RS-14-082, ECCS Evaluation Model Error - 10 CFR 50.46 30-Day Report

ECCS Evaluation Model Error - 10 CFR 50.46 30-Day Report

ML14059A234
Person / Time
Site:	Byron, Braidwood
Issue date:	02/27/2014
From:	Gullott D Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RS-14-082
Download: ML14059A234 (8)

Text

RS-14-082 February 27, 2014 U.S. Nuclear Regulatory Commission A TIN: Document Control Desk Washington, DC 20555-0001 Braidwood Station, Unit 1 Facility Operating License No. NPF-72 NRC Docket No. STN 50-456 Byron Station, Unit 1 Facility Operating License No. NPF-37 NRC Docket No. STN 50-454

Subject:

ECCS Evaluation Model Error-10 CFR 50.46 30-Day Report 10 CFR 50.46

Reference:

Letter from D. M. Gullett (Exelon Generation Company, LLC) to U.S. NRC, "Annual10 CFR 50.46 Report of the Emergency Core Cooling System Evaluation Model Changes and Errors," dated AprilS, 2013 In accordance with 10 CFR 50.46, "Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors," paragraph (a)(3)(ii), Exelon Generation Company, LLC, (EGC) is submitting the attached information to fulfill the 30-day reporting requirement for Braidwood Station, Unit 1 and Byron Station, Unit 1.

In the referenced letter, Exelon Generation Company, LLC (EGC) submitted the most recent 10 CFR 50.46 report for Braidwood and Byron Stations, Units 1 and 2, describing the cumulative changes in the peak cladding temperature (PCT) for the current emergency core cooling systems evaluation model. Subsequent to issuance of the referenced report, by letters dated May 2, 2013, July 30, 2013, July 31, 2013, October 4, 2013, and January 30, 2014, Westinghouse Electric Company notified EGC of changes and errors in the ASTRUM Large Break Loss of Coolant Accident (LBLOCA) methodology.

Upon receipt of the Westinghouse Electric Company letter dated January 30, 2014, which addressed the HOTSPOT burst strain error (see Attachment 2 Note 18), these changes and errors resulted in a cumulative PCT impact of greater than 50°F for the Braidwood Station, Unit 1 and Byron Station, Unit 1 LBLOCA analysis. The above noted letters did not identify model errors or changes to the Small Break Loss of Coolant Accident (SBLOCA) analysis which would necessitate NRC notification. The cumulative PCT impact for Unit 1 is 66°F whereas the cumulative PCT impact for Unit 2 is 46°F (i.e., did not exceed the 10 CFR 50.46 reporting

U.S. Nuclear Regulatory Commission February 27, 2014 Page2 criteria of 50°F); therefore, Braidwood Station, Unit 2 and Byron Station, Unit 2 are not addressed in this 30-day report.

Although the subject evaluation model changes and errors resulted in an increase in the limiting PCT for the LBLOCA, the PCT value remains well below the 10 CFR 50.46 acceptance criteria of 2200°F; therefore, a reanalysis is not required due to these new model changes and errors. provides updated information regarding changes to the PCT values for the limiting LBLOCA analysis for Braidwood Station, Unit 1 and Byron Station, Unit 1. As previously noted, the SBLOCA analyses were not affected. Attachment 2 contains the associated Assessment Notes.

There are no regulatory commitments contained in this submittal. Should you have any questions regarding this letter, please contact Joseph A. Bauer at (630) 657-2804.

Respectfully, David M. Gullatt Manager - Licensing Exelon Generation Company, LLC Attachments:

1. Braidwood Station Unit 1 and Byron Station Unit 1 - 1 0 CFR 50.46 Report

2. Braidwood Station Unit 1 and Byron Station Unit 1 -10 CFR 50.46 Report Assessment Notes cc:

NRC Regional Administrator-Region Ill NRC Senior Resident Inspector-Braidwood Station NRC Senior Resident Inspector-Byron Station NRR Project Manager, Braidwood and Byron Stations Illinois Emergency Management Agency-Division of Nuclear Safety Braidwood Station Unit 1 and Byron Station Unit 1 -10 CFR 50.46 Report PLANT NAME:

Braidwood Station Unit 1 ECCS EVALUATION MODEL:

REPORT REVISION DATE:

Large Break Loss of Coolant Accident {LBLOCA}

02/27114 CURRENT OPERATING CYCLE:

~

AOR Evaluation Model: ASTRUM (2004)

