AEP-NRC-2013-68, Day Report of Changes to or Errors in an Evaluation Model

Day Report of Changes to or Errors in an Evaluation Model

ML13247A174
Person / Time
Site:	Cook
Issue date:	08/30/2013
From:	Gebbie J Indiana Michigan Power Co
To:	Document Control Desk
References
AEP-NRC-2013-68
Download: ML13247A174 (17)

Text

INDIANA Indiana Michigan Power MICHIGAN Cook Nuclear Plant POWER One Cook Place Bridgman, MI 49106 A unit of American Electric Power Indiana Michigan Power,com August 30, 2013 AEP-NRC-2013-68 10 CFR 50.46 Docket No.: 50-315 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Donald C. Cook Nuclear Plant Unit 1 30-DAY REPORT OF CHANGES TO OR ERRORS INAN EVALUATION MODEL Pursuant to 10 CFR 50.46, Indiana Michigan Power Company (I&M), the licensee for Donald C. Cook Nuclear Plant (CNP) Unit 1, is reporting significant changes to or errors in Emergency Core Cooling System evaluation models (EM), or in the application of such models that affect the calculated peak fuel cladding temperature. By report dated July 31, 2013, Westinghouse notified I&M of EM errors which significantly affected the Best-Estimate Large-Break Loss-of-Coolant Accident (LBLOCA) analysis for CNP Unit 1. The impact of these errors is not significant to the CNP Unit 2 LBLOCA Analysis Calculated Peak Cladding Temperature (PCT). The CNP Unit 1 and Unit 2 'Small-Break LOCA analyses are not affected by this error.

The enclosure to this letter provides a description of each LBLOCA EM error correction and the associated impact to the CNP Unit 1 LBLOCA analysis of record. Based on information provided by Westinghouse, an assessment of these errors resulted in a PCT decrease of 55 0F for Unit 1.

Previously I&M had provided a schedule for a reanalysis resulting from an unrelated error associated with thermal conductivity degradation. Based on the previously provided schedule for reanalysis and since the changes from these errors resulted in a decrease in PCT, there are no plans for a reanalysis as a result of these errors. This condition has been entered into CNP's corrective action program.

There are no new or revised commitments in this letter. Should you have any questions, please contact Mr. Michael K. Scarpello, Regulatory Affairs Manager, at (269) 466-2649.

Sincerely, Joel P. Gebbie Site Vice President HLE/kmh OOZ

U. S. Nuclear Regulatory Commission AEP-NRC-2013-68 Page 2

Enclosure:

Donald C. Cook Nuclear Plant Unit 1 Report of Error Corrections on Westinghouse Large-Break Loss-of-Coolant Analysis (LBLOCA) Emergency Core Cooling System (ECCS) Evaluation Model (EM) c: J. T. King, MPSC S. M. Krawec, AEP Ft. Wayne, w/o enclosures MDEQ - RMD/RPS NRC Resident Inspector C. D. Pederson, NRC Region III T.J. Wengert, NRC Washington, DC

ENCLOSURE TO AEP-NRC-2013-68 DONALD C. COOK NUCLEAR PLANT UNIT 1 REPORT OF ERROR CORRECTIONS ON WESTINGHOUSE LARGE-BREAK LOSS-OF-COOLANT ANALYSIS (LBLOCA)

EMERGENCY CORE COOLING SYSTEM (ECCS) EVALUATION MODEL (EM)

Abbreviations:

OF degrees Fahrenheit FAH nuclear enthalpy rise hot channel factor FQ heat flux hot channel factor LOCA loss of coolant accident MWt megawatts - thermal PCT peak cladding temperature SGTP steam generator tube plugging Summary:

By Westinghouse letter LTR-LIS-13-360, "D. C. Cook Units 1 and 2 10 CFR 50.46 Report for Revised Heat Transfer Multiplier Distributions," dated July 31, 2013, Westinghouse Electric Company notified Indiana Michigan Power (I&M), the licensee for Donald C. Cook Nuclear Plant (CNP) Unit 1, of significant errors in the evaluation model (EM) for the Large-Break (LB) LOCA analysis of record for CNP Unit 1. By Westinghouse letter LTR-LIS-13-406, "Additional Information on the Evaluation of Revised Heat Transfer Multiplier Distributions," dated August 14, 2013, Westinghouse Electric Company provided I&M additional detail on the nature of the errors and the corrections made. This report contains three parts. The first part discusses the nature of the change or error. The second part discusses error corrections incorporated into the latest release code version that was used to estimate impact to calculated PCT. Although these corrections had no impact on PCT (Estimated Effect = 0°F) they are included in this report for completeness. The third part is the summary of changes and errors and their estimated effect on the calculated PCT of CNP Unit 1 LBLOCA analysis of record which also includes a PCT impact associated with the Unit 1 Cycle 25 operating cycle that began in May 2013.

Nature Of The Change Or Error:

An excerpt of the information provided by Westinghouse letter LTR-LIS-13-406, "Additional Information on the Evaluation of Revised Heat Transfer Multiplier Distributions," dated August 14, 2013, which is relevant to CNP Unit 1 LBLOCA analysis of record is provided below.

This excerpt describes the nature of the errors and the corrections made. For the purposes of this report, CNP Unit 1 is a "Late Reflood Limited Plant." The references contained in this excerpt are included as the References section at the end of the excerpt text. Following the text there is also a Figure 1 that is included as a part of the excerpt. The excerpt is as follows:

Enclosure to AEP-NRC-2013-68 Page 2 "1.0 Background on Error Identification and Reporting "As a result of code development and maintenance, several errors in the WCOBRA/TRAC 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 WCOBRA/TRAC, the revised blowdown heatup, blowdown cooling, refill and reflood heat transfer multiplier distributions were determined.

