ML13281A803

30-Day Report of Emergency Core Cooling System Model Changes Pursuant to the Requirements of 10 CFR 50.46

ML13281A803
Person / Time
Site:	North Anna
Issue date:	10/02/2013
From:	Sartain M D Dominion, Virginia Electric & Power Co (VEPCO)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
13-522
Download: ML13281A803 (7)

Text

VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 October 2, 2013 U.S. Nuclear Regulatory Commission Serial No.13-522 Attention:

Document Control Desk NL&OS/ETS RO Washington, DC 20555 Docket No. 50-339 License No. NPF-7 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

NORTH ANNA POWER STATION UNIT 2 30-DAY REPORT OF EMERGENCY CORE COOLING SYSTEM (ECCS) MODEL CHANGES PURSUANT TO THE REQUIREMENTS OF 10 CFR 50.46 In accordance with 10 CFR 50.46(a)(3)(ii), Dominion hereby submits information regarding changes and errors in AREVA's Large Break Loss of Coolant Accident (LBLOCA)Emergency Core Cooling System (ECCS) Evaluation Model for North Anna Power Station (NAPS) Unit 2 on Peak Clad Temperature (PCT). The sum of the absolute magnitudes of the PCT changes since the last 30-day report is 55 0 F, which is greater than the 50°F limit for reporting as defined in 10 CFR 50.46(a)(3)(i).

Attachment 1 provides a report describing the changes and errors associated with the AREVA LBLOCA ECCS Evaluation Model for North Anna Unit 2 since the last 30-day report. Information regarding the effect of the PCT changes to the reported AREVA LBLOCA analyses is provided for North Anna Unit 2 in Attachment

2. The licensing basis PCT is 1909 0 F for North Anna Unit 2.10 CFR 50.46(a)(3)(ii) requires the licensee to provide a report within 30 days, which includes a proposed schedule for providing a reanalysis or taking other action as may be needed to show compliance with 10 CFR 50.46. Dominion has reviewed the information provided by AREVA and determined that the adjusted LBLOCA PCT values and the manner in which they were derived continue to conform to the requirements of 10 CFR 50.46. As such, Dominion considers the scheduler requirements of 10 CFR 50.46(a)(3)(ii) to be satisfied with the submission of this notification.

Dominion routinely tracks adjustments to the LBLOCA calculated PCT values to ensure that reasonable margins to the acceptance value set by 10 CFR 50.46 are maintained.

North Anna Unit 2 is in the process of transitioning from AREVA fuel to Westinghouse fuel.North Anna Unit 2 is currently operating with the first batch of Westinghouse RFA-2 fuel that relies on a different LBLOCA Evaluation Model. Upon reaching an equilibrium core of Westinghouse fuel in spring 2016, the AREVA Realistic LBLOCA analysis will no longer support operation of North Anna Unit 2. With this fuel transition plan and the reported 291°F margin to the 2200°F regulatory limit, a Realistic LBLOCA reanalysis for AREVA fuel is not planned.

Serial No.13-522 Docket No. 50-339 50.46(a)(3)(ii).

If please contact This information satisfies the 30-day reporting requirements of 10 CFR you have any further questions regarding this submittal, Mr. Thomas Shaub at (804) 273-2763.Very truly yours, VA44DSJ-'J Mark D. Sartain Vice President

-Nuclear Engineering and Development Commitments made in this letter: None Attachments:

(2)1) Report of Changes and Errors in AREVA Realistic Large Break LOCA ECCS Evaluation Model 2) 30 Day Reporting of 10 CFR 50.46 Margin Utilization

-North Anna Power Station Unit 2.cc: U.S. Nuclear Regulatory Commission Region II Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, GA 30303-1257 Dr. V. Sreenivas NRC Project Manager U.S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Stop 08 G-9A Rockville, MD 20852-2738 NRC Senior Resident Inspector North Anna Power Station Serial No.13-522 Docket No. 50-339 NAPS Unit 2 50.46 30-day Report ATTACHMENT I REPORT OF CHANGES AND ERRORS IN AREVA LARGE BREAK LOCA ECCS EVALUATION MODEL NORTH ANNA POWER STATION UNIT 2 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

Serial No.13-522 Docket No. 50-339 NAPS Unit 2 50.46 30-day Report Identification of ECCS Evaluation Model Change Since the last 30-day report, AREVA has informed Dominion of three non-zero changes to the PCT from the Realistic LBLOCA licensing basis for North Anna Power Station Unit 2 and these changes are described below. Pursuant to requirements of 1 OCFR50.46, Dominion has reviewed these changes and found that they represent a significant change in PCT, as defined in 10 CFR 50.46(a)(3)(i).