Calculation: Westinghouse WCAP-16841-P, November 2007 Fuel: VANTAGE+ 17 x 17 Limiting Fuel Type: VANTAGE+ 17 x 17 Limiting Single Failure: Loss of one train of ECCS flow Limiting Break Size and Location: Guillotine break in the Cold Leg Reference PCT PCT = 1913.0 oF MARGIN ALLOCATION A. PRIOR LOSS OF COOLANT ACCIDENT (LOCA) MODEL ASSESSMENTS 10 CFR 50.46 report dated March 15, 2011 (Note 1) ilPCT = 0 °F 10 CFR 50.46 report dated April 6, 2011 (Note 2) ilPCT = 0 °F 10 CFR 50.46 report dated April6, 2012 (Note 3) ilPCT = 0 °F 10 CFR 50.46 report dated May 21,2012 (Note 4) ilPCT =44 °F 10 CFR 50.46 report dated April 5, 2013 (Note 5) ilPCT = 0 °F NETPCT PCT = 1957.0 °F B. CURRENT LOCA MODEL ASSESSMENTS Initial Fuel Pellet Average Temperature Uncertainty Calculation ilPCT = 0 °F (Note 6)

Heat Transfer Model Error Corrections (Note 7)

I8PCT = 0 °F Correction to Heat Transfer Node Initialization (Note 8)

UEcT=0°F Mass Conservation Error Fix (Note 9) ilPCT = 0 °F Correction to Split Channel Momentum Equation (Note 10) ilPCT = 0 °F Heat Transfer Logic Correction For Rod Burst Calculation (Note 11) ilPCT = 0 °F Changes to Vessel Superheated Steam Properties (Note 12) ilPCT = 0 °F Update to Metal Density Reference Temperatures (Note 13) 8PCT = 0 °F Decay Heat Model Error Corrections {Note 14) ilPCT = 0 °F Correction to The Pipe Exit Pressure Drop Error {Note 15) 8PCT = 0 °F Revised Heat Transfer Multiplier Distributions {Note 16) ilPCT = 5°F Changes to Grid Blockage Ratio and Porosity (Note 17) ilPCT = 24 °F HOTSPOT Burst Strain Error Correction (Note 18) ilPCT = 37 °F Total PCT change from current assessments L ilPCT = +66 °F Cumulative PCT change from current assessments L lilPCT I= 66 °F NETPCT PCT = 2023.0 °F 1 of2 Braidwood Station Unit 1 and Byron Station Unit 1 -10 CFR 50.46 Report PLANT NAME:

Byron Station Unit 1 ECCS EVALUATION MODEL:

REPORT REVISION DATE:

Large Break Loss of Coolant Accident (LBLOCA) 02/27/14 CURRENT OPERATING CYCLE:

19 AOR Evaluation Model: ASTRUM (2004)

Calculation: Westinghouse WCAP-16841-P, November 2007 Fuel: VANTAGE+ 17 x 17 Limiting Fuel Type: VANTAGE+ 17 x 17 Limiting Single Failure: Loss of one train of ECCS flow Limiting Break Size and Location: Guillotine break in the Cold Leg Reference PCT PCT = 1913.0 oF MARGIN ALLOCATION A. PRIOR LOSS OF COOLANT ACCIDENT (LOCA) MODEL ASSESSMENTS 10 CFR 50.46 report dated March 15, 2011 (Note 1) 10 CFR 50.46 report dated April 6, 2011 (Note 2) 10 CFR 50.46 report dated AprilS, 2012 (Note 3) 10 CFR 50.46 report dated May 21, 2012 (Note 4) 10 CFR 50.46 report dated AprilS, 2013 (Note 5)

NET PCT B. CURRENT LOCA MODEL ASSESSMENTS Initial Fuel Pellet Average Temperature Uncertainty Calculation Note 6 Split Channel Momentum Equation (Note 10) er Logic Correction For Rod Burst Calculation N uperheated Steam Properties (Note 12)

I Density Reference Temperatures (Note 13)

Correction to The Pipe Exit Pressure Oro Error (Note 15)

Revised Heat Transfer Multiplier Distributions (Note 16)

Changes to Grid Blockage Ratio and Porosity Note 17)

HOTSPOT Burst Strain Error Correction (Note 18)

Total PCT change from current assessments Cumulative PCT change from current assessments NETPCT 2 of2

.llPCT = 0 °F

.llPCT = 0 °F

.llPCT = 0 °F

.llPCT = 44 °F ilPCT = 0 °F PCT = 1957.0°F

.llPCT = 0 °F

.llPCT = 0 °F

.llPCT = 0 °F

.llPCT = 0 °F CT = 0 °F PCT = 2023.0 °F Braidwood Station Unit 1 and Byron Station Unit 1 -10 CFR 50.46 Report Assessment Notes

1. Prior LOCA Model Assessment The 30-day 10 CFR 50.46 report dated March 15, 2011 reported a new ASTRUM Best Estimate Large-Break Loss-of-Coolant Accident (LBLOCA} analysis to support operations for Byron and Braidwood Stations Units 1 and 2. The same report assessed the impact from several errors, issues, and code enhancements. Each of these errors/issues/code enhancements had a ooF PCT impact with a net OoF PCT impact.

