ML20206U535
| ML20206U535 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 05/17/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20206U523 | List: |
| References | |
| GL-95-07, GL-95-7, NUDOCS 9905250249 | |
| Download: ML20206U535 (5) | |
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e" k UNITED STATES g ^g NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 i
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l STAFF EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION !
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REGTRDING RESPONSE TO GENERIC LETTER 95-07
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NIAGARA MOHAWK POWER CORPORATION
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NINE MILE POINT NUCLEAR STATION. UNIT NOS.1 AND 2 DOCKET NOS. 50-220 AND 50-410 l
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1.0 INTRODUCTION
i Pressure locking and thermal binding represent potential common-cause failure mechanisms 1 that can render redundant safety systems incapable of performing their safety functions. The identification of susceptible valves and the determination as to when the phenomena might i occur require a thorough knowledge of components, systems, and plant operations. Pressure l 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 tha 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 i valve is subjected to specific pressures and temperatures during various modes of plant l 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 Title 10 of the Code of Federal Reaulations (10 CFR) Part 50 (Appendix A General Design i 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 l 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 those safety-related power-operated gate valves that are 9905250249 990517 PDR ADOCK 05000220 P PDR j
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susceptible to pressure locking or thermal binding are capable of performing their safety l functions within the current licensing bases / 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 i analyses and take needed corrective actions (or justify longer schedules) to ensure that these susceptible valves are capable of performing their intended safety functions undar all modes of plant operation, including test configurations. In addition, the NRC requested in GL 95-07 that )
within 180 days of the date of issuance of the generic letter, licensees provide to the NRC a i 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 I 50.109(a)(4)(i) because modification may be necessary to bring facilities into compliance with the rules of the Commission referenced above.
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In letters dated October 16,1995, and February 13,1996, Niagara Mohawk Power Corporation l, (NMPC and licensee) submitted its 60-day and 180-day responses to GL 95-07 for Nine Mile Point Nuclear Station, Unit Nos.1 and 2 (NMP1 and NMP2). In a letter dated June 20,1996, NMPC provideo additionalinformation requested by the NRC staff. In letters dated November 21,1996, and August 29,1997, NMPC revised its letters of February 13 and j June 20,1996. The NRC staff performed two inspections to review specific aspects of the !
information summarized in the NMPC's responses to GL 95-07, as documented in NRC Inspection Reports 50-410/96-11 and 50-210,410/98-10. In a letter dated April 21,1999, NMPC again provided additional information requested by the NRC staff.
3.0 NRC STAFF EVALUATION 3,1 Scoce 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. NMPC's letters of February 13, June 20, and November 21,1996, August 29,1997, and April 21,1999, described the scope of valves evaluated in response to GL 95-07. The NRC staff has reviewed the scope of the NMPC's susceptibility evaluation performed in response to GL 95-07 and found it to be complete and acceptable. Normally open, safety-related power-operated gate valves that are closed for test or surveillance, but must return 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 technical specifications. The criteria for determining the scope of pcwer-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."
3.2 Corrective Actions in GL 95-07, the NRC requested that, within 180 days, licensees 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 functions under t
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I all modes of plant operation, including test configurations. NMPC's submittals discussed proposed corrective actions to address potential pressure-locking and thermal-binding problems. The NRC staff's evaluation of the NMPC's actions is discussed in the following !
paragraphs:
- a. NMPC stated that NMP1 Core Spray System valves 40-01,40-02,40-05,40-06,40-09, 40-10,40-11 and 40-12 were modified to eliminate the potential for pressure locking.
NMPC stated inat the following NMP2 valves were modified to eliminate the potential for pressure locking:
2CHS*MOV107 High Pressure Core Spray (HPCS) Pump Injection 2CSH*MOV105 HPCS Pump Minimum Flow Bypass isolation 2CSH*MOV118 HPCS Pump Suction From Seppression Pool isolation 2CSL*MOV104 Low Pressure Core Spray (LPCS) Injection Isolation 2iCS*MOV126 Reactor Core Isolation Cooling (RCIC) injection Isolation 2lCS*MOV136 RCIC Pump Suction From Suppression Pool isolation 2RHS*MOV15A/B Containment Spray Loop Outboard Isolation 2RHS*MOV24A/B/C Residual Heat Removal (RHR) Loop Inlet Isolation 2RHS*MOV25A/B Containment Spray Loop inboard Isolation The NRC staff finds that physical modification to valves susceptible to pressure locking is an appropriate corrective action to ensure operability of the valves. Thus, NMPC's response to GL 95-07 with respect to these valves is acceptable.
