RS-12-037, Response to Request for Information Regarding Thermal Conductivity Degradation and 10 CFR 50.46 Report
| ML12079A112 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood  |
| Issue date: | 03/19/2012 |
| From: | Simpson P Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RS-12-037 | |
| Download: ML12079A112 (14) | |
Text
Exelon Generation Company, LLC www.exeloncorp,com 4300 Winfield Road Warrenville, IL 60555 10 CFR 50.54(f) 10 CFR 50.46 RS-12-037 March 19, 2012 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk 11555 Rockville Pike Rockville, MD 20852 Braidwood Station, Unit 2 Facility Operating License No. NPF-77 NRC Docket No. STN 50-457 Byron Station, Unit 2 Facility Operating License No. NPF-66 NRC Docket No. STN 50-455
Subject:
Response to Request for Information Regarding Thermal Conductivity Degradation and 10 CFR 50.46 Report
Reference:
Letter from Michele G. Evans (U.S. NRC) to Michael J. Pacilio (Exelon Nuclear), "Information Request Pursuant to 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 (TAC No. M99899)," dated February 16, 2012 In the Reference letter, the NRC issued an information request for Exelon Generation Company, LLC, (EGC) associated with the effect of thermal conductivity degradation on peak cladding temperature in the Westinghouse Electric Company furnished realistic emergency core cooling system evaluation models for Braidwood Station, Unit 2 and Byron Station, Unit 2. The NRC requested the information be provided within 30 days of the date of the Reference letter. In accordance with 10 CFR 50.4, "Written communications," paragraph (a), the response is due by March 19, 2012. to this letter contains the requested information.
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March 19, 2012 U.S. Nuclear Regulatory Commission Page 2 In accordance with 10 CFR 50.46, "Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors," paragraph (a)(3)(ii), EGC is submitting of this letter to fulfill the 30 day reporting requirement for Braidwood Station, Unit 2 and Byron Station, Unit 2.
Regulatory commitments are contained in Attachment 3 of this letter.
If you have any questions about this letter, please contact me at (630) 657-2823.
I declare under penalty of perjury that the foregoing is true and correct. Executed on the 19th day of March 2012.
Res,,ectfuIly, Patrick R. Simpson Manager - Licensing Exelon Generation Company, LLC Attachments:
- 1. Response to NRC Request for Information 2.
10 CFR 50.46 30 Day Report for Braidwood Station Unit 2 and Byron Station Unit 2 3.
Regulatory Commitments cc: Director of NRR, U.S. NRC Regional Administrator, U.S. NRC Region III U.S. NRC Senior Resident Inspector, Braidwood Station U.S. NRC Senior Resident Inspector, Byron Station
ATTACHMENT 1 Response to NRC Request for Information The NRC in Reference 1 requested that Exelon Generation Company, LLC, (EGC) respond to the following specific issues associated with the effect of thermal conductivity degradation (TCD) on peak cladding temperature (PCT) in the Westinghouse Electric Company furnished realistic emergency core cooling system evaluation models for Braidwood Station, Unit 2 and Byron Station, Unit 2:
1)
An estimation of the effect of the thermal conductivity degradation error on the peak fuel cladding temperature calculation for the emergency core cooling system evaluations at Braidwood Station, Unit No. 2 and Byron Station, Unit No. 2.
2)
A description of the methodology and assumptions used to determine the estimates. This description shall include consideration of experimental data relevant to thermal conductivity degradation and specific information regarding any computer code model changes which were necessary to address these data.
Response
Reference 2 contains the latest 10 CFR 50.46 annual report submitted to the NRC for Braidwood and Byron Stations and provided the most recent results of the large break loss of coolant accident (LBLOCA) Analysis of Record (AOR). The PCT reported for the LBLOCA was 2041 OF for Braidwood Station Unit 2 and Byron Station Unit 2.
The 2004 Westinghouse Realistic Large Break LOCA Evaluation Model Using ASTRUM (i.e., Reference 3) is based on the PAD 4.0 fuel performance code (i.e., Reference 4).
PAD 4.0 was approved without explicitly considering fuel pellet TCD with burnup.