Enclosure to AEP-NRC-2013-68 Page 3 "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 25th 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 25th 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 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.

• 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 impacts from the changes to the heat transfer multiplier CDFs on each of these transient types are discussed in the following subsections.

Enclosure to AEP-NRC-2013-68 Page 4 "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.

"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

Enclosure to AEP-NRC-2013-68 Page 5 behavior for the evaluation of the revised heat transfer multiplier distributions. The representative transients chosen were early reflood limited, mid 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 ASTRUM EM "For each calculation in the ASTRUM 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 same, 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 2.

"3.3 Estimates of Effect "Three representative plants were identified and a representative WCOBRA/TRAC calculation was performed for each representative plant (as described in Section 3.1).

These WCOBRAITRAC calculations provided the boundary condition input for a matrix 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 matrix 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 limiting 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

Enclosure to AEP-NRC-2013-68 Page 6 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 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 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 Summary of Effects and Observed Trends "For plants licensed with the ASTRUM EM, 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, limiting 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 penalties tended to be slightly higher than the benefits. As a result, small penalties were generally observed 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 ASTRUM EM, limiting runs tend to sample low reflood heat transfer multipliers. In addition, the impact of the reflood heat transfer multiplier 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 runs 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.

Enclosure to AEP-NRC-2013-68 Page 7 "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-13-346, "10 CFR 50.46 Notification and Reporting for WCOBRAFIRAC 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.

1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0r 0.55 O.SO E -Old d3 0.45

- - . New 0.40 0.35 0.30 0.25 0.20.

0.15 0.10 0.05 0.00 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 Heat Transfer Multiplier 1-1 "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)"

Enclosure to AEP-NRC-2013-68 Page 8 Error Corrections Incorporated into the Latest Release Code Version:

The text below is an excerpt from Westinghouse letter LTR-LIS-13-346, "10 CFR 50.46 Notification and Reporting for WCOBRA/TRAC Changes and Error Corrections," dated July 30, 2013. It describes the correction of errors that were incorporated into the latest released code version (Version MOD7A, Revision 8) that was used to determine the impact to LBLOCA calculated PCT in Westinghouse letter LTR-LIS-13-360, "D. C. Cook Units 1 and 2 10 CFR 50.46 Report for Revised Heat Transfer Multiplier Distributions," dated July 31, 2013.

The excerpt is as follows:

"ELEVATIONS FOR HEAT SLAB TEMPERATURE INITIALIZATION "Background "An error was discovered in WCOBRA/ITRAC 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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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 "Background "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

Enclosure to AEP-NRC-2013-68 Page 9 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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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.

"CORRECTION TO HEAT TRANSFER NODE INITIALIZATION "Background "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.

"Affected Evaluation Models

"1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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 "Background "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

Enclosure to AEP-NRC-2013-68 Page 10 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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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 "Background "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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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.

Enclosure to AEP-NRC-2013-68 Page 11 "HEAT TRANSFER LOGIC CORRECTION FOR ROD BURST CALCULATION "Background "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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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 "Background "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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "The updates to the calculations of the superheated steam properties had generally less than 1IF impact on the resulting steam temperature values, leading to an estimated peak cladding temperature impact of 0°F.

Enclosure to AEP-NRC-2013-68 Page 12 "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 "Background "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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect 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.

"DECAY HEAT MODEL ERROR CORRECTIONS "Background "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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM

Enclosure to AEP-NRC-2013-68 Page 13 "Estimated Effect "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 "Background "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.

"Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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 "Background "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 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.

Enclosure to AEP-NRC-2013-68 Page 14 "Affected Evaluation Models "1996 Westinghouse Best Estimate Large Break LOCA Evaluation Model 1999 Westinghouse Best Estimate Large Break LOCA Evaluation Model, Application to PWRs with Upper Plenum Injection 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM "Estimated Effect "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."

Enclosure to AEP-NRC-2013-68 Page 15 Estimated Effect On The Calculated PCT For CNP Unit 1 Larae Break LOCA:

Evaluation Model: ASTRUM (2004)

FQ= 2.15 FAH = 1.55 SGTP = 10%(a) Break Size: Split Analysis Date: November 20, 2007 LICENSING BASIS Analysis-of-Record PCT = 2128°F MARGIN ALLOCATIONS (Delta PCT)

A. PREVIOUS 10 CFR 50.46 ASSESSMENTS 3840F(a.)

B. PLANNED PLANT MODIFICATION EVALUATIONS -381 oF(a.)

1. PBOT/PMID Evaluation for Cycle 25 140F(b.)

C. NEW 10 CFR 50.46 ASSESSMENTS -55 0 F D. OTHER 0OF LICENSING BASIS PCT + MARGIN ALLOCATIONS PCT = 2090'F Notes:

a. These assessments are coupled via an evaluation of burnup effects which include thermal conductivity degradation, peaking factor burndown and design input changes (e.g., reduction in the maximum allowed steam generator tube plugging from 10% to 2%

and maximum FdH reduced to 1.545). Evaluation details provided in a letter dated March 19, 2012, (ADAMS Accession No. ML12088A104), and supplemented by letter dated June 11, 2012, (ADAMS Accession No. ML12173A025), and subsequently found acceptable by NRC letter dated March 7, 2013 (ADAMS Accession No. ML13077A137).

b. This PCT impact is only applicable to the Unit 1 Cycle 25 operating cycle, which began in May 2013 and is scheduled to end in October 2014.