ML103640257
| ML103640257 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 01/18/2011 |
| From: | Chernoff H Plant Licensing Branch 1 |
| To: | Heacock D Dominion Nuclear Connecticut |
| Sandeers, Carleen, NRR/DORL, 415-1603 | |
| References | |
| TAC ME4476, FOIA/PA-2011-0115 | |
| Download: ML103640257 (8) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 January 18, 2011 Mr. David A. Heacock President and Chief Nuclear Officer Dominion Nuclear Connecticut, Inc.
Innsbrook Technical Center 5000 Dominion Boulevard Glen Allen, VA 203060-6711 SUB~IECT:
MILLSTONE POWER STATION, UNIT NO.2-ISSUANCE OF RELIEF REQUEST RR-04-08 REGARDING USE OF ALTERNATIVE PRESSURE TESTING REQUIREMENTS (TAC. NO. ME4476)
Dear Mr. Heacock:
By letter dated July 29, 2010, as supplemented by letter dated August 5, 2010 (Agencywide Document Access and Management System Accession Nos. ML1025802040 and ML102220527, respectively), Dominion Nuclear Connecticut, Inc. (DNC or the licensee) submitted relief requests for the fourth 10-year inservice inspection (lSI) interval program at Millstone Power Station, Unit NO.2 (MPS2). DNC requested use of alternatives to certain American Society of Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI requirements. Included in this submittal is Relief Request RR-04-08 which proposes to perform VT-2 visiual examination of the reactor pressure vessel (RPV) flange seal leak-off piping as an alternative to performing the ASME-required system pressure test. Each relief request contained in the July 29, 2010, submittal will be addressed separately.
The Nuclear Regulatory Commission (NRC) staff has reviewed the subject request and concludes, as set forth in the enclosed Safety Evaluation, that performance of an ASME Code system pressure test would result in a hardship without a compensating increase in the level of quality and safety. The NRC staff's review also concludes that the visual examination described in RR-04-08 is acceptable because it provides reasonable assurance of structural integrity of the RPV flange seal leak-off piping.
Therefore, pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Part 50, Section 50.55a(a)(3)(ii), the NRC authorizes the use of visual examination as an alternative to the ASME Code,Section XI, required system leakage test of the RPV flange seal leaf-off piping for the remainder of the fourth 10-year lSI interval for MPS2. The fourth 1O-year lSI interval at MPS2 began on April 1, 2010, and is scheduled to be completed on March 31, 2020.
All other ASME Code,Section XI, requirements for which relief was not specifically requested and approved remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
D. Heacock
- 2 If you have any question, please contact the Project Manager, Carleen Sanders, at 301-415-1603.
Sincerely, J. -f253G r
Harold K. Chernoff, Chief Plant Licensing Branch 1-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-336
Enclosure:
As stated cc w/encl: Distribution via Listserv
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION THIRD 10-YEAR INSERVICE INSPECTION INTERVAL REQUEST FOR RELIEF NO. RR-04-08 MILLSTONE POWER STATION, UNIT NO.2 DOMINON NUCLEAR CONNECTICUT. INC.
DOCKET NUMBER 50-336
1.0 INTRODUCTION
By letter dated July 29, 2010, as supplemented by letter dated August 5, 2010 (Agencywide Document Access and Management System Accession Nos. ML1025802040 and ML102220527, respectively), Dominion Nuclear Connecticut, Inc. (DNC or the licensee) submitted relief requests for the fourth 1O-year inservice inspection (lSI) interval program at Millstone Power Station, Unit NO.2 (MPS2). DNC requested use of alternatives to certain American Society of Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI requirements. Included in this submittal is Relief Request RR-04-08 which proposes to perform VT-2 visual examination of the reactor pressure vessel (RPV) flange seal leak-off piping as an alternative to performing the ASME-required system pressure test.
2.0 REGULATORY EVALUATION
The lSI of ASME Code Class 1, 2, and 3 components is performed in accordance with Section XI of the ASME Code and applicable addenda as required by Title 10 of the Code of Federal Regulations (10 CFR), Part 50, Section 50.55a(g), except where specific relief has been granted by the Nuclear Requlatory Commission (NRC) pursuant to 10 CFR 50.55a(g)(6)(i).