- b. NMPC stated that it used a thrust-prediction methodology developed by Commonwealth Edison Company (Comed) to demonstrate that the fo' lowing NMP2 valves could open under pressure-locking conditions:
2lCS*MOV122 RCIC Steam Exhaust to Suppression Pool 2lCS*MOV128 RCIC Steam Supply Inboard Isolation 2SWP*MOV17A/B Sen/ ice Water (SW) to Spent Fuel Cooling (SFC) Heat Exchanger Isolation 2SWP*MOV18A/B SW from SFC Heat Exchanger Isolation ,
2SWP*MOV67A/B SW To Control Room Chiller Isolation 2SWP*MOV94A SW Return From HPCS Diesel Generator (DG) Cooler On April 9,1997, the NRC staff held a public meeting to discuss the technical adequacy of the ComEo pressure-locking thrust prediction methodology and its generic use by licensees in their submittals responding to GL 96-07. The minutes of the public meeting were issued on April 25,1997, and are publicly available 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. The NRC considers the use of the Comed pressure-locking methodology acceptable provided these margins, diagnnstic equipment accuracy requirements, and methodology limitations are incorporated into the pressure-locking calculations. Comed indicated that its methodology may be revised. The NRC staff considers that calculations that are used to demonstrate that valves can overcome pressure locking are required to meet the requirements of 10 CFR
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4 Part 50, Appendix B," Quality Assurance Criteria for Nuclear Power Plants," and therefore, controls are required to be in place to ensure that any industry pressure- !
locking thrust prediction methodology requirements and revisions are properly l implemented. Under this condition, the NRC staff finds that the Comed methodology ]
provides a technically-sound basis for assuring that valves susceptible to pressure i locking are capable of performing their intended safety-related functions. l
- c. NMPC stated that it used a thrust-prediction methodology developed by Comed to ,
demonstrate that the following NMP2 valves could open under precsure-locking conditions, and that the valves will be modified to increase the margin between calculated pressure-locking thrust and actuator capability during the NMP2 refueling ,
outage scheduled for the spring of 2000. ,
2CSL*MOV107 LPCS Pump Minimum Flow Bypass isolation ,
2RHS*MOV4A/B/C RHR Loop Pump Minimum Flow Isolation 2RHS*MOV115 SW/RHR Containment Flooding Cross Tie .
2RHS*MOV116 SW/RHR Containment Flooding Cross Tie isolation 2SWP*MOV66A/B SW Return isolation From DG Cooler 2SWP*MOV94B SW Return From HPCS DG Cooler Isolation J
i The NRC staff finds that the use of Comed thrust-prediction pressure-locking methodology provides reasonable assurance that the valves will be operable until the {
planned modifications to increase the margin between calculated pressure-locking thrust and actuator capability are implemented.
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- d. NMPC stated that all flexible and solid wedge gate valves within the scope of GL 95-07 were evaluated for thermal binding. When evaluating whether valves were susceptible l to thermal binding, NMPC assumed that thermal binding would not occur below specific temperature thresholds. The conditions for NMP1 Emorgency Cooling Valves,39-07R,39-08R,39-09R, and 39-10R could potentially exceed the temperature thresholds during surveillance testing. As corrective action, NMPC revised procedures that require that the applicable Technical Specification Action Statement be entered whenever the valves are closed for surveillance testing.
The screening criteria used by NMPC provides 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 NMPC's actions to address thermal binding of gate valves are acceptable.
4.0 CONCLUSION
On the basis of this evaluation, the NRC staff finds that NMPC has performed appropriate evaluations of the operational configurations of safety-related power-operated gate valves to
identify valves at NMP1 and NMP2 that are susceptible to pressure locking or thermal binding.
In addition, the NRC staff finds that NMPC has taken, or is scheduled to take, the appropriate corrective actions to ensure that these valves are capable of performing their intended safety -
functions. Therefore, the NRC staff concludes that NMPC has adequately addressed the requested actions discussed in GL 95-07.
Principal Contributor: S. Tingen Date: May 17,1999 l
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