ML13246A103

Units 1 and 2 - Report Per 10 CFR 50.46, Changes to or Errors in an Evaluation Model

ML13246A103
Person / Time
Site:	Mcguire, Catawba, McGuire
Issue date:	08/29/2013
From:	Duncan R Duke Energy Carolinas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML13246A103 (19)

Text

Robert J. Duncan

~DUKE Sr Vice President Nuclear Operations NERGY.

Catawba, McGuire 526 S. Church Street Charlotte, NC 28202 Mailing Address:

EC07H / P.O. Box 1006 Charlotte, NC 28202 o: 704-382-4098 c: 919-812-7226 f: 704-382-6056 Bob. Duncan(.duke-eoneroqy. com 10 CFR 50.4 10 CFR 50.46 August 29, 2013 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001 Catawba Nuclear Station, Units 1 and 2 Docket Numbers 50-413 and 50-414/Renewed License Numbers NPF-35 and NPF-52 McGuire Nuclear Station, Units 1 and 2 Docket Numbers 50-369 and 50-370/Renewed License Numbers NPF-9 and NPF-17

Subject:

Duke Energy Carolinas, LLC (Duke Energy): Report Pursuant to 10 CFR 50.46, Changes to or Errors in an Evaluation Model

References:

1) Letter, D. C. Culp (Duke Energy) to USNRC,

Subject:

Catawba Nuclear Station Units 1 and 2, and McGuire Nuclear Station Units 1 and 2, Response to Information Request Pursuant to 10 CFR 50.54(f) Related to the Estimated Effect on Peak Cladding Temperature Resulting from Thermal Conductivity Degradation in the Westinghouse-Furnished Realistic Emergency Core Cooling System Evaluation and 30-Day Report Pursuant to 10 CFR 50.46, Changes to or Errors in an Evaluation Model," March 16, 2012. [ADAMS ML12079A180]

10 CFR 50.46 (a)(3)(ii) requires the reporting of changes to or errors in Emergency Core Cooling (ECCS) evaluation models (EMS). On July 31, 2013, Duke Energy received a letter from Westinghouse Electric Company identifying errors in the heat transfer multiplier uncertainty distributions which affect the Best Estimate Large Break Loss of Coolant Accident (BELOCA) analysis of record for Catawba Nuclear Station (CNS) Units 1 & 2 and McGuire Nuclear Station (MNS) Units 1 & 2. Small Break LOCA analyses for CNS and MNS are not impacted by these errors.

The enclosed Attachment 1 provides a description of the errors, and the associated impact to the Catawba and McGuire BELOCA analysis of record. Based on information supplied by Westinghouse, an assessment of this error results in a peak cladding temperature (PCT) decrease of 85"F for the limiting transient. These impacts to the BELOCA analyses are discussed in Table 1, and are included on the PCT reporting sheets, Tables 2 through 4.

Aob6

U.S. Nuclear Regulatory Commission August 29, 2013 Page 2 provides additional information on how Westinghouse performed the PCT evaluation of changes to the heat transfer multiplier uncertainty distributions.

Since the absolute value of the change in PCT is greater than 50 OF, this is considered to be a significant change. 10 CFR 50.46(a)(3)(ii) states: "... If the change or error is significant, the applicant or licensee shall provide this report within 30 days and include with the report a proposed schedule for providing a reanalysis or taking other action as may be needed to show compliance with 50.46 requirements In Reference 1, Duke Energy has previously committed to submit by December 15, 2016 a LBLOCA analysis that applies an NRC-approved ECCS evaluation model that includes the effects of fuel pellet thermal conductivity degradation. Since this report identifies a reduction in PCT, there are no adverse impacts to safety as a result of the ECCS evaluation model errors described herein, and all 10 CFR 50.46 acceptance criteria are met. Therefore, the existing LBLOCA reanalysis commitment discussed in Reference 1 is sufficient to address the requirements of 10 CFR 50.46(a)(3)(ii) pertaining to the most recent ECCS evaluation model errors described herein.

Several other changes were made to the WCOBRAITRAC computer code used within the BELOCA evaluation model. The specific details of these changes are also provided in Table 1, and were evaluated by Westinghouse as having no impact on the calculated PCTs. Since there was no PCT impact due to these WCOBRA/TRAC code changes, they are not included in the PCT reporting sheets, Tables 2 through 4.