10 CFR 50.55a(a)(3) states that alternatives to the requirements of paragraph (g) may be used, when authorized by the NRC, if the licensee demonstrates that: (i) the proposed alternatives would provide an acceptable level of quality and safety; or (ii) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Pursuant to 10 CFR 50.55a(g)(4), ASME Code Class 1, 2, and 3 components (including supports) shall meet the requirements, except the design and access provisions and the pre service examination requirements, set forth in the ASME Code,Section XI, "Rules for Inservice Inspection of Nuclear Power Plant Components," to the extent practical within the limitations of design, geometry, and materials of construction of the components. The regulations require that inservice examination of components and system pressure tests conducted during the first 1O-year interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code incorporated by reference in 10 CFR 50.55a(b)
Enclosure
- 2 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein.
The ASME Code of Record for the fourth 10-year lSI interval at MPS2 is the 2004 Edition with no Addenda.
3.0 TECHNICAL EVALUATION
3.1 System/Component(s) for Which Relief is Requested Reactor Pressure Vessel (RPV) Flange Seal Leak-Off Piping 3.2 Applicable ASME Code Requirements IWB-2500, Table IWB-2500-1, Examination Category B-P, Item Number B15.10, requires that all Class 1 pressure retaining components be VT-2 visually examined each refueling outage and system leakage tested in accordance with IWB-5220, System Leakage Test.
IWB-5221, Pressure, states:
(a) The system leakage test shall be conducted at a pressure not less than the pressure corresponding to 100% rated reactor power.
(b) The system test pressure and temperature shall be attained at a rate in accordance with the heat-up limitation specified for the system.
IWB-5222, Boundaries, states:
(a) The pressure retaining boundary during the system leakage test shall correspond to the reactor coolant boundary, with all valves in the position required for normal reactor operation startup. The visual examination shall, however, extend to and include the second closed valve at the boundary extremity.
(b) The pressure retaining boundary during the system leakage test conducted at or near the end of each inspection interval shall extend to all Class 1 pressure retaining components within the system boundary.
3.3 Licensee's Request for Relief Relief is requested from performing the system leakage test at a pressure corresponding to nominal operating pressure. Instead, DNC proposes to perform VT-2 visual examination of the unpressurized RPV flange seal leak-off piping during each outage.
3.4 Licensee's Basis for Requesting Relief The RPV flange seal leak detection piping is separated from the reactor pressure boundary by one passive membrane, which is an O-ring located on the vessel flange. A second O-ring is located on the outside of the tap in the inner vessel flange. This piping is required during plant
- 3 operation in order to indicate failure of the inner flange seal O-ring. If the inner O-ring should leak during the operating cycle, it is identified by an increase in the temperature of the leak-off line above ambient temperature, which results in the annunciation of a high temperature alarm in the Control Room. Failure of the inner O-ring is the only condition under which this line is pressurized. Following the alarm, procedurally-controlled operator actions allow identification of any further compensatory actions required. The leakage is collected in the primary drain transfer tank.
The configuration of this piping precludes system pressure testing while the vessel head is removed because the configuration of the vessel tap, coupled with the high test pressure requirement prevents the tap in the flange from being temporarily plugged or connected to other piping. The opening in the flange is smooth walled, making the effectiveness of a temporary seal very limited. Failure of this seal could possibly cause ejection of the device used for plugging or connecting to the vessel.
The configuration also precludes pressuring the line externally with the head installed. The top head of the vessel contains two grooves that hold the O-rings. The O-rings are held in place by a series of retainer clips that are housed in recessed cavities in the flange face. If a pressure test was performed with the head on, the inner O-ring would be pressurized in a direction opposite to what it would see in normal operation. This test pressure would result in a net inward force on the inner O-ring that would tend to push it into the recessed cavities that house the retainer clips. The thin O-ring material would likely be damaged by this inward force.
Purposely failing the inner O-ring to perform the ASME Code-required test would require the 0 rings to be replaced. This would result in additional time needed during the outage and additional radiation exposure to personnel associated with the removal and reinstallation of the RPV head.
