ML13260A251

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Day Report for Emergency Core Cooling System Model Changes Pursuant to the Requirements of 10 CFR 50.46
ML13260A251
Person / Time
Site: Millstone Dominion icon.png
Issue date: 09/09/2013
From: Grecheck E
Dominion, Dominion Nuclear Connecticut
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
13-501
Download: ML13260A251 (21)
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Dominion Nuclear Connecticut, Inc.

5000 Dominion Boulevard, Glen Allen, VA 23060 Dominion Web Address: www.dorn.corn September 9, 2013 U.S. Nuclear Regulatory Commission Serial No.13-501 Attention: Document Control Desk NL&OS/MAE R1 Washington, DC 20555 Docket No. 50-423 License No. NPF-49 DOMINION NUCLEAR CONNECTICUT. INC.

MILLSTONE POWER STATION UNIT 3 30-DAY REPORT FOR EMERGENCY CORE COOLING SYSTEM MODEL CHANGES PURSUANT TO THE REQUIREMENTS OF 10 CFR 50.46 In accordance with 10 CFR 50.46(a)(3)(ii), Dominion Nuclear Connecticut, Inc. (DNC) hereby submits information regarding changes and errors in Westinghouse's Large Break Loss of Coolant Accident (LBLOCA) Emergency Core Cooling System (ECCS) Evaluation Model that impacts the calculated peak clad temperature (PCT) for Millstone Power Station Unit 3 (MPS3). provides a report describing the changes and errors associated with the Westinghouse LBLOCA ECCS evaluation model for MPS3. provides information regarding the effect of the changes to the reported LBLOCA PCT for MPS3. To summarize the information in Attachment 2, the calculated PCT for the LBLOCA analyses is changed by an absolute value of 91'F to a new PCT value of 1912°F for MPS3. This result represents a significant change in PCT, as defined in 10 CFR 50.46(a)(3)(i).

DNC requested clarifying information from Westinghouse in support of the evaluation of the reported errors. Westinghouse responded to DNC's request in a letter dated August 14, 2013 (LTR-LIS-13-406). Attachment 3 contains the part of the Westinghouse letter applicable to MPS3 (i.e., Attachment 2 of the letter) that provides information on the single, non-zero PCT error for the MPS3 LBLOCA Evaluation Model.

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. DNC has reviewed the information provided by Westinghouse 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, DNC considers the schedular requirements of 10 CFR 50.46(a)(3)(ii) to be satisfied with the submission of this notification. DNC 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.

In a letter dated November 29, 2012 (Serial No.12-705), DNC committed to a reanalysis of the LBLOCA prior to November 30, 2017 for issues concerning fuel pellet thermal conductivity degradation. The LBLOCA Evaluation Model PCT assessments identified herein do not adversely affect the reanalysis schedule of November 30, 2017. The PCT has

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Serial No.13-501 Docket No. 50-423 Page 2 of 3 decreased by 91 OF to a licensing basis PCT of 1912 OF. In light of the new issues herein, the reanalysis schedule of November 30, 2017 remains acceptable. The reanalysis will address the known PCT changes and errors at the time of the analysis.

This information satisfies the 30-day reporting requirements of 10 CFR 50.46(a)(3)(ii).

If you have any further questions regarding this submittal, please contact Wanda Craft at (804) 273-4687.

Sincerely, Eugene S. Grecheck Vice President - Nuclear Engineering and Development Commitments made in this letter: None Attachments:

1) Report of Changes and Errors in Westinghouse ASTRUM Large Break LOCA ECCS Evaluation Model
2) 30-Day Reporting of 10 CFR 50.46 Margin Utilization - Millstone Power Station Unit 3
3) Additional Information on Westinghouse ASTRUM Large Break LOCA Revised Heat Transfer Multiplier Distributions

Serial No.13-501 Docket No. 50-423 Page 3 of 3 cc: U.S. Nuclear Regulatory Commission Region 1 2100 Renaissance Blvd Suite 100 King of Prussia, PA 19406-2713 J. S. Kim Project Manager - Millstone Power Station U.S. Nuclear Regulatory Commission One White flint North 11555 Rockville Pike Mail Stop 08-C2 Rockville, MD 20852-2738 NRC Senior Resident Inspector Millstone Power Station

Serial No.13-501 Docket No. 50-423 ATTACHMENT I REPORT OF CHANGES AND ERRORS IN WESTINGHOUSE ASTRUM LARGE BREAK LOCA ECCS EVALUATION MODEL MILLSTONE POWER STATION UNIT 3 DOMINION NUCLEAR CONNECTICUT, INC.