There are no new regulatory commitments contained in this letter.

Please address any comments or questions regarding this matter to Tom Byrne at (980) 373-3249 (Tom. Byrne@duke-energy. com).

Sincerel, Senior Vice President Nuclear Operations Catawba, McGuire Table 1 - Errors/Evaluation Model Changes Table 2 - Peak Cladding Temperature Summary - McGuire Units 1 & 2 Table 3 - Peak Cladding Temperature Summary - Catawba Unit 1 Table 4 - Peak Cladding Temperature Summary - Catawba Unit 2 - Additional Information on the Evaluation of Revised Heat Transfer Multiplier Distributions for Plants Licensed with the CQD Evaluation Model

U.S. Nuclear Regulatory Commission August 29, 2013 Page 3 xc (with attachments):

V. M. McCree, Region II Administrator U.S. Nuclear Regulatory Commission Marquis One Tower 245 Peachtree Center Avenue NE, Suite 1200 Atlanta, GA 30303-1257 J. C. Paige, Senior Project Manager (CNS & MNS)

U. S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop 0-8C2A Rockville, MD 20852-2738 J. Zeiler, NRC Senior Resident Inspector McGuire Nuclear Station G. A. Hutto, NRC Senior Resident Inspector Catawba Nuclear Station

ATTACHMENT 1 Table I - Errors/Evaluation Model Changes Table 2 - Peak Cladding Temperature Summary - McGuire Units 1 & 2 Table 3 - Peak Cladding Temperature Summary - Catawba Unit I Table 4 - Peak Cladding Temperature Summary - Catawba Unit 2, Page 1 of 10

References:

A) Letter, G. R. Peterson (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," April 11, 2001. [ADAMS ML011070266]

B) Letter, M. S. Tuckman (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," April 3, 2002. [ADAMS ML021070672]

C) Letter, W. R. McCollum, Jr. (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," July 291, 2003. [ADAMS ML032170639]

D) Letter, W. R. McCollum, Jr. (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," May 26, 2004. [ADAMS ML041560349]

E) Letter, J. R. Morris (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," June 21, 2005. [ADAMS ML051790210]

F) Letter, T. C. Geer (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," March 13, 2007. [ADAMS ML070800546]

G) Letter, T. C. Geer (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," May 22, 2007. [ADAMS ML071500297]

H) Letter, D. C. Culp (Duke Energy) to USNRC,

Subject:

Catawba Nuclear Station Units 1 and 2, and McGuire Nuclear Station Units 1 and 2, Response to Information Request Pursuant to 10 CFR 50.54(f) Related to the Estimated Effect on Peak Cladding Temperature Resulting from Thermal Conductivity Degradation in the Westinghouse-Furnished Realistic Emergency Core Cooling System Evaluation and 30-Day Report Pursuant to 10 CFR 50.46, Changes to or Errors in an Evaluation Model," March 16, 2012. [ADAMS ML12079A180]

I) Letter, J. Thompson (USNRC) to K. Henderson and S. D. Capps (Duke Energy),

Subject:

Catawba Nuclear Station Units 1 and 2, and McGuire Nuclear Station Units 1 and 2, Closure Evaluation for Report Pursuant to Title 10 of the Code of Federal Regulations, Part 50, Section 50.46, Paragraph (a)(3)(ii) Concerning Significant Emergency Core Cooling System Evaluation Model Error Related to Nuclear Fuel Thermal Conductivity Degradation (TAC Nos.

ME8447, ME8448, ME8449, and ME8450)" November 16, 2012. [ADAMS ML12314A031]

J) Letter, M. J. Annacone, (Duke Energy) to USNRC, "Report Pursuant to 10 CFR 50.46, Changes to or Errors in an ECCS Evaluation Model," July 11, 2013. [ADAMS ML13199A279]

K) Westinghouse Electric Company Letter LTR-LIS-13-348; McGuire Units 1 & 2 and Catawba Units 1 & 2 - 10 CFR 50.46 Report for Revised Heat Transfer Multiplier Distributions, July 31, 2013.

