temperatures for thermal binding concerns were 100*F for flexible wedge gate valves and 50*F for solid wedge gate valves. 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 therr.al binding of gate valves are acceptable.
4.0 CONCLUSION
Sasec on the above evaluation the NRC staff finds-that the licensee has performed appropriate evaluations of the operational configurations of safety-related pcwer-operated gate valves to identify valves that are susceptible to pressure locking or thermal binding for THI-1.
In addition, the NRC staff finds that the licensee has taken, or is scheduled to take, apprcariate corrective actions to ensure that these valves are capaole of perf(c.ing 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. Tinger WR Date: January 22, 1998 1
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