ML121370963

Millstone Power Station, Unit No. 2 - Review of the Core Operating Limits Report (Colr), Cycle 21 (TAC No. ME6365)

ML121370963
Person / Time
Site:	Millstone
Issue date:	05/22/2012
From:	Kim J S Plant Licensing Branch 1
To:	Heacock D A Dominion Nuclear Connecticut
Kim J, NRR/DORL, 415-4125
References
TAC ME6365
Download: ML121370963 (6)

Text

UNITED NUCLEAR REGULATORY WASHINGTON, D.C. 20555-0001 May 22,2012 Mr. David President and Chief Nuclear Officer Dominion Nuclear Connecticut, Inc. Innsbrook Technical Center 5000 Dominion Boulevard Glen Allen, VA 23060-6711 MILLSTONE POWER STATION, UNIT NO.2 -REVIEW OF THE CORE OPERATING LIMITS REPORT (COLR), CYCLE 21 (TAC NO. ME6365)

Dear Mr. Heacock:

By letter dated May 19, 2011 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML 11152A 148), Dominion Nuclear Connecticut, Inc. (DNC) submitted the Millstone Power Station, Unit 2 (MPS2) Cycle 21 Core Operating Limits Report (COLR) in accordance with Technical Specification (TS) Section 6.9.1.8.d. DNC provided additional information regarding the COLR by letters dated November 30, 2011 (ML 11342A 122), January 18, 2012 (ML 12025A225), and May 1, 2012 (ML 12129A200). The U.S. Nuclear Regulatory Commission (NRC) staff has completed its review of the report and concludes that the revision to the Cycle 21 COLR is acceptable for MPS2 Cycle 21 operation. The NRC staff's safety evaluation of the report is enclosed. Please contact me at (301) 415-4125 if you have any questions on this issue.

Sincerely,James Kim, Project Manager Plant Licensing Branch 1-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-336

Enclosure:

As stated cc w/encl: Distribution via Listserv UNITED NUCLEAR REGULATORY WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BYTHE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO CYCLE 21 CORE OPERATING LIMITS REPORT REVISION DOMINION NUCLEAR CONNECTICUT, INC. MILLSTONE POWER STATION, UNIT 2 DOCKET NO. 50-336 1.0 INTRODUCTION By letter dated May 19, 2011 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML 11152A148), Dominion Nuclear Connecticut, Inc. (DNC) submitted the Millstone Power Station, Unit 2 (MPS2) Cycle 21 Core Operating Limits Report (COLR) in accordance with Technical Specification (TS) Section 6.9.1.8.d. DNC provided additional information regarding the COLR by letters dated November 30, 2011 (ML 11342A 122), January 18, 2012 (ML 12025A225), and May 1, 2012 (ML 12129A200). The proposed revision to the MPS2 COLR includes: (1) revision to page headers to reflect Cycle 21; (2) addition of a penalty factor to account for the impact of offset incore instrument (lCI) detectors on the linear heat rate measurement; and (3) revision to Figure 2.5-1 based on the analysis supporting the offset ICI detectors. 2.0 REGULATORY EVALUATION The U.S. Nuclear Regulatory Commission (NRC) staff considered the following regulatory requirements and guidance in its review of the proposed revision. Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Section 50.59, "Changes, tests, and experiments," require: (1) under 50.59 (a)(1) a modification or addition to, or removal from, the facility or procedures that affects a design function, method of performing or controlling the function, or an evaluation that demonstrates that intended function will be accomplished; and (2) under 50. 59(a)(2) departure from a method of evaluation described in the FSAR [Final Safety Analysis Report] unless the results of the analysis are conservative or essentially the same 10 CFR 50.92, "Issuance of amendment," requires involvement of the material alternation of a licensed facility. 1 0 CFR 50.36, "Technical specifications," provides: (1) Safety limits, limiting safety system settings, and limiting control settings, (2) Limiting conditions for operation, (3) Surveillance requirements, (4) Design Features, (5) Administrative controls, (6) Decommissioning, (7) Initial notification, and (8) Written Reports.