L) Westinghouse Electric Company Letter LTR-LIS-13-346; 10 CFR 50.46 Notification and Reporting for WCOBRA/TRAC Changes and Error Corrections, July 30, 2013., Page 2 of 10

Table 1 Errors I Evaluation Model Changes Revised Heat Transfer Multiplier Distributions (Data provided in Reference K)

Affected Evaluation Model(s) Applicable to Catawba/McGuire:

1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model Several changes and error corrections were made to WCOBRA/TRAC 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 WCOBRA/TRAC 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 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.

Resolution of these issues represents a closely related group of Non-Discretionary Changes in accordance with Section 4.1.2 of WCAP-13451.

A plant transient calculation representative of McGuire Units 1 and 2 and Catawba Units 1 and 2 transient behavior was performed with the latest version of WCOBRA/TRAC. Using this transient, HOTSPOT calculations were performed with both the original and revised heat transfer multiplier distributions. Based on the change in the 95 th percentile results, estimated PCT effects of -40°F for Reflood 1 and -85°F for Reflood 2 have been established for 10 CFR 50.46 reporting purposes for McGuire Units 1 and 2 and Catawba Units 1 and 2. For Catawba and McGuire, the limiting PCT occurs late in the reflood phase (Attachment 2, Section 2.6, Late Reflood Limited).

Please see Attachment 2 for additional details on how Westinghouse performed the PCT evaluation of changes to the heat transfer multiplier distributions., Page 3 of 10

The following changes are described in Reference L, and are included herein for completeness, since these changes were incorporated into the WCOBRA/TRAC model used to assess the PCT impact due to the revised heat transfer multiplier distributions described above.

ELEVATIONS FOR HEAT SLAB TEMPERATURE INITIALIZATION Affected Evaluation Model(s) Applicable to Catawba/McGuire:

1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model An error was discovered in WCOBRAITRAC whereby an incorrect value would be used in the initial fuel rod temperature calculation for a fuel rod heat transfer node if that node elevation was specified outside of the bounds of the temperature initialization table. This problem has been evaluated for impact on existing analyses and its resolution represents a Discretionary Change in accordance with Section 4.1.1 of WCAP-1 3451.

Based on inspection of plant analysis input, it was concluded that the input decks for existing analyses are not impacted by this error, leading to an estimated peak cladding temperature impact of 0°F.

HEAT TRANSFER MODEL ERROR CORRECTIONS Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA Several related changes were made to WCOBRA/TRAC 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. Correction of these errors represents a closely-related group of Non-Discretionary Changes in accordance with Section 4.1.2 of WCAP-1 3451.

Based on the results of representative plant calculations, separate effects and integral effects test simulations, it is concluded that the error corrections have a negligible local effect on heat transfer, leading to an estimated peak cladding temperature impact of 0°F., Page 4 of 10

CORRECTION TO HEAT TRANSFER NODE INITIALIZATION Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA An error was discovered in the heat transfer node initialization logic in WCOBRA/TRAC 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 represents a Non-Discretionary Change in accordance with Section 4.1.2 of WCAP-13451.

Based on engineering judgment and the results from a matrix of representative plant calculations, it is concluded that the effect of this error is within the code resolution, leading to an estimated peak cladding temperature impact of 0°F.

MASS CONSERVATION ERROR FIX Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA It was identified that mass was not conserved in WCOBRA/TRAC 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. Correction of this problem represents a Non-Discretionary Change in accordance with Section 4.1.2 of WCAP-1 3451.

This error was observed to primarily affect the mass on the secondary side of the steam generator. This issue was judged to have a negligible impact on existing LBLOCA analysis results, leading to an estimated peak cladding temperature impact of 0°F.

CORRECTION TO SPLIT CHANNEL MOMENTUM EQUATION Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA An error was discovered in the momentum equation calculations for split channels in WCOBRAITRAC. 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 and its resolution represents a Non-Discretionary Change in accordance with Section 4.1.2 of WCAP-13451.

Based on the results from a matrix of representative plant calculations, it is concluded that the effect of this error on the quantities directly impacted by the momentum equation calculations for split channels (velocities, flows, etc.) is negligible, leading to an estimated peak cladding temperature impact of 0°F., Page 5 of 10

HEAT TRANSFER LOGIC CORRECTION FOR ROD BURST CALCULATION Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA A change was made to the WCOBRAITRAC coding to correct an error which had disabled rod burst in separate effect test simulations. This change represents a Discretionary Change in accordance with Section 4.1.1 of WCAP-1 3451.