Serial No.13-501 Docket No. 50-423 Attachment 1, Page 1 of 6 Report of Changes in Westinghouse ASTRUM Large Break LOCA ECCS Evaluation Model Millstone Power Station Unit 3 Since the last 30-day report under 10 CFR 50.46, Westinghouse has informed Dominion Nuclear Connecticut, Inc. (DNC) of thirteen changes to the licensing basis peak cladding temperature (PCT) for the Westinghouse Automated Statistical Treatment of Uncertainty Method (ASTRUM) Best-Estimate (BE) Large Break Loss of Coolant Accident (LBLOCA) Evaluation Model (EM) for Millstone Power Station Unit 3 (MPS3). These changes are described below. Pursuant to requirements of 10CFR50.46, DNC has reviewed these changes and found that they represent a significant change in PCT, as defined in 10 CFR 50.46(a)(3)(i).

INITIAL FUEL PELLET AVERAGE TEMPERATURE UNCERTAINTY CALCULATION In the ASTRUM BE LBLOCA 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 in Section 25-4-2-4 of WCAP-12945-P-A, "Code Qualification Document for Best-Estimate LOCA Analysis." However, the initial fuel pellet average temperature uncertainty range analyzed at higher burnups 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 represents a Non-Discretionary Change in accordance with Section 4.1.2 of WCAP-13451, "Westinghouse Methodology for Implementation of 10 CFR 50.46 Reporting."

The issue described above is judged to have either no effect or a negligible effect on existing MPS3 LBLOCA analysis results, leading to an estimated PCT impact of 0°F.

Additionally, Westinghouse has informed DNC that as a result of code development and maintenance, several errors in the WCOBRA/TRAC code used for best estimate large break loss of coolant analysis in the ASTRUM EM 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. There were eleven identified issues, each with an estimated 0°F PCT impact. These changes to WCOBRA/TRAC are described in the eleven discussions below.

ELEVATIONS FOR HEAT SLAB TEMPERATURE INITIALIZATION An error was discovered in WCOBRA/TRAC 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 the bounds of the temperature initialization table. This

Serial No.13-501 Docket No. 50-423 Attachment 1, Page 2 of 6 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-13451.

Based on inspection of plant analysis input, it was concluded that the input decks for the existing MPS3 analysis are not impacted by this error, leading to an estimated PCT impact of 0°F.

HEAT TRANSFER MODEL ERROR CORRECTIONS Several related changes were made to WCOBRAITRAC to correct errors 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, including separate effects tests (SET) and integral effects test (lET) simulations, it is concluded that the error corrections have a negligible local effect on heat transfer, leading to an estimated PCT impact of 00 F for MPS3.

CORRECTION TO HEAT TRANSFER NODE INITIALIZATION 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 PCT impact of 0°F for MPS3.

MASS CONSERVATION ERROR FIX It was identified that mass was not conserved in WCOBRAITRAC 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

Serial No.13-501 Docket No. 50-423 Attachment 1, Page 3 of 6 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 the MPS3 LBLOCA analysis results, leading to an estimated PCT impact of 0°F.

CORRECTION TO SPLIT CHANNEL MOMENTUM EQUATION 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-1 3451.

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 PCT impact of 0°F for MPS3.

HEAT TRANSFER LOGIC CORRECTION FOR ROD BURST CALCULATION A change was made to the WCOBRA/TRAC 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 EM requirements for plant modeling in Westinghouse BE LBLOCA analyses with WCOBRA/TRAC, it is judged that the existing MPS3 analyses are not impacted by this change, leading to an estimated PCT impact of 0°F.