Explicit modeling of fuel pellet TCD in the fuel performance code leads to changes in the fuel rod design parameters beyond beginning of life which are input to the LBLOCA analysis. The effects of explicitly modeling fuel pellet TCD on the Braidwood Station Unit 2 and Byron Station Unit 2 LBLOCA analysis have been considered as described in Reference 5.
Fuel performance data that accounts for fuel pellet TCD (using an unlicensed model) was used as input to the Braidwood Station Unit 2 and Byron Station Unit 2 evaluation.
The new PAD fuel performance data was generated with a representative model that includes explicit modeling of fuel pellet TCD. Therefore, the evaluations performed consider the fuel pellet TCD effects cited in Reference 6.
Evaluation of Fuel Pellet TCD and Peaking Factor Burndown Fuel pellet TCD and peaking factor burndown were not explicitly considered in the Braidwood Station Unit 2 and Byron Station Unit 2 LBLOCA AOR. The NRC requested EGC to provide an estimated effect of TCD on the PCT calculation for Braidwood Station Unit 2 and Byron Station Unit 2. This change affects the LBLOCA evaluation model using ASTRUM (i.e., Reference 3).
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A quantitative evaluation as discussed in Reference 5 was performed to assess the PCT effect of TCD and peaking factor burndown with other considerations of burnup on the Braidwood Station Unit 2 and Byron Station Unit 2 LBLOCA analysis and concluded that the estimated PCT impact is +148°F for 10 CFR 50.46 reporting purposes. The peaking factor burndown included in the evaluation is provided below in Tables 1, 2, and 3. EGC and its vendor, Westinghouse Electric Company LLC, utilize processes which ensure that the LOCA analysis input values conservatively bound the as-operated plant values for those parameters. Specifically, these tables are already included in the cycle specific Reload Safety Analysis Checklist limits.
Table 1: FDH Burndown Considered in the Evaluation of TCD Rod Burnup MWD/MTU FDH(1)(2) 0 1.7 28,000 1.7 40,000 1.513 50,000 1.411 60,000 1.326 70,000 1.258 (1) Includes uncertainties.
(2) Hot assembly average power uses same burndown, since it is a function of FDH.
Rod Burnup MWD/MTU FQ Steady-State 0
2.1 30,000 2.1 40,000 1.953 50,000 1.806 60,000 1.722 70,000 1,701 (1) Includes uncertainties.
Evaluation of Design Input Changes With Respect to Plant Operation To demonstrate compliance with the 10 CFR 50.46(b)(1) acceptance criterion concerning PCT when explicitly considering fuel pellet TCD and peaking factor burndown in the Braidwood Station Unit 2 and Byron Station Unit 2 LBLOCA analysis, design input values were revised, yet still bound the as-operated plant values. These input changes are not changes to the approved evaluation model in Reference 3. The updated inputs for Braidwood Station Unit 2 and Byron Station Unit 2 include:
Table 2: Steady St FQ Burndown Considered in the Evaluation of TCD Table 3: Transien Rod Burnup (MWD/MTU)
FQ Transient(')
0 2.6 40,000 2.6 50,000 2.314 60,000 2.184 70,000 2.106 2
Reduction in upper bound steam generator tube plugging from 10% to 5%,
Reduction in upper bound nominal vessel average temperature from 588°F to 583.5°F, and Increase in the conservatively low assumed containment pressure boundary condition (see Figure 1).
By/Br Unit 2 ASTRUM TC Evaluation G/T INPUT C/T INPUT PN la.
Figure 1: Comparison of AOR assumed containment pressure boundary condition with that of the Design Input Changes and TCD evaluation EGC and its vendor, Westinghouse Electric Company LLC, utilize processes which ensure that the LOCA analysis input values conservatively bound the as-operated plant values for those parameters. This change affects the LBLOCA evaluation model using ASTRUM (i.e., Reference 3).
A quantitative evaluation as discussed in Reference 5 was performed to estimate an overall PCT change due to changes in design input parameters. The evaluation concluded that the estimated PCT impact of these analysis input changes is -190°F for 10 CFR 50.46 reporting purposes.