Based on the nature of the change and the evaluation model requirements for plant modeling in Westinghouse best estimate large break LOCA analyses with WCOBRA/TRAC, it is judged that existing analyses are not impacted by this change, leading to an estimated peak cladding temperature impact of 0°F.

CHANGES TO VESSEL SUPERHEATED STEAM PROPERTIES Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA Several related changes were made to the WCOBRA/TRAC coding for the vessel super-heated water properties, including updating the HGAS subroutine coding to be consistent with Reference 1 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 represent a closely-related group of Non-Discretionary Changes in accordance with Section 4.1.2 of WCAP-1 3451.

The updates to the calculations of the superheated steam properties had generally less than 1 OF impact on the resulting steam temperature values, leading to an estimated peak cladding temperature impact of 0°F.

Reference

1. WCAP-12945-P-A, Volume 1, Revision 2, and Volumes 2 through 5, Revision 1, "Code Qualification Document for Best Estimate LOCA Analysis," 1998.

UPDATE TO METAL DENSITY REFERENCE TEMPERATURES Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA 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 represents a Non-Discretionary Change in accordance with Section 4.1.2 of WCAP-1 3451.

This change primarily impacts the reactor coolant system loop piping modeled in the large break loss-of coolant accident (LBLOCA) WCOBRA/TRAC models. It was judged that the effect of this change on the peak cladding temperature results was negligible, leading to an estimated peak cladding temperature impact of 0°F., Page 6 of 10

DECAY HEAT MODEL ERROR CORRECTIONS Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA The decay heat model in the WCOBRA/TRAC 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 represents a closely-related group of Non-Discretionary Changes in accordance with Section 4.1.2 of WCAP-1 3451.

These changes have a negligible impact on the calculated decay heat power, leading to an estimated peak cladding temperature impact of 0°F.

CORRECTION TO THE PIPE EXIT PRESSURE DROP ERROR Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA 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 represents a Non-Discretionary Change in accordance with Section 4.1.2 of WCAP-13451.

Based on the results from a matrix of representative plant calculations, it is concluded that the effect of this error on the pressure at the break and the break flow is negligible, leading to an estimated peak cladding temperature impact of 0°F.

WCOBRA/TRAC U19 FILE DIMENSION ERROR CORRECTION Affected Evaluation Model(s) Applicable to Catawba/McGuire: 1996 BELOCA A problem was identified in the dimension of an array used to generate the u19 file in WCOBRA/TRAC. The u19 file is read during HSDRIVER execution and provides information needed to generate the HOTSPOT thermal-hydraulic history and user input files. The array used to write the desired information to the u19 file is dimensioned to 2000 in WCOBRAITRAC. It is possible, however, for more than 2000 curves to be written to the u19 file. If that is the case, it is possible that the curves would not be stored correctly on the ul 9 file. A survey of current Best Estimate Large Break LOCA analyses indicated that the majority of plants had less than 2000 curves in their u19 files; therefore these plants are not affected by the change. For those plants with more than 2000 curves, plant-specific sensitivity calculations indicated that resolution of this issue does not impact the peak cladding temperature (PCT) calculation for prior analyses. This represents a Discretionary Change in accordance with Section 4.1.1 of WCAP-1 3451.

As discussed in the Background section, resolution of this issue does not impact the peak cladding temperature calculation for prior LBLOCA analyses, leading to an estimated peak cladding temperature impact of 0°F., Page 7 of 10

Table 2 Peak Cladding Temperature Summary - McGuire Units 1 & 2 LBLOCA Cladding Temp Comments (OF)

Evaluation model : WCOBRA/TRAC, CQD 1996 MNS/CNS Analysis of record PCT (Reflood 2) 2028 Composite Model Prior errors (APCT)

1. Decay heat in Monte Carlo calculations 8

Reference A

2. MONTECF power uncertainty correction 20 Reference B

3. Safety Injection temperature range 59 Reference C

4. Input error resulting in an incomplete solution matrix 25 Reference D

5. Revised Blowdown Heatup Uncertainty Distribution 5

Reference E

6. Vessel Unheated Conductor Noding 0

Reference F

7. Thermal Conductivity Degradation with Peaking 15 References H, I Factor Burndown Prior evaluation model changes (APCT)

1. Revised Algorithm for Average Fuel Temperature 0

Reference F

2.