2. Prior LOCA Model Assessment The 10 CFR 50.46 report dated April 6, 2011 reported no evaluations for the LBLOCA model.

3. Prior LOCA Model Assessment The 10 CFR 50.46 report dated April6, 2012 reported evaluations for LOCA model changes and errors. The report documents general code maintenance for LBLOCA which had a 0°F PCT penalty associated with it.

4. Prior LOCA Model Assessment The 30-day 10 CFR 50.46 report dated May 21, 2012 applicable to Braidwood Unit 1 and Byron Unit 1 reported an assessment of Thermal Conductivity Degradation (TCD} with an associated peaking factor bumdown and an analysis input change consisting of a reduction in conservatism in analyzed FQ values and an increase in the assumed containment pressure boundary condition.

As a result, Westinghouse estimated the effect of the TCD w/ burndown to be +110°F and estimated the effect of the analysis input changes to be -66°F. These two assessments are coupled together via their evaluations of burnup effects which include thermal conductivity degradation, peaking factor burndown and analysis input changes.

Therefore, the combined affect of these two changes results in a net change in the reported LBLOCA PCT of +44°F.

5. Prior LOCA Model Assessment The 10 CFR 50.46 report dated April5, 2103 reported evaluations for LOCA model changes and errors. The report documents general code maintenance, errors in the HOTSPOT code pertaining to burst temperature and fuel pellet average temperature calculations, an error in the WCOBRAJTRAC Automated Restart Process logic, an error in the calculation of rod internal pressure, and an error in the thermal hydraulic history file used in HSDRIVER for LBLOCA which all have a oaF PCT penalty associated with them.

6. Current LOCA Model Assessment In the ASTRUM LBLOCA Evaluation Model (EM}, uncertainties are applied to the gap heat transfer coefficient and pellet thermal conductivity to capture the uncertainty in the initial fuel pellet average temperature. This approach was compared to the initial fuel pellet average temperature uncertainties predicted by the PAD code at beginning-of-life conditions and found to be conservative. However, the initial fuel pellet average 1 of 4 Braidwood Station Unit 1 and Byron Station Unit 1 -10 CFR 50.46 Report Assessment Notes temperature uncertainty range analyzed at higher bumups in the ASTRUM EM is much wider than the uncertainty range predicted by the PAD code, which may result in excessively low or high analyzed initial fuel pellet average temperatures. This issue has been evaluated to estimate the impact on existing ASTRUM LBLOCA analysis results.

The resolution of this issue led to an estimated PCT impact of 0°F.

7. Current LOCA Model Assessment Several related changes were made to WCOBRAffRAC to correct errors discovered which affected the heat transfer models. These errors included calculation of the entrained liquid fraction used in calculation of the drop wall heat flux, application of the grid enhancement factor for grid temperature calculation, calculation of the Reynold's number used in the Wong*Hochrieter correlation for the heat transfer coefficient from fuel rods to vapor, fuel rod initialization and calculation of cladding inner radius with creep, application of grid and two phase enhancement factors and radiation component in single phase vapor heat transfer, and reset of the critical heat flux temperature when J=2.

These errors have been evaluated to estimate the impact on existing LBLOCA analysis results. These changes led to an estimated PCT impact of oaF.

8. Current LOCA Model Assessment An error was discovered in the heat ~ransfer node initialization logic in WCOBRAffRAC whereby the heat transfer node center locations could be inconsistent with the geometric node center elevations. The primary effects of this issue are on the interpolated fluid properties and grid turbulent mixing enhancement at the heat transfer node. This problem has been evaluated for impact on existing analyses and its resolution led to an estimated PCT impact of 0°F.

9. Current LOCA Model Assessment It was identified that mass was not conserved in WCOBRAffRAC one*dimensional component cells when void fraction values were calculated to be slightly out of the physical range (greater than 1.0 or smaller than 0.0}. This was observed to result in artificial mass generation on the secondary side of steam generator components. This correction led to an estimated PCT impact of 0°F.

