ML24192A323
| ML24192A323 | |
| Person / Time | |
|---|---|
| Issue date: | 01/31/2024 |
| From: | Joseph Messina NRC/NRR/DSS/SFNB |
| To: | Van Wert C NRC/NRR/DSS |
| References | |
| Download: ML24192A323 (9) | |
Text
January 31, 2024
MEMORANDUM TO: Christopher Van Wert, Senior Technical Advisor for Reactor Fuel Division of Safety Systems Office of Nuclear Reactor Regulation
FROM: Joseph Messina, Reactor Systems Engineer /RA/
Nuclear Methods and Fuel Analysis Branch Division of Safety Systems Office of Nuclear Reactor Regulation
SUBJECT:
50.46C ECCS SAFETY ASSESSMENT: 2022 - 2023 UPDATE
The purpose of this memorandum is to provide an annual update to the 50.46c
emergency core cooling system (ECCS) performance safety assessment. This update captures
ECCS model changes and errors reported within the 50.46(a)(3) annual and 30- day reports and
new loss-of-coolant accident (LOCA) analysis-of-record. The 50.46c ECCS performance safety
assessment documents plant-specific safety margin relative to the proposed requirements,
confirms continued safe operation for the entire fleet, and informs the implementation plan for
the proposed 10 CFR 50.46c rule.
Enclosure:
2022 - 2023 ECCS Performance Safety
CONTACT: Joseph Messina, NRR/DSS/SFNB 301 -415-4226 2
SUBJECT:
50.46C ECCS SAFETY ASSESSMENT: 2022-2023 UPDATE DATED: January 31, 202 4
DISTRIBUTION:
PUBLIC JMessina CVanWert RidsNrrDss
ADAMS Accession Number: ML24192A323 *via email OFFICE NRR/DSS NAME JMessina DATE 01/31/2023 OFFICIAL RECORD COPY 2022 - 2023 50.46c ECCS Performance Safety Assessment
- 1. Scope and Purpose
The purpose of this memorandum is to provide an annual update to the 50.46c ECCS performance safety assessment. This update captures ECCS model changes and errors reported within the 50.46(a)(3) annual and 30-day reports as well as new LOCA analysis-of-record. The 50.46c ECCS performance safety assessment documents plant-specific safety margin relative to the proposed requirements, confirms continued safe operation for the entire fleet, and informs the implementation plan for the proposed 10 CFR 50.46c rule.
In response to the research findings in Research Information Letter (RIL) 0801, Technical Basis for Revision of Embrittlement Criteria in 10 CFR 50.46, (ADAMS Accession No. ML081350225), the staff performed a preliminary safety assessment of currently operating reactors (ADAMS Accession No. ML081620302 Proprietary, ML090340073 Non-Proprietary).
This assessment found that, due to measured cladding performance under loss-of-coolant accident (LOCA) conditions, realistic fuel rod power history, and current analytical conservatisms, sufficient safety margin exists for operating reactors. Therefore, the NRC staff determined that immediate regulatory action was not required, and that changes to the emergency core cooling system (ECCS) acceptance criteria to account for these new findings can reasonably be addressed through the rulemaking process.
Recognizing that finalization and implementation of the new ECCS requirements would take several years, the staff decided that a more detailed safety assessment was necessary.
Working with the PWROG and BWROG, the staff completed a comprehensive ECCS performance safety assessment which confirmed, on a plant-specific basis, the safe operation of the U.S. commercial nuclear fleet. The ECCS performance safety assessment was issued in a memorandum dated September 27, 2011 (ADAMS Accession number ML11262A017) along with the staffs audit report of the PWR Owners Group (ADAMS Accession No. ML11140A159) and BWR Owners Group (ADAMS Accession No. ML111950139) ECCS margin assessment reports.
The 2011 ECCS performance safety assessment represents a snapshot of the available post-quench ductility (PQD) and breakaway oxidation margin at the time the plant specific information was compiled. Since that time, changes to and errors discovered in ECCS models, as well as planned license amendment requests (e.g., power uprates, fuel transitions),
challenge the continued applicability of the 2011 ECCS performance safety assessment. To ensure continued safe operation until the proposed 10 CFR 50.46c requirements are implemented, DSS committed to perform annual updates.
- 2. ECCS Performance Safety Assessment
In accordance with 10 CFR 50.46(a)(3)(iii), licensees are required to report any change to or error discovered in an acceptable evaluation model or in the application of such a model that affects the temperature calculation. Within these reports, the licensee provides a rack up of the changes and errors including an estimated change in peak cladding temperature (PCT).
The current regulation does not require an estimate to the previously reported maximum local oxidation. Note that the proposed 50.46c rule requires reporting changes in ECR.
Enclosure The following assumptions were used in assessing the impact of reported changes and errors on available ECR margin:
- 1. If no errors or changes were reported (i.e., PCT unchanged), then prior annual ECCS margin assessment remains applicable.
- 2. If summation of estimated impacts of errors and changes equaled zero (i.e., PCT unchanged), then prior annual ECCS margin assessment remains applicable.
- 3. If summation of estimated impacts of errors and changes was negative (i.e., PCT reduced), then prior annual ECCS margin assessment remains applicable.
- 4. If summation of estimated impacts of errors and changes was positive (i.e., PCT increased), then an assessment of residual ECCS margin was performed.
If an assessment of residual ECCS margin is necessary, then the change in CP-ECR (ECR) is estimated based upon reported changes in PCT. The following steps are taken to complete the annual assessment.