Serial No.13-501 Docket No. 50-423 Attachment 1, Page 4 of 6 CHANGES TO VESSEL SUPERHEATED STEAM PROPERTIES 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 Equation 10-6 of the Code Qualification Document (CQD) topical report WCAP-12945-P-A, 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-13451.

The updates to the calculations of the superheated steam properties had less than 1OF impact on the resulting steam temperature values, leading to an estimated PCT impact of 0°F for MPS3.

UPDATE TO METAL DENSITY REFERENCE TEMPERATURES 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-13451.

This change primarily impacts the reactor coolant system loop piping modeled in the LBLOCA WCOBRA/TRAC models. It was judged that the effect of this change on the PCT results was negligible, leading to an estimated PCT impact of 0°F for MPS3.

DECAY HEAT MODEL ERROR CORRECTIONS 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-13451.

These changes have a negligible impact on the calculated decay heat power, leading to an estimated PCT impact of 0°F for MPS3.

Serial No.13-501 Docket No. 50-423 Attachment 1, Page 5 of 6 CORRECTION TO THE PIPE EXIT PRESSURE DROP ERROR An error was discovered in WCOBRA/TRAC 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 PCT of 0°F for MPS3.

WCOBRAITRAC U19 FILE DIMENSION ERROR CORRECTION 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 WCOBRA/TRAC. 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 u19 file. A survey of current BE LBLOCA 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 PCT calculation for prior analyses. This represents a Discretionary Change in accordance with Section 4.1.1 of WCAP-13451.

As discussed above, resolution of this issue does not impact the PCT calculation for prior LBLOCA analyses, leading to an estimated PCT impact of 0°F for MPS3.

One of the related WCOBRA/TRAC errors had a non-zero PCT impact. Westinghouse informed DNC of that error under separate cover and provided further technical information to support DNC evaluation of the change. The additional information applicable to the ASTRUM methodology was provided by Westinghouse and is included as Attachment 3 of this letter.

Serial No.13-501 Docket No. 50-423 Attachment 1, Page 6 of 6 REVISED HEAT TRANSFER MULTIPLIER DISTRIBUTIONS Some of the changes and error corrections described above affected the WCOBRA/TRAC heat transfer models, the heat transfer node initialization, or the heat transfer renoding logic. This lead to an investigation of the heat transfer multiplier distributions using the results for the SET and lETs. 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.

The blowdown, heatup, blowdown cooling, refill, and reflood heat transfer multiplier distributions were redeveloped. 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 MPS3 transient behavior was performed with the latest version of WCOBRA/TRAC. Using this transient, a matrix of HOTSPOT calculations was performed to estimate the effect of the heat transfer multiplier distribution changes. The limiting runs for the MPS3 analysis were identified, including consideration of the fuel pellet thermal conductivity degradation effects and other evaluations on the analysis of record, which substantially impacted the ranking or PCTs of the limiting cases. The set of limiting runs for MPS3 were selected such that less limiting runs which were not explicitly considered would not become limiting due to the estimated PCT impact from the change in heat transfer multipliers. The heat transfer multipliers for each run were used to categorize the multipliers, and an estimated PCT impact for that individual multiplier was assigned. The individual estimated PCT impacts for the run (based on the four multipliers) were summed to estimate the overall impact on the run. Finally, the run results were re-ranked based on the estimated impacts on each run. The change between the estimated 95/95 PCT before and after this process was reported as the estimate of effect for the MPS3 analysis.

Using these results and considering the heat transfer multiplier uncertainty attributes from limiting cases for MPS3, an estimated PCT effect of -91OF has been established for 10 CFR 50.46 reporting purposes for MPS3.

Serial Number 13-501 Docket No. 50-423 ATTACHMENT 2 30-DAY REPORTING OF 10 CFR 50.46 MARGIN UTILIZATION MILLSTONE POWER STATION UNIT 3 DOMINION NUCLEAR CONNECTICUT, INC.