Change: Sleicher-Rouse single phase vapor heat transfer correlation

(+14 0 F)An internal AREVA corrective action item was written to evaluate the impact modifying the formulation of the Sleicher-Rouse single phase vapor heat transfer correlation in the S-RELAP5 code. In developing a BWR LOCA analysis methodology using S-RELAP5, AREVA noticed that the behavior of the Sleicher-Rouse single phase vapor heat transfer correlation differed from other correlations.

Additionally, it was discovered that the formulation of the correlation in the S-RELAP5 code different from the formulation of the correlation used in other industry codes. AREVA prepared an S-RELAP5 code version with the alternative formulation.

The alternative formulation of the Sleicher-Rouse agreed more closely with the formulation of the correlation used in the other industry codes.The results of plant sample problems indicate that the predicted RLBLOCA PCT for North Anna Unit 2 should be increased by 14 0 F.Change Liquid Fallback into Surrounding 6 Assemblies

(+31'F)The limiting case revealed non-conservatism in the core exit modeling relative to the form loss coefficient (FLC) between the upper plenum and the central core region. Liquid communication from the central core region, which represents six assemblies surrounding the hot channel, to the hot channel resulted in steam cooling and a reduction in the hot rod PCT at elevations associated with the peaked axial power.This Condition Report is related to a previous issue in which it was discovered that the RLBLOCA reactor vessel model, in particular, the upper plenum, was producing a non-physical phenomenon.

This non-physical phenomenon was manifesting itself by producing an influx of liquid from the upper plenum into the hot assembly.

The previous 50.46 evaluation identified the cases with liquid fallback in the hot assembly and reran these cases with a modification to the reverse FLC for the hot assembly and central core. The selection of the cases to be evaluated was made by verifying the occurrence of liquid fallback at the Hot Assembly exit junction.

For each case evaluated, further investigation was deemed necessary if liquid fallback at the hot assembly junction was detected to have occurred subsequent to the beginning of the core reflood. For cases where larger amounts of liquid fallback were detected and the case had the potential of becoming limiting, the selection was made without regard to the timing of the liquid fallback.The issue is now expanded to the area of concern for liquid fallback from the hot assembly to include the surrounding six assemblies.

Additional cases were identified that had liquid downflow into the central core and these were rerun with the high reverse FLC for the hot assembly and central core region. It was discovered in the calculation supporting this issue that the results for the previous issue were incorrect.

This is due to incorrectly applying a Page 1 of 2 Serial No.13-522 Docket No. 50-339 NAPS Unit 2 50.46 30-day Report high reverse FLC to the average core. The current evaluation applied the correction correctly to the hot assembly and the central core region.Upon further evaluation of the reverse FLC being applied only to the hot assembly and central core region, the estimated PCT impact for North Anna Unit 2 is + 31 OF.Change -Issue with S-RELAP5 routine associated with the RODEX3a fuel rod model (-10°F)While performing code restructuring activities a code developer reported an issue in an S-RELAP5 routine associated with the RODEX3a fuel rod model in the code. In realistic large break loss of coolant accident (RLBLOCA) analyses, RODEX3a is used to calculate the fuel rod conditions.