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LBLOCA Description of Evaluation The evaluation method discussed in Reference 5 was used to determine the estimated effect of fuel pellet TCD and peaking factor burndown. First, the integrated PCT was calculated to demonstrate compliance with the 10 CFR 50.46(b)(1) criterion when the Design Input Changes and TCD and burndown were considered. Then, the margin PCT was calculated, including only the Design Input Changes.
For the integrated PCT calculation, a total of 26 WCOBRA/TRAC executions were performed. The uncertainty attributes of these executions were taken from among the most limiting cases from the original 124-run ASTRUM analysis. The evaluation considered an adequate range of burnup such that the effects of TCD and related burnup effects were captured. HOTSPOT executions were performed for each WCOBRA/TRAC case to consider the effect of local uncertainties for both IFBA (Integral Fuel Burnable Absorber) and non-IFBA fuel.
For the margin PCT calculation, a total of 14 WCOBRA/TRAC executions were performed. Again, the uncertainty attributes were taken from among the most limiting cases from the original 124-run ASTRUM analysis.
The estimated effect of TCD with burndown was then taken as the difference between the integrated PCT and the margin PCT.
LBLOCA Results Consistent with Reference 3, the most limiting PCT from each evaluation was taken as the representative PCT. The limiting integrated PCT case, considering all Design Input Changes and TCD and burndown, was 1999°F, less than the 2200°F acceptance criterion. Considering only the Design Input Changes, the margin PCT was 1851 OF.
Given the current AOR PCT of 2041°F, the estimate of effect of the Design Input Changes for 10 CFR 50.46 reporting purposes is -190°F. The estimate of effect of TCD and burndown is the difference between the margin PCT and the integrated PCT, or
+148°F. These changes are inherently linked and cannot be separated out, so the results of the estimated net PCT change for LBLOCA is -42 OF, creating a new limiting PCT value of 1999°F. The updated PCT value remains within the NRC 10CFR 50.46 acceptance criterion of 2200°F.
Methodology and Assumptions With respect to a description of the methodology and assumptions used to determine the above estimate, Westinghouse has provided this information directly to the NRC in Reference 5. This description includes consideration of experimental data relevant to TCD and specific information regarding computer code model changes which were necessary to address these data.
10 CFR 50.46 Reporting The estimated impact on the Braidwood Station, Unit 2 and Byron Station, Unit 2 LBLOCA evaluation model from fuel TCD represents a significant change in PCT, as 4
defined in 10 CFR 50.46(a)(3)(i). 10 CFR 50.46(a)(3)(ii) requires the licensee to provide a report within 30 days, including a proposed schedule for providing a reanalysis or taking other action as may be needed to show compliance with 10 CFR 50.46. of this submittal contains the 30 day report required by 10 CFR 50.46(a)(3)(ii) for Braidwood Station, Unit 2 and Byron Station, Unit 2. EGC has evaluated the requirement for reanalysis specified in 10 CFR 50.46(a)(3)(ii) and proposes the following schedule for reanalysis.
By December 15, 2016, EGC will submit to the NRC for review and approval a LBLOCA analysis that applies NRC approved methods that include the effects of fuel TCD for Braidwood Station, Unit 2 and Byron Station, Unit 2. The date for the analysis submittal is contingent on the following milestones which must be completed in order to perform a revised licensing basis LBLOCA analysis with an NRC approved ECCS evaluation model that explicitly accounts for TCD:
- 1) NRC approval of a fuel performance analysis methodology that includes the effects of TCD. The new methodology for developing inputs to the LBLOCA evaluation model would replace the current licensing basis methodology for Braidwood Station, Unit 2 and Byron Station, Unit 2 that is described in WCAP-15063-P-A, Revision 1 with Errata, "Westinghouse Improved Performance Analysis and Design Model (PAD 4.0)."
2)
NRC approval of a LBLOCA evaluation model that includes the effects of TCD and accommodates the rulemaking associated with the proposed 10 CFR 50.46c (Docket ID NRC-2008-0332). The new methodology would replace the current licensing basis methodology, WCAP-16009-P-A, "Realistic Large-Break LOCA Evaluation Methodology Using the Automated Statistical Treatment of Uncertainty Method (ASTRUM)." of this submittal contains the regulatory commitment associated with the proposed schedule of reanalysis. This information and that contained in Attachment 2 satisfies the reporting requirements of 10 CFR 50.46(a)(3)(ii).