Serial Number 13-501 Docket No. 50-423 Attachment 2, Page 1 of 2 10 CFR 50.46 MARGIN UTILIZATION - LARGE BREAK LOCA Plant Name: Millstone Power Station, Unit 3 Utility Name: Dominion Nuclear Connecticut, Inc.

Analysis Information EM: ASTRUM (2004)

Limiting Break Size: Guillotine Analysis Date: 04/17/07 Vendor: Westinghouse FQ: 2.6 FdH: 1.65 Fuel: RFA-2 SGTP(%): 10 Notes: None Clad Temp (*F)

LICENSING BASIS Analysis of Record PCT 1781 PCT ASSESSMENTS (Delta PCT)

A. Prior ECCS Model Assessments

1. HOTSPOT burst Temperature Logic Errors 0
2. CCFL Global Volume Error 0
3. HOTSPOT Gap Heat Transfer Logic 0
4. Discrepancy in Metal Masses Used From Drawings 0
5. Error in ASTRUM Processing of Average Rod Burnup 0 and Rod Internal Pressure
6. Treatment of Vessel Average Temperature Uncertainty 0
7. Error in ASTRUM Processing of Average Rod Burnup 0
8. PBOT and PMID Evaluation 0
9. Evaluation of Fuel Pellet Thermal Conductivity 222 Degradation
10. HOTSPOT Burst Temperature Calculation 0 For ZIRLO Cladding
11. Rod Internal Pressure Calculation 0
12. HOTSPOT Iteration Algorithm for Calculating the 0 Initial fuel Pellet Average Temperature
13. WCOBRA/ITRAC Thermal-Hydraulic History File 0 Dimension used in HSDRIVER Background
14. WCOBRA/ITRAC Automated Restart Process Logic Error 0 B. Planned Plant Modification Evaluations
1. None 0

Serial Number 13-501 Docket No. 50-423 Attachment 2, Page 2 of 2 Clad Temp (OF)

C. 2013 ECCS Model Assessments

1. Initial Fuel Pellet Average Temperature Uncertainty 0 Calculation
2. Elevations for Heat Slab Temperature Initialization 0
3. Heat Transfer Model Error Corrections 0
4. Correction to Heat Transfer Node Initialization 0
5. Mass Conservation Error Fix 0
6. Correction to Split Channel Momentum Equation 0
7. Heat Transfer Logic Correction for Rod Burst Calculation 0
8. Changes to Vessel Superheated Steam Properties 0
9. Update to Metal Density Reference Temperatures 0
10. Decay Heat Model Error Corrections 0
11. Correction to the Pipe Exit Pressure Drop Error 0
12. WCOBRA/TRAC U19 File Dimension Error Correction 0
13. Revised Heat Transfer Multiplier Distribution -91 D. Other
1. None 0 LICENSING BASIS PCT + PCT ASSESSMENTS PCT 1912°F

Serial Number 13-501 Docket No. 50-423 ATTACHMENT 3 ADDITIONAL INFORMATION ON WESTINGHOUSE ASTRUM LARGE BREAK LOCA REVISED HEAT TRANSFER MULTIPLIER DISTRIBUTIONS MILLSTONE POWER STATION UNIT 3 DOMINION NUCLEAR CONNECTICUT, INC.

Serial Number 13-501 Docket No. 50-423 Attachment 3, Page 1 of 7 Westinghouse Non-Proprietar' Class 3 of LTR-LIS-13-406 Aum*ust 14, 2013 Page 1 of 7 Attachlnent 2:

Additional Infornrntion on the Evaluation of Revised Heat Transfer Multiplier Distributions for Plants Licensed with the ASTRUM EM (7 pages, including cover page)