PAD 3.4 to PAD 4.0

-75 References H, I

3. Peak FQ = 2.7 in bottom third of core 0

References H, I

4. MUR Uprate to 101.7% of 3411 MWt 16 References H, I Current Errors (APCT)

1. Revised Heat Transfer Multiplier Distribution

-85 Reference K Current Evaluation model changes (APCT)

1. None Absolute value of errors/changes for this report (APCT) 85 Net change in PCT for this report

-85 Final PCT 2016 SBLOCA Evaluation model: NOTRUMP Analysis of record PCT 1323 2 inch break Reference G Prior errors (APCT)

1. Evaluation of Fuel Pellet Thermal Conductivity 0

Reference J Degradation Prior evaluation model changes (APCT)

1. None 0

Current Errors (APCT)

1. None 0

Current Evaluation model changes (APCT)

1. None 0

Absolute value of errors/changes for this report (APCT) 0 Net change in PCT for this report 0

Final PCT 1323, Page 8 of 10

Table 3 Peak Cladding Temperature Summary - Catawba Unit I LBLOCA Cladding Temp Comments

('IF)

Evaluation model : WCOBRAITRAC, CQD 1996 MNS/CNS Analysis of record PCT (Reflood 2) 2028 Composite Model Prior errors (APCT)

1. Decay heat in Monte Carlo calculations 8

Reference A

2. MONTECF power uncertainty correction 20 Reference B

3. Safety Injection temperature range 59 Reference C

4. Input error resulting in an incomplete solution matrix 25 Reference D

5. Revised Blowdown Heatup Uncertainty Distribution 5

Reference E

6. Vessel Unheated Conductor Noding 0

Reference F

7. Thermal Conductivity Degradation with Peaking 15 References H, I Factor Burndown Prior evaluation model changes (APCT)

1. Revised Algorithm for Average Fuel Temperature 0

Reference F

2. PAD 3.4 to PAD 4.0

-75 References H, I

3. Peak FQ = 2.7 in bottom third of core 0

References H, I Current Errors (APCT)

1. Revised Heat Transfer Multiplier Distribution

-85 Reference K Current Evaluation model changes (APCT)

1. None Absolute value of errors/changes for this report (APCT) 85 Net change in PCT for this report

-85 Final PCT.

2000 SBLOCA Evaluation model: NOTRUMP Analysis of record PCT 1323 2 inch break Reference G Prior errors (APCT)

1. Evaluation of Fuel Pellet Thermal Conductivity 0

Reference J Degradation Prior evaluation model changes (APCT)

1. None 0

Current Errors (APCT)

1. None 0

Current Evaluation model changes (APCT)

1. None 0

Absolute value of errors/changes for this report (APCT) 0 Net change in PCT for this report 0

Final PCT 1323, Page 9 of 10

Table 4 Peak Cladding Temperature Summary - Catawba Unit 2 LBLOCA Cladding Temp Comments (OF)

Evaluation model: WCOBRA/TRAC, CQD 1996 MNS/CNS Analysis of record PCT (Reflood 2) 2028 Composite Model Prior errors (APCT)

1. Decay heat in Monte Carlo calculations 8

Reference A

2. MONTECF power uncertainty correction 20 Reference B

3. Safety Injection temperature range 59 Reference C

4. Input error resulting in an incomplete solution matrix 25 Reference D

5. Revised Blowdown Heatup Uncertainty Distribution 5

Reference E

6. Vessel Unheated Conductor Noding 0

Reference F

7. Thermal Conductivity Degradation with Peaking 15 References H, I Factor Burndown Prior evaluation model changes (APCT)

1. Revised Algorithm for Average Fuel Temperature 0

Reference F

2. PAD 3.4 to PAD 4.0

-75 References H, I

3. Peak FQ = 2.7 in bottom third of core 0

References H, I Current Errors (APCT)

1. Revised Heat Transfer Multiplier Distribution

-85 Reference K Current Evaluation model changes (APCT)