The issue involves the trapped stack model in subroutine mdatr3, which is part of the RODEX3a fuel rod model in the code. The error affects any RODEX3a based S-RELAP5 analysis which contains a "trapped stack" of fuel pellets. A "trapped stack" condition exists in any fuel rod containing a "locked" gap with open gaps lying at lower axial levels. A gap is locked when the calculated gap dimension is less than 0.5 mils. That dimension was chosen for the locked criteria to account for roughness, pellet cocking, and cladding ovality effects. All axial levels below the lowest locked gap are part of a trapped stack.The erroneous coding in mdatr3 involves incorrect variable addressing which essentially deactivates the trapped stack model. The effect of this error would not be obvious in existing analyses since preliminary assessments indicate the effect of a functioning trapped stack model is very small. Although the effect is small it was determined that it can be conservative or non-conservative depending of the steady-state initial stored energy.A development version of S-RELAP5 was prepared with the correct evaluation of the trapped stack model and several code validation and plant sample problems were repeated.

The assessments included analyses for RLBLOCA Rev 0. The SBLOCA analysis is not affected by this change because RODEX2, as opposed to RODEX3a, is used in the analysis.The estimated impact of this change on the North Anna Unit 1 and Unit 2 RLBLOCA analyses calculated peak cladding temperature is -10 0 F.Conclusion Dominion has performed an evaluation of PCT for comparison to 10 CFR 50.46 requirements.

The Analysis of Record (AOR) PCT for NAPS 2 is 1789 0 F. Considering the current PCT changes as well as all previously reported changes and errors, the licensing basis LBLOCA PCT is 1909OF for North Anna Unit 2. The LBLOCA results have sufficient margin to the 2200°F limit specified in 10 CFR 50.46(b)(1).

The sum of the absolute magnitudes of the respective PCT changes since the last 30-day report is 55 0 F, which is greater than the 50°F limit for reporting as defined in 10 CFR 50.46(a)(3)(i);

Page 2 of 2 Serial No.13-522 Docket No. 50-339 NAPS Unit 2 50.46 30-day Report ATTACHMENT 2 30-DAY REPORTING OF 10 CFR 50.46 MARGIN UTILIZATION NORTH ANNA POWER STATION UNIT 2 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

Serial No.13-522 Docket No. 50-339 NAPS Unit 2 50.46 30-day Report 10 CFR 50.46 MARGIN UTILIZATION

-AREVA LARGE BREAK LOCA Plant Name: North Anna Power Station, Unit 2 Utility Name: Virginia Electric and Power Company Analysis Information EM: AREVA RLBLOCA EM Limiting Break Size: DEGB Analysis Date: 2004 Vendor: AREVA FQ: 2.32 FAH: 1.65 Fuel: Advanced Mark-BW SGTP (%): 12 Notes: None Clad Temp (*F)LICENSING BASIS Analysis of Record PCT 1789 PCT ASSESSMENTS (Delta PCT)A. Prior ECCS Model Assessments

1. Forslund-Rohsenow Correlation Modeling 64 2. RWST Temperature Assumption 8 3. LBLOCA/Seismic SG Tube Collapse 0 4. ICECON Code Errors 0 5. RLBLOCA Choked Flow Disposition 22 6. RLBLOCA Changes in Uncertainty Parameters 10 7. Advanced Mark-BW Top Nozzle Modification 65 8. GSI-191 Sump Strainer 0 9. Mixture Level Model Limitation in the S-RELAP5 Code -19 10. Point Kinetics Programming Issue with RELAP5-Based Computer Codes -20 11. Cold Leg Condensation Under Predicted by S-RELAP5 Following Accumulator Injection 0 12. Cross-Flow Junction Area in S-RELAP Model 0 13. Radiation to Fluid Heat Transfer Model Change -32 14. S-RELAP5 Kinetics and Heat Conduction Model -29 15. RODEX3A -Thermal Conductivity Degradation 0 16. Steam Generator Entrainment Bias Factor (FIJ) Change -4 17. MUR Implementation 20 18. RLBLOCA Upper Plenum Modeling 0 19. Sleicher-Rouse Correlation Modeling 14 20. Liquid Fallback into Surrounding 6 Assemblies 31 21. Cathcart-Pawel Uncertainty Implementation 0 in RLBLOCA Applications B. Planned Plant Modification Evaluations

1. None 0 C. 2013 ECCS Model Assessments I. Issue with S-RELAP5 routine associated with the RODEX3a fuel rod model -10 D. Other 1. None 0 LICENSING BASIS PCT + PCT ASSESSMENTS PCT = 1909 Page 1 of 1