©2013 We-;nnghoiure Electric Conmpm LLC AlliRihtrserved

Serial Number 13-501 Docket No. 50-423 Attachment 3, Page 2 of 7 of LTR-LIS- 13-406 August 14,2013 Page 2 of 7 1.0 Background on Error Identification and Reporting As a result of code development and maintenance, several errors in the WCOBRA,1TRAC 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 WCOBRAiTRAC 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 v 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 Tie results for the Separate Effects Tests (SETs) and Integral Effects Tests (JETs) used to determine each of the potentially impacted uncertainty distributions were examined, comparing results bet-ween the latest version of WCOBLATR.AC (Version MOD7A Revision 8. with all of the errors listed in Reference [3]

corrected) and WCOBRA;TR-C 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 IETs 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 WCOBRATRAC 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 WCOBRA,TRAC, the revised blowdown heatup, blowdown cooling, refill and reflood heat transfer multiplier distributions were determined.

2.0 Revised Disntibutions and Expected Effects 2.1 Backg-ound 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 fof the heat transfer multipliers, but is used simply for denionstration). For example, if the 25": 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.

Serial Number 13-501 Docket No. 50-423 Attachment 3, Page 3 of 7 of LIR-LIS-13-406 August 14,2013 Page 3 of 7 2.2 Changes to the Heat Transfer Multiplier Distibutions The CDFs of the heat transfer multipliers changed as follows:

  • Blowdown heatup heat transfer multipliers increa&sed for low multipliers and across most of die 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 strongly dependent on the behavior of individual transients. For the assessment, plants were classified as follows:

  • Blowdown limited: A limiting PCT typically within the first 20 seconds of the transient.
  • Earlyv 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 impacts from the changes to the heat transfer multiplier CDFs on each of these transient types are 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 PCI since the blowdown heanup time period is short.

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 PCI time.

2.5 M[id 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

Serial Number 13-501 Docket No. 50-423 Attachment 3, Page 4 of 7 of LTR-LIS-13-406 August 14, 2013 Page 4 of 7 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 tie change to the reflood heat transfer nmltiplier 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 M[ethodolog" for the Estimate of Effect 3.1 Selection and Desciiption 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 themial hydraulic boundary conditions taken from WCOBR.A'TRAC. Plant characteristics determine the typical PCT transient behavior fbr 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 tranisfer multiplier distributions. The representative transients chosen were early reflood limited, mrid reflood limited, or late reflood limited. The blowdown PCT impact was taken from the most conservative results of the representative transients.

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.

3.2 Background of the ASTRUMN EM For each calculation in the ASTRIJI uncertainty analysis, the blowdown cooling. blowdown heatup, refill, and reflood heat transfer multipliers are independently sampled using the methodology discussed in Section 2.1. With the new CDFs, then, for a given analysis with an associated seed, the randomly sampled percentile for each heat transfer multiplier in each run is the sanme. but it is translated to a different multiplier based on the new distributions.

The revised heat transfer multiplier CDFs changed in different ways, such that PCT penalties or benefits would be expected, depending on the nature of the PCT transient and where the multipliers were sampled for a given run. as described in Section'-).

Serial Number 13-501 Docket No. 50-423 Attachment 3, Page 5 of 7 of LTR-LIS-13-406 August 14. 2013 Page 5 of 7 3.3 Estimates of Effect Three representative plants were identified and a representative WCOBR.!TRA.C calculation was performed for each representative plant (as described in Section 3.1). These __COBRAI/TR.AC calculations provided the boundary condition input for a natrLx of representative HOTSPOT calculations.

The matrix of HOTSPOT calculations was developed by dividing each heat transfer multiplier distribution into a discrete number of bins. The heat transfer multipliers representative of that bin for the old distribution and the new distribution were identified. Then a pair of HOTSPOT calculations was performed. where the only difference between the two was that heat transfer multiplier. For example, in the reflood multiplier representing the 30-50% bin, the value of the old multiplier might be 0.8 while the new multiplier is 0.9. Those reflood multipliers are specified in the two different HOTSPOT calculations while the other heat transfer multipliers are set at nominal values. This process was performed for each heat transfer multiplier distribution (blowdown cooling, blowdown heatup. refill, reflood).