-2 3.0 TECHNICAL The technical review of the proposed the Cycle 21 COLR for MPS2 includes: (1) revision to page headers to reflect Cycle 21; (2) addition of a penalty factor to account for the impact of ICI detectors on the linear heat rate measurement; and (3) revision to Figure 2.5-1 based on the analysis supporting the offset ICI detectors. Revision to Page Headers to Reflect Cycle 21 The NRC staff reviewed the proposed revision to page header to reflect Cycle 21 is acceptable because the revision is administrative in nature. Addition of a Penalty Factor to Account for the Impact of Offset ICI Detectors on the Linear Heat Rate Measurement MPS2 has a fixed ICI system. The ICI system consists of 45 arrays and each array consists of four levels of Rhodium detector segments with nominal positioning at 20%, 40%,60%, and 80% of the core height. Within the core, the ICls are located within Zircaloy thimble tubes. The thimble tubes are conduits which provide a means for quick removal and reinsertion of ICls during refueling outages and for centering and cooling of the ICls within them. The industry has experienced radiation induced growth of Zircaloy instrument thimble tubes. Dominion contracted Westinghouse to replace the 45 instrument thimble tubes with tubes that are 10.5 inches shorter than the original design. The shorter replacement thimble tubes are necessary to ensure that the thimble tube do not contact the fuel assembly lower end fitting due to radiation induced growth at the end of plant life. The replacement of the thimble tubes took place during the fall 2009 refueling outage (2R19) with Cycle 20 being the first cycle of operation with the replaced thimble tubes. During field fabrication of the replacement tubes in 2R19, Westinghouse cut 26 of the 45 thimble tubes shorter than intended by 1.375 inches. By design, the ICls should be free hanging within the thimble tubes. However, the shortened thimble tubes raised the possibility that some of the ICI strings were bottomed out and slightly misaligned from the ideal location. While some of the ICls may still have been free hanging in the shortened thimble tubes, Dominion conservatively instructed AREVA to quantify the potential impact on the indications of core power distribution by assuming that 26 affected ICI strings were misaligned by the maximum amount of 1.375 inches. Any potential impacts were addressed in the AREVA cycle-specific setpoint analysis. For Cycle 20 operation, no change was needed to the acceptable operation regions in the COLR figures and the impact on FaN (or linear Heat Generation Rate (LHGR)) was accommodated within the known conservatism of the methodology because the total FaN uncertainty of 1.07 used for the Cycle 20 is higher than 1.0690 determined by AREVA in the letter dated May 1, 2012. For Cycle 21 operation, a slight change in the Linear Heat Rate limiting condition for operation (LCO) monitoring tent (COLR Figure 2.5-1, used only when monitoring with excore detectors) and the use of a FaN penalty factor (used when monitoring with incore detectors) were needed to account for the maximum possible misalignment of the ICls. An associated 1.0025 penalty factor was included in COLR Section 2.5 for Cycle 21.

-The NRC staff reviewed the responses to the staff's request for additional information for the proposed MPS2 Cycle 21 COLR and summarized the staff's evaluation on the issues as follow: Approved methodologies used to determine the linear heat rate measurement. The MPS2 listed the approved methodology (Reference 1) used to validate the INPAX-II method using PRISM results. A detailed description of the INPAX-II method which converts measured signals to power distributions is given in Reference 2, which was cited for the use of INPAX-II for SAV95 application to one of incore monitoring of Combustion Engineering design plants that used fixed incore detectors similar to MPS2 design. The NRC staff reviewed the methodology issue and found it acceptable because an approved methodology was used. Methodology used to compute a penalty factor to account for the impact of the misaligned ICI detectors on the linear heat rate measurement. The NRC-approved core simulator code PRISM (Reference 5) was used to generate predicted nodal power and activation rate information specific to the MPS2 Cycle 21 reactor core. Nodal power and activation rate information was generated at numerous axial points for each instrumented fuel assembly and at numerous times during core life. The PRISM-generated activation information was used to generate pseudo-measured (or simulated) incore detector signals at both "nominal" and "offset" ICI detector conditions throughout core life. The "nominal" detector configurations were centered at the standard positions of core height. In the "offset" detector configuration, the 26 identified incore detectors were conservatively offset by the maximum amount of 1.375 inches. For each incore detector, a pseudo-measured signal was generated in the nominal and offset configurations, which was used to generate a nominal pseudo-measured 3-D power distribution (reconstructed nominal nodal power distribution) and an offset pseudo-measured 3-D power distribution (reconstructed offset nodal power distribution), respectively. The relative difference between the reconstructed "nominal" and "offset" power distributions represents the potential error due to the misaligned detectors. This error was calculated for limiting reactor core locations which are instrumented. The maximum under-prediction difference for limiting measured locations during anytime in core life defines the maximum potential error due to the offset detectors. This maximum error was applied to the uncertainty calculated in Reference 5 and the amount over the TS measurement-calculational uncertainty factor was the additional penalty applied for this reload. The TS measurement-calculational uncertainty factor for FaN (or LHGR) is 1.07 for the INPAX-IJ core monitoring system installed at MPS2. The additional penalty factor of 1.0025 provided in the letter dated January 18, 2012, will apply to peak measured FaN, as determined by the INPAX-II core monitoring system, to account for the potential misaligned incore detectors. The NRC staff reviewed the licensee's justification for using a conservative penalty factor of 1.0025 for FaN and found it acceptable due to the approved methodologies used.

-3.3 Revision to Figure 2.5-1 Based on the Analysis Supporting the Offset ICI Detectors The penalty factor of 1.0025 for the offset ICI detectors was applied to the setpoint verification calculations as a conservative bias on FaN. The setpoint verification calculations were performed in accordance with AREVA Topical Report EMF-1961 (P)(A), Statistical SetpointfTransient Methodology for Combustion Engineering Type Reactors. The operating region provided in Figure 2.5-1 of the Cycle 21 COLR was updated to provide adequate margin with the application of the penalty. The NRC staff reviewed the licensee's justification for the conservative revised acceptable operating region shown in Figure 2.5-1 and found it acceptable because the updated operating region included penalty factor of 1.0025 for the offset ICI detectors. In summary, the NRC staff concluded that the proposed COLR report for MPS2 Cycle 21 operation is acceptable. 4.0 CONCLUSION The NRC staff has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) there is reasonable assurance that such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public. 5.0 REFERENCES EMF-96-029(P)(A) Volumes 1 and 2, "Reactor Analysis System for PWRs, Volume Methodology Description, Volume 2 -Benchmarking Results," Siemens Power Corporation, January 1997. XN-NF-83-01 (P), "Exxon Nuclear Analysis of Power Distribution Measured Uncertainty for st. Lucie Unit 1, "Exxon Nuclear Company, January 1983. Principal Contributor: THuang Date: May 22, 2012 May 22, 2012 Mr. David