1. None Absolute value of errors/changes for this report (APCT) 85 Net change in PCT for this report

-85 Final PCT 2000 SBLOCA Evaluation model: NOTRUMP Analysis of record PCT 1243 4 inch break Reference G Prior errors (APCT)

1. Evaluation of Fuel Pellet Thermal Conductivity 0

Reference J Degradation Prior evaluation model changes (APCT)

1. None 0

Errors (APCT)

1. Evaluation of Fuel Pellet Thermal Conductivity 0

Degradation Evaluation model changes (APCT)

1. None 0

Absolute value of errors/changes for this report (APCT) 0 Net change in PCT for this report 0

Final PCT 1243, Page 10 of 10

ATTACHMENT 2 Additional Information on the Evaluation of Revised Heat Transfer Multiplier Distributions for Plants Licensed with the CQD EM

(

Reference:

Westinghouse Electric Company Letter LTR-LIS-13-406; Additional Information on the Evaluation of Revised Heat Transfer Multiplier Distributions, August 14, 2013.)

1.0 Background on Error Identification and Reporting As a result of code development and maintenance, several errors in the WCOBRAITRAC code used for best estimate large break loss of coolant (BELOCA) analysis in the Code Qualification Document (CQD, Reference [1]) and ASTRUM (Reference [2]) evaluation models (EMs) were identified. Some of the errors affected the WCOBRA/TRAC heat transfer models, the heat transfer node initialization or the heat transfer renoding logic, as well as other models. These changes to WCOBRA/TRAC were described in Reference [3].

As a result of these changes, the following uncertainty distributions used in the CQD and ASTRUM EMs were investigated for potential impact:

Critical flow Downcomer condensation Upper plenum drain distribution (condensation and interfacial drag for upper plenum injection)

Blowdown heatup heat transfer Blowdown cooling heat transfer Refill heat transfer Reflood heat transfer The results for the Separate Effects Tests (SETs) and Integral Effects Tests (lETs) used to determine each of the potentially impacted uncertainty distributions were examined, comparing results between the latest version of WCOBRA/TRAC (Version MOD7A Revision 8, with all of the errors listed in Reference [3] corrected) and WCOBRA/TRAC Version MOD7A Revision 6 (which was used in the licensing of the ASTRUM EM in Reference [2]). It was determined that the results for the SETs and lETs used to develop the critical flow, downcomer condensation, and upper plenum drain uncertainty distributions were sufficiently similar; therefore, those distributions did not require changes. It was also confirmed that emergency core cooling (ECC) bypass predictions remain conservative. However, it was determined that the heat transfer multiplier distributions required additional investigation.

During the investigation into the potential impact on the heat transfer multiplier distributions, errors were identified in the development of the original multiplier distributions, including errors in the grid locations specified in the WCOBRA/TRAC models for the G2 Refill and G2 Reflood SETs, and errors in processing test data used to develop the reflood heat transfer multiplier distribution. These errors were also corrected and, using latest released version of WCOBRAITRAC, the revised blowdown heatup, blowdown cooling, refill and reflood heat transfer multiplier distributions were determined., Page 1 of 6

2.0 Revised Distributions and Expected Effects 2.1 Background on Heat Transfer Multiplier Sampling In order to sample heat transfer multipliers, a percentile for each time period heat transfer multiplier is sampled. That point is then converted to the heat transfer multiplier value based on the cumulative distribution function (CDF) of the time period heat transfer multiplier. Figure 1 illustrates this concept for a change from an old distribution to a new one (note that this CDF does not represent any actual CDF for the heat transfer multipliers, but is used simply for demonstration). For example, if the 2 5 th percentile is sampled, Figure 1 shows that a multiplier of about 0.65 would be obtained for the old distribution. For the new distribution, the sampled 25t" percentile would result in a multiplier of about 1.15.

2.2 Changes to the Heat Transfer Multiplier Distributions The CDFs of the heat transfer multipliers changed as follows:

Blowdown heatup heat transfer multipliers increased for low multipliers and across most of the middle of the sampling range, and were mostly unchanged for the highest multipliers.

Blowdown cooling heat transfer multipliers decreased slightly from the top of the range through the middle, and were mostly unchanged for low multipliers.