This matrLx of HOTSPOT calculations produced a table of PCT deltas that were estimates of an individual change for a bin of each multiplier distribution. The limiting runs for each plant analysis were identified, including consideration of the TCD effects and other evaluations on the analysis of record (AOR) which substantially impacted the ranking or PCTs of the linmiting cases- The set of limiting runs for each plant were selected such that less limiting runs which were not explicitly considered would not become limiting due to the estimated PCT impact from the change in heat transfer multipliers. The heat transfer multipliers for each run were used to identify which bin that multiplier falls into, and an estimated PCT impact for that individual multiplier was assigned. The individual estimLated PCT impacts for the run (based on the four multipliers) were summed to estimate the overall impact on the run. Finally. the run results were re-ranked based on the estimated impacts on each run. The change between the estimated 95?95 PCT before and after this process was reported as the estimate of effect for each plant analysis.

It is noted that for some analyses, the limiting runs were a mixture of different transient behaviors (some limiting runs were early reflood, some mid. etc.). In these cases, results from the appropriate representative transient were used on a case-by-case basis.

4.0 Summaa" of Effects and Observed Trends For plants licensed with the ASTRUM E.M. it is noted that these discussions give observable trends and expected behavior, but the ultimate estimates of effect for some plants did deviate. For example. a limiting case that had a high sampled reflood heat transfer multiplier for a late reflood transient would have a penalty, whereas a more typical limiting case with a low sampled multiplier would have a benefit.

In addition, the limiting runs in some analyses were a mixture of different transient behaviors. In those cases. these descriptions will generally apply on a run-by-run basis, but the overall PCT estimate of effect may vary.

For blowdown limited plants licensed with the ASTRUM EM. tlmitmig runs typically do not have high blowdown heatup heat transfer multipliers. therefore., the blowdown limited plants received benefits from the change to the heat transfer multiplier CDFs.

For early reflood limited plants licensed with the ASTRUM EM. the heat transfer multipliers sampled in the various time periods of the limiting runs can vary. In addition, the impacts for each heat transfer multiplier CDF are of similar magnitude due to similar time spent in each time period, though the

Serial Number 13-501 Docket No. 50-423 Attachment 3, Page 6 of 7 of LTR-LIS-13-406 August 14. 2013 Page 6 of 7 penalties tended to be slightly hi*her thw. the benefits. As a result, small penalties were generally obsen*'ed for early reflood limited plants for the change to the heat transfer multiplier CDFs. All estimates of effect were small.

For mid reflood limited plants licensed with the ASTRUXM EM. limiting runs tend to sample low reflood heat transfer multipliers. In addition. the impact of the reflood heat transfer nmltiplier CDF on mid-reflood limited plants was higher than the impact of the other heat transfer multiplier CDFs. As a result, mid reflood plants tended to receive small to moderate benefits from the change to the heat transfer multiplier CDFs.

For late reflood limited plants licensed with the ASTRUM EM, limiting rumn tend to sample low reflood heat transfer multipliers. As a result- late reflood plants tended to receive large benefits from the change to the heat transfer multiplier CDFs.

5.0 References I. 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)." Januar., 2005.
3. LTR-LIS-13-346. "10 CFR 50.46 Notification and Reporting for WCOBRA/TRAC Changes and Error Corrections,"' July 2013.
4. LTR-NRC-12-217. "Westinghouse Input Supporting Licensee Response to NRC 10 CFR 50.54(f)

Letter Regarding Nuclear Fuel Thermal Conductivity Degradation (Proprietary,;Non-Proprietatry),"

March 2012.

Serial Number 13-501 Docket No. 50-423 Attachment 3, Page 7 of 7 of LTR-LIS-13-406 August 14, 2013 Plge 7 of' 7 01)30.

0.85 S

5 U.IsJ

- Nnw 11.1.a 0301, Oi100~ .! 1.00 *..i!, 01 1)0, 2.W0 2.J5 Z'5)

HC*t Trr c,s MulliplieoI.

Figure 1: Example Heat Transfer Multiplier Cumulative Distribution Function (Note that this CDF does not represent any actual CDF for the heat transfer multipliers, but is used simply for illustrative purposes)

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