3.5 Licensee's Proposed Alternative In lieu of the requirements of IWB-5222(b), a VT-2 visual examination will be performed each outage on the unpressurized subject piping as part of the ASME Code Class 1 leakage test. If the inner O-ring should leak during the operating cycle, it will be identified by an increase in temperature of the leak-off line above ambient temperature, actuating an alarm in the Control Room.
The flange seal leak-off line is essentially a leakage collection/detection system and the line would only function as an ASME Code Class 1 pressure boundary if the inner O-ring fails, thereby pressurizing the line. If any significant leakage does occur in the leak-off line piping itself during this time of pressurization, the line would clearly exhibit boric acid accumulation and be discernable during the proposed VT-2 visual examination.
3.6
NRC Staff Evaluation
The ASME Code of Record requires that all ASME Code Class 1 components undergo a system leakage test each refueling outage prior to plant startup. Relief Request RR-04-08 requests relief from performing a system leakage test of the RPV seal leak detection piping at the ASME Code-required test pressure corresponding to 100% rated reactor power. The piping is located between the inner and the outer O-ring seals of the vessel flange and is required
- 4 during plant operation in order to detect failure of the inner flange seal O-ring. The design of this line makes the ASME Code-required system leakage test difficult either with the vessel head in place or removed. The piping cannot be filled completely with water since it cannot be vented to remove entrapped air from the line either with the vessel head in place or removed due to its configuration.
As discussed above, the configuration of this piping precludes system pressure testing while the vessel head is removed. The configuration prevents the tap in the flange from being temporarily plugged or connected to other piping and the opening in the flange is smooth walled, making the effectiveness of a temporary seal very limited. Failure of this seal could possibly cause ejection of the device used for plugging or connecting to the vessel.
If a pressure test were to be performed with the head in place, the space between the inner and the outer O-ring seals would be pressurized. As discussed above, the test pressure would exert a net inward force on the inner O-ring that would tend to push it into the recessed cavities that house the retainer clips with the possibility of damaging the inner O-ring seal. The NRC staff concurs with the licensee's finding that each pressure test at Code-required pressure, with the RPV head on, would require replacing at least one new O-ring. The processes of de-tensioning and removing the RPV head, replacing of the inner O-ring and installing the RPV head would result in loss-of-outage time and additional radiation exposure to personnel. Therefore, the NRC staff concludes that performing the ASME Code system pressure test would result in hardship or unusual difficulty to the licensee without a compensating increase in the level of quality and safety.
The leak detection line is essentially a leakage collection and detection system. The line would only function as a pressure boundary if the inner O-ring fails and pressurizes the line. If the inner O-ring should leak during the operating cycle, it will be identified by an increase in temperature of the leak-off line above ambient temperature. This high temperature would actuate an alarm in the Control Room, which would be closely monitored by procedurally controlled operator actions allowing identification of any further compensatory actions required.
Additionally, if any significant leakage does occur in the leak-off piping itself during this time of pressurization then it would clearly exhibit boric acid accumulation which would be discernable during the proposed VT-2 visual examination that will be performed each outage. Therefore, the NRC staff concludes that visual examination provides reasonable assurance of structural integrity of the RPV flange seal leak-off piping.
4.0 CONCLUSION
Based on review presented above, the NRC staff concludes that a system leakage test of the RPV flange seal leak detection piping at the ASME Code-required test pressure corresponding to 100% rated reactor power would result in a hardship to the licensee without a compensating increase in the level of quality and safety. The NRC staff also concludes that the proposed alternative provides reasonable assurance of structural integrity. Therefore, pursuant to 10 CFR 50.55a(a)(3(ii), the NRC authorizes the use of Relief Request RR-04-08 for the remainder of the fourth 10-year lSI interval for MPS2. The fourth 10-year lSI interval for MPS2 began on April 1, 2010, and is scheduled to be completed on March 31, 2020.
- 5 All other requirements of the ASME Code,Section XI for which relief has not been specifically requested and approved remain applicable, including a third party review by the Authorized Nuclear Inservice Inspector.
Principal Contributor: P. Patnaik Date: January 18, 2011
- By Memo Dated
- via email OFFICE LPL1-2/PM LPL 1-2/LA DCI/CPNB LPL1-2/BC NAME CSanders ABaxter TLupold*
HChernoff (REnnis for)
DATE 1/10/2011 1/5111 12115/2010 1118111