Conclusions EGC 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 comply with the requirements of 10 CFR 50.46. EGC concludes, consistent with 10 CFR 50.46(a)(1)(i), there remains a high level of probability that the acceptance criterion of 10 CFR 50.46(b)(1), concerning the fuel PCT, would not be exceeded, when the model is corrected for TCD.
References Letter from Michele G. Evans (U.S. NRC) to Michael J. Pacilio (Exelon Nuclear),
"Information Request Pursuant to 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 (TAC No. M99899)," dated February 16, 2012 5
2.
Letter from Jeffrey L. Hansen (EGG) to U.S. NRC, "Annual 10 CFR 50.46 Report of the Emergency Core Cooling System Evaluation Model Changes and Errors,"
dated April 6, 2011 3.
WCAP-16009-P-A, "Realistic Large-Break LOCA Evaluation Methodology Using the Automated Statistical Treatment of Uncertainty Method (ASTRUM)," dated January 2005 4.
WCAP-15063-P-A Revision 1 with Errata, "Westinghouse Improved Performance Analysis and Design Model (PAD 4.0)," dated July 2000 5.
Letter from J. A. Gresham (Westinghouse) to U.S. NRC, "Westinghouse Input Supporting Licensee Response to NRC 10 CFR 50.54(f) Letter Regarding Nuclear Fuel Thermal Conductivity Degradation," dated March 7, 2012 6.
NRC Information Notice 2011-21, "Realistic Emergency Core Cooling System Evaluation Model Effects Resulting From Nuclear Fuel Thermal Conductivity Degradation," dated December 13, 2011 6
ATTACHMENT 2 10 CFR 50.46 30 Day Report for Braidwood Station Unit 2 and Byron Station Unit 2
Peak Cladding Temperature Rack-Up Sheet PLANT NAME:
Braidwood Station Unit 2 ECCS EVALUATION MODEL:
Large Break Loss of Coolant Accident (LBLOCA)
REPORT REVISION DATE:
03/19/12 CURRENT OPERATING CYCLE: 16 ANALYSIS OF RECORD Evaluation Model:
ASTRUM (2004)
Calculation:
Westinghouse WCAP-1 6841 -P, November 2007 Fuel:
VANTAGE+ 17 x 17 Limiting Fuel Type:
VANTAGE+ 17 x 17 Limiting Single Failure:
Loss of one train of ECCS flow Limiting Break Size and Location: Guillotine break in the Cold Leg Reference PCT 2041.0°F MARGIN ALLOCATION A. PRIOR LOSS OF COOLANT ACCIDENT (LOCA) MODEL ASSESSMENTS 10 CFR 50.46 report dated April 6, 2011 (see note 2)
APCT PCT =
= 0°F 10 CFR 50.46 report dated March 15, 2011 (see note 1)
NET PCT B. CURRENT LOCA MODEL ASSESSMENTS Design Input Changes with Respect to Plant Operation (see note 3) c1PCT = -190°F Evaluation of Pellet Thermal Conductivity Degradation and Peaking Factor Burndown see note 3 APCT = +148°F Total PCT change from current assessments APCT = -42°F Cumulative PCT change from current assessments APCT = 338°F NET PCT PCT = 1999.0°F PCT = 2041.0°F 1
Peak Cladding Temperature Rack-Up Sheet PLANT NAME:
Byron Station Unit 2 EGGS EVALUATION MODEL:
Large Break Loss of Coolant Accident (LBLOCA)
REPORT REVISION DATE:
03/19/12 CURRENT OPERATING CYCLE: 17 ANALYSIS OF RECORD Evaluation Model:
ASTRUM (2004)
Calculation:
Westinghouse WCAP-1 6841 -P, November 2007 Fuel:
VANTAGE+ 17 x 17 Limiting Fuel Type:
VANTAGE+ 17 x 17 Limiting Single Failure:
Loss of one train of ECCS flow Limiting Break Size and Location:
Guillotine break in the Cold Leg Reference PCT PCT = 2041.0°F MARGIN ALLOCATION A. PRIOR LOSS OF COOLANT ACCIDENT (LOCA) MODEL ASSESSMENTS 10 CFR 50.46 report dated March 15, 2011 (see note 1) dPCT = 0°F 10 CFR 50.46 report dated April 6, 2011 (see note 2)
NET PCT B. CURRENT LOCH MODEL ASSESSMENTS Design Input Changes with Respect to Plant Operation (see note 3)
APCT = -190°F Evaluation of Pellet Thermal Conductivity Degradation and Peaking Factor Burndown see note 3 APCT = +148°F Total PCT change from current assessments APCT = -42°F Cumulative PCT change from current assessments APCT
= 338°F NET PCT PCT = 1999.0°F PCT = 2041.0°F 2
Assessment Notes 1.