Refill heat transfer multipliers decreased considerably at the top end of the range and gradually reduced to a slight decrease at the bottom end of the range. Although the magnitude of the change to the refill multiplier distribution was larger than that observed in the other distributions, the PCT impact is small because heat transfer rates are low during the nearly adiabatic refill time period.

Reflood heat transfer multipliers increased at the bottom end of the range and the middle, and then decreased at the top end of the range.

The implications of these changes are dependent on the behavior of plant transients. For the assessment, plants were classified as follows:

Blowdown limited: A limiting PCT typically within the first 20 seconds of the transient.

Early reflood limited: A limiting PCT after the end of the refill time period, but within about the first 70 seconds of the transient.

Mid reflood limited: A limiting PCT that is between the early and late reflood time periods.

Late reflood limited: A limiting PCT generally after about 200 seconds.

The impact from the change to the heat transfer multiplier CDFs on each of these transient types is discussed in the following subsections.

2.3 Blowdown Limited Blowdown limited plants are only affected by the changes to the blowdown heatup heat transfer multiplier CDF. The increased heat transfer multipliers have a small benefit on PCT since the blowdown heatup time period is short., Page 2 of 6

2.4 Early Reflood Limited Early reflood limited plants are affected by the changes to all of the heat transfer multiplier CDFs. The effects of the changes to the blowdown heatup and blowdown cooling heat transfer multiplier CDFs are limited since much of their effect diminishes through refill and the beginning of reflood. The effects of the changes to the refill heat transfer multiplier CDF are more pronounced since the early reflood PCT occurs shortly after the end of refill. The effects of the changes to the reflood heat transfer multiplier CDF are limited since the run spends very little time in the reflood time period prior to the PCT time.

2.5 Mid Reflood Limited Mid reflood limited plants are affected by the changes to all of the heat transfer multiplier CDFs.

The effects of the changes to the blowdown heatup and blowdown cooling heat transfer multiplier CDFs are very limited since most of their effect diminishes through refill and early reflood. The effects of the changes to the refill heat transfer multiplier CDF are limited since most of their effect diminishes through early reflood. The effects of the changes to the reflood heat transfer multiplier CDF are more pronounced due to the time over which the multiplier is applied prior to the PCT time.

2.6 Late Reflood Limited Late reflood limited plants are predominately affected by the change to the reflood heat transfer multiplier CDF. The effects of the changes to the blowdown heatup, blowdown cooling, and refill heat transfer multiplier CDFs are negligible since their effect diminishes entirely throughout the lengthy reflood period. The effect of the change to the reflood heat transfer multiplier CDF can be significant due to the longer time over which the multiplier is applied prior to the PCT time.

3.0 Methodology for the Estimate of Effect 3.1 Selection and Description of Representative Transients Representative PCT transients were used in determining the estimated PCT effect due to the revised heat transfer multiplier distributions. Heat transfer multipliers are applied in HOTSPOT; the HOTSPOT code performs a one-dimensional conduction calculation modeling the effect of local uncertainties on the hot rod, using thermal hydraulic boundary conditions taken from WCOBRAITRAC. Plant characteristics determine the typical PCT transient behavior for the plant. Transients from different plants with similar PCT behavior tend to have fairly consistent thermal hydraulic characteristics around the hot rod. As a result, the choice of representative plant was based on PCT transient behavior for the evaluation of the revised heat transfer multiplier distributions.

The representative transients discussed above were performed with the latest released version of WCOBRA/TRAC, which incorporated correction of all of the errors listed in Reference [3].

The representative transients were similar to Reference Transient calculations. Fuel performance data which explicitly reflects burnup-dependent effects of thermal conductivity degradation (TCD), calculated as described in Reference 4, was used for the representative calculations., Page 3 of 6

3.2 Background of the CQD EM Section 2.1 gives a high level description of sampling methodology. In the CQD EM, HOTSPOT runs 1000 calculations with randomly sampled local uncertainty attributes and produces 9 5 th percentile results. For the CQD plant evaluations, two representative transients were executed to assess the early/mid reflood plants and late reflood plants; because the CQD EM individually tracks the Reflood 1 and Reflood 2 PCTs, one representative plant was sufficient to represent both early and mid reflood plants.