10. Current LOCA Model Assessment An error was discovered in the momentum equation calculations for split channels in WCOBRAffRAC. This error impacts the ( 1) continuity area of the phantom/boundary bottom cell; (2) bottom and top continuity area correction factors for the channel inlet at the bottom of a section and for the channel outlet at the top of a section; and (3) drop entrainment mass rate per unit volume and drop de-entrainment mass rate per unit volume contributions to the momentum calculations for split channels. This problem has been evaluated for impact on existing analyses which led to an estimated PCT impact of 0°F.

2of4 Braidwood Station Unit 1 and Byron Station Unit 1 -10 CFR 50.46 Report Assessment Notes

11. Current LOCA Model Assessment A change was made to the WCOBRAITRAC coding to correct an error which had disabled rod burst in separate effect test simulations. This change led to an estimated PCT impact of O"F.

12. Current LOCA Model Assessment Several related changes were made to the WCOBRAITRAC coding for the vessel super-heated water properties, including updating the HGAS subroutine coding to be consistent with WCAP-12945-P-A Equation 10-6, updating the approximation of the enthalpy in the TGAS subroutine to be consistent with the HGAS subroutine coding, and updating the temperature iteration method and convergence criteria in the TGAS subroutine. These changes led to an estimated PCT impact of O"F.

13. Current LOCA Model Assessment It was identified that for one-dimensional components in which heat transfer to stainless steel 304 or 316 is modeled, the reference temperature for the metal density calculation was allowed to vary; as a result the total metal mass was not preserved. Correction of this problem led to an estimated PCT impact of O"F.

14. Current LOCA Model Assessment The decay heat model in the WCOBRAITRAC code was updated to correct the erroneously coded value of the yield fraction directly from fission for Group 19 of Pu-239, and to include the term for uncertainty in the prompt energy per fission in the calculation of the decay heat power uncertainty. Correction of these errors led to an estimated PCT impact of O"F.

15. Current LOCA Model Assessment An error was discovered in WCOBRAITRAC whereby the frictional pressure drop at the split break TEE connection to the BREAK component was incorrectly calculated using the TEE hydraulic diameter instead of the BREAK component length input. This error has been evaluated for impact on existing analyses and its resolution led to an estimated PCT impact of O"F.

16. Current LOCA Model Assessment Several changes and error corrections were made to WCOBRAITRAC and the impacts of these changes on the heat transfer multiplier uncertainty distributions were investigated.

During this investigation, errors were discovered in the development of the original multiplier distributions, including errors in the grid locations specified in the WCOBRAITRAC models for the G2 Refill and G2 Reflood tests, and errors in processing test data used to develop the reflood heat transfer multiplier distribution. Therefore, the blowdown heatup, blowdown cooling, refill, and reflood heat transfer multiplier distributions were redeveloped. For the reflood heat transfer multiplier development, the evaluation time windows for each set of test experimental data and each test simulation were separately defined based on the time at which the test or simulation exhibited 3 of4 Braidwood Station Unit 1 and Byron Station Unit 1 - 10 CFR 50.46 Report Assessment Notes dispersed flow film boiling heat transfer conditions characteristic of the reflood time period. The revised heat transfer multiplier distributions have been evaluated for impact on existing analyses. These changes led to an estimated PCT impact of 5°F for Byron and Braidwood Unit 1.

17. Current LOCA Model Assessment A change in the methodology used to calculate grid blockage ratio and porosity for Westinghouse fuel resulted in a change to the grid inputs used in the Byron Unit 1 and Braidwood Unit 1 large break loss-of-coolant accident (LBLOCA) analysis. Grid inputs affect heat transfer in the core during a LBLOCA. This change resulted in an estimated PCT effect of 24°F.

18. Current LOCA Model Assessment An error in the application of the burst strain was discovered in HOTSPOT. In the equation the outer radius of the cladding after burst occurs should be calculated based on the burst strain, and the inner radius of the cladding should be calculated based on the outer radius. In HOTSPOT, the burst strain is applied to the calculation of the cladding inner radius. The cladding outer radius is then calculated based on the inner radius. As such, the burst strain is incorrectly applied to the inner radius rather than the outer radius, which impacts the resulting cladding geometry at the burst elevation after burst occurs.

Correction of the erroneous calculation results in thinner cladding at the burst node and more fuel relocating into the burst node, leading to an increase in the Peak Cladding Temperature (PCT) at the burst node. This issue has been evaluated to estimate the impact on existing Best-Estimate (BE) Large-Break Loss-of-Coolant Accident (LBLOCA) analysis results. The error led to an estimated PCT impact of 37°F for Byron and Braidwood Unit 1.

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