Prior LOCA Model Assessment The 30 day 10 CFR 50.46 report in Reference 1 reported a new large break BELOCA (ASTRUM) analysis to support operations for Braidwood and Byron Stations Units 1 and 2. The same report assessed the impact from several errors, issues, and code enhancements. Each of these errors/issues/code enhancements had a 0°F PCT impact with a net 0°F PCT impact.
2.
Prior LOCA Model Assessment The 10 CFR 50.46 report in Reference 2 reported no changes, corrections, or enhancements for the LBLOCA model.
3.
Current LOCA Model Assessment Two changes were performed, affecting the Unit 2 Byron and Braidwood Stations LBLOCA analysis. The first is an error in the evaluation model concerning the modeling of thermal conductivity degradation (TCD) and associated peaking factor burndown. The second is a design input change consisting of a reduction in upper bound steam generator tube plugging, a reduction in upper bound nominal vessel average temperature, and an increase in the assumed containment pressure boundary condition.
As a result, Westinghouse estimated the effect of the TCD with burndown to be
+148°F and estimated the effect of the design input changes to be -190°F. These two assessments are coupled together via their evaluations of burnup effects which include TCD, peaking factor burndown and design input changes. Therefore, the combined affect of these two changes results in a net change in the reported LBLOCA PCT of -42°F.
References 1.
Letter from Jeffrey L. Hansen (EGC) to U.S. NRC, "ECCS Evaluation Model Error 10 CFR 50.46 Reports," dated March 15, 2011 2.
Letter from Jeffrey L. Hansen (EGC) to U.S. NRC, "Annual 10 CFR 50.46 Report of the Emergency Core Cooling System Evaluation Model Changes and Errors,"
dated April 6, 2011 3
ATTACHMENT 3 Regulatory Commitments The following list identifies those actions committed to by Exelon Generation Company, LLC, (EGG) in this submittal. Any other actions discussed in the submittal represent intended or planned actions by EGC, are described only for information, and are not regulatory commitments.
COMMITMENT TYPE COMMITMENT COMMITTED DATE ONE-TIME PROGRAM-OR "OUTAGE" ACTION MATIC (YES/NO)
(YES/NO)
EGG will submit to the NRC December 15, 2016 Yes No for review and approval a LBLOCA analysis that applies NRC approved methods that include the effects of fuel TCD for Braidwood Station, Unit 2 and Byron Station, Unit 2.
The date for the analysis submittal is contingent on the following milestones which must be completed in order to perform a revised licensing basis LBLOCA analysis with an NRC approved EGGS evaluation model that explicitly accounts for TCD:
1)
NRC approval of a fuel performance analysis methodology that includes the effects of TCD. The new methodology for developing inputs to the LBLOCA evaluation model would replace the current licensing basis methodology for Braidwood Station, Unit 2 and Byron Station, Unit 2 that is described in WCAP-15063-P-A, Revision 1 with Errata, "Westinghouse Improved Performance Analysis and 1
COMMITMENT TYPE COMMITMENT COMMITTED DATE ONE-TIME PROGRAM-OR "OUTAGE" ACTION MATIC (YES/NO)
(YES/NO)
Design Model (PAD 4.0)."
2)
NRC approval of a LBLOCA evaluation model that includes the effects of TCD and accommodates the rulemaking associated with the proposed 10 CFR 50.46c (Docket ID NRC-2008-0332). The new methodology would replace the current licensing basis methodology, WCAP-16009-P-A, "Realistic Large-Break LOCA Evaluation Methodology Using the Automated Statistical Treatment of Uncertainty Method (ASTRUM)."
2