3.3 Estimates of Effect For each representative transient, the WCOBRA/TRAC calculation described in Section 3.1 was executed. The results from this WCOBRA/TRAC calculation provided boundary conditions for execution of the HOTSPOT code with the old and new heat transfer multiplier distributions. The estimated effect for the Blowdown, Reflood 1, and Reflood 2 time periods for each representative plant calculation was determined from the 95th percentile HOTSPOT results using the old heat transfer multiplier distributions, and the 95th percentile HOTSPOT results using the revised heat transfer multiplier distributions. For these evaluation calculations, the two latest released versions of HOTSPOT were used; the only difference between these HOTSPOT versions that affects the calculated results is the heat transfer multiplier distributions.

3.4 Applicability of the TCD Evaluations It has been previously observed that explicitly considering TCD does not significantly impact the nature of the overall plant transient behavior and thermal-hydraulic response. In addition, the WCOBRAITRAC calculations described in Section 3.1 were performed using fuel performance data which explicitly accounted for the effects of TCD. Therefore, the revised heat transfer multiplier distributions would be expected to have similar effect on the base and sensitivity calculations executed to evaluate the effects of TCD and peaking factor burndown. The revised heat transfer multiplier distributions do not invalidate the prior estimated effects for TCD.

3.5 Applicability of the Uncertainty Calculations HOTSPOT runs are used in several steps of the uncertainty calculations in the CQD methodology; thus, the changes in heat transfer multiplier distributions could have impact on the final Monte Carlo simulations. However, based on Section 28-3-2 of Reference [1], as long as an EM change does not substantially change the nature of the transient, an estimate of effect based on a Reference Transient for a representative plant is sufficient. The heat transfer multiplier changes are only applied in HOTSPOT; therefore, the nature of the transient remains unchanged. Because the representative calculations are meant to represent Reference Transient conditions (altered for the effects of TCD), the method used herein is consistent with the approach described in Reference [1].

4.0 Summary of Effects and Observed Trends As described in Section 3.2, in the CQD EM, each HOTSPOT calculation is comprised of 1000 calculations where the local uncertainty attributes are sampled for each iteration from their respective distributions; the overall 95th percentile PCT results from the 1000 iterations are the result of interest for the heat transfer multiplier evaluations. As such, the results are indicative of generic trends due to the overall changes in the distributions., Page 4 of 6

For the early/mid reflood limited representative transient, the Reflood 1 PCT may be considered representative of early reflood PCT and Reflood 2 PCT may be considered representative of mid-reflood PCT. For the early/mid reflood limited representative transient, the Reflood 1 PCT experienced a small penalty, which is consistent with the expectations from Section 2.4 due to the reduction in refill heat transfer multipliers. The Reflood 2 PCT of this representative transient experienced a moderate benefit, which is consistent with expectations from Section 2.5 due to the increase in reflood heat transfer multipliers over the low end of the range.

For the late reflood limited representative transient, the Reflood 1 PCT may be considered representative of mid-reflood PCT and Reflood 2 PCT may be considered representative of late-reflood PCT. For the late reflood limited representative transient, the Reflood 1 PCT experienced a moderate benefit, which is consistent with expectations from Section 2.5 due to the increase in reflood heat transfer multipliers over the low end of the range. The Reflood 2 PCT of this representative transient experienced a large benefit, which is consistent with expectations from Section 2.6 due to the increase in reflood heat transfer multipliers over the low end of the range and the longer time for which the multiplier is applied.

5.0 References

1. WCAP-12945-P-A, Volume 1, Revision 2, and Volumes 2 through 5, Revision 1, "Code Qualification Document for Best Estimate LOCA Analysis," March 1998.

2. WCAP-16009-P-A, "Realistic Large-Break LOCA Evaluation Methodology Using the Automated Statistical Treatment Of Uncertainty Method (ASTRUM)," January 2005.

3. LTR-LIS-1 3-346, "10 CFR 50.46 Notification and Reporting for WCOBRA/TRAC Changes and Error Corrections," July 2013.

4. LTR-NRC-12-27, "Westinghouse Input Supporting Licensee Response to NRC 10 CFR 50.54(f) Letter Regarding Nuclear Fuel Thermal Conductivity Degradation (Proprietary/Non-Proprietary)," March 2012., Page 5 of 6

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