- 1. If a new ECCS calculation was performed during past 12 months (e.g., LAR involving ECCS), then record predicted PCT, ECR, burst/no burst, and time above 800C. Update AOR portion of ECCS margin database.
- 2. Compute margin relative to proposed requirements (alloy-specific). Update margin assessment portion of ECCS margin database.
- 3. Assess need for analytical credits, similar to Owners Group margin report. Update Owners Group portion of ECCS margin database.
-- OR -
- 1. Record the estimated change in PCT from 50.46(a)(3) reports.
- 2. Record the predicted time above 1600 °F for the limiting UFSAR AOR transient scenario (separate SB and LB for PWRs)
- 3. If burst predicted, perform 2-sided ECR calculation assuming 30% strain. Otherwise, 1-sided ECR calculation.
- 4. As shown in Figure 2-1, perform ECR calculation for simplified AOR (10degC ramp up to PCT, hold for time duration above 1600F, followed by 10 degC ramp down)
- 5. As shown in Figure 2-1, perform ECR calculation for modified AOR (10degC ramp up to new estimated PCT, hold for time duration above 1600F, followed by 10 degC ramp down)
- 8. If positive ECR margin exists, then assessment complete. If negative margin exists, then investigate possible analytical credits (similar to OG report), perform a more detailed ECR assessment, or contact the licensee or fuel vendor.
- 9. Evaluate breakaway oxidation margin.
A simplified ECR calculation is necessary since the impact of the change/error on the accident progression and time-temperature history is unknown. The above simplified ECR calculation is inherently conservative since the duration at the peak temperature is artificially extended for both the simplified AOR PCT and the estimated PCT. Thus, maximizing the ECR calculation.
This is illustrated in Figure 2-1 where the solid blue line represents the actual AOR peak cladding temperature profile and the dotted lines represent the simplified temperature profiles.
For each plant, the updated ECCS Margin Database provides the following information:
- Plant design
- Licensed power
- Fuel vendor
- Fuel rod cladding alloy
- Evaluation model
- Plant grouping
- Rebaseline ECR (if available)
- Margin to PQD analytical limit
- Margin to breakaway oxidation analytical limit
- Identify analytical credit(s)
- New AOR (Y/N)
- Reported changes
- Impact on margin assessment
- ADAMS number of 50.46(a)(3) report
Figure 2-1: Estimated Change in ECR Margin
2022 - 2023 Annual Update:
The ECCS margin database was updated to capture all 50.46(a)(3)(iii) reports and relevant LARs over the period from January 2022 to January 2023. In summary, the following impacts were reported:
Boiling Water Reactors (31 total reactors):
- 23 plants reported no change in PCT
- 0 plants reported a reduction in PCT
- 5 plants reported an increase in PCT
Pressurized Water Reactors (64 total reactors):
Large Break LOCA:
- 59 plants reported no change in PCT
- 0 plants reported a reduction in PCT
- 1 plant reported an increase in PCT
- 55 plants reported no change in PCT
- 0 plants reported a reduction in PCT
- 1 plant reported an increase in PCT
The revised database is listed below. Table 2-1 provides a record of revision for the ECCS margin database.
ECCS_Margin_Database_2022 _2023_r0.pdf
Examination of the 2022-2023 ECCS Margin Database reveals that the majority of plants needed no adjustments to show a positive margin to the revised analytical limits. In summary:
Revised PQD Analytical Limit:
o Remaining BWR performed new LOCA calculations which credit COLR Thermal-Mechanical Operating Limits (TMOL) reduced rod power at higher burnup to satisfy new analytical limits.
o No change from 2021-2022 margin assessment
- For PWR small break LOCA, 62 of 64 plants (97% of PWR fleet) needed no adjustment or new calculations:
o Remaining 2 PWRs credit rod peaking factor burndown
o No change from 2021-2022 margin assessment
- For PWR large break LOCA, 44 of 64 plants (69% of PWR fleet) needed no adjustment or new calculations:
o Remaining 20 PWRs credit either new LOCA calculations (including rebase lined PCTs) or identified credits to satisfy new analytical limits.
o Reduction from 23 plants in 2021-2022 margin assessment
New Breakaway Oxidation Analytical Limit:
- All 95 plants needed no adjustments or new calculations
o No plants had a time-at-temperature duration greater than 2000 seconds.
o Oyster Creek (shutdown in 2019) was the only plant with minimal margin.
Table 2-1: 50.46c ECCS Margin Database Record of Revision
Revision Date Database Description Initial release. Data collected from 0 9/2011 ECCS_Margin_Database PWROG Report OG 143, BWROG Report TP 010, and via vendor audits.
1st annual revision. PWR LBLOCA and 1 9/2012 ECCS_Margin_Database_ SBLOCA assessments separated.
Sept2012 Revisions to AOR and error reports captured. ECR tables added to assess impact of PCT change on CP-ECR.
Revise dECR/dT calculations with 1a 10/2012 ECCS_Margin_Database_ corrected cladding thickness. Added Sept2012_r 1 ECR estimates for SBLOCA based on bounding 1000 second time-at-temperature.
2nd annual revision. Revisions to AOR 2 9/2013 ECCS_Margin_Database_ and 50.46 change/error reports Sept2013_r0 captured. dECR calculated based upon dPCT and AOR time-at-temperature profile.
Capture revision to PWROG margina 2a 1/2014 ECCS_Margin_Database_ assessment (PA-ASC-1094).
2013_r1 Westinghouse evaluated impact of TCD and past rack-ups, documented new credits, and re-grouped plants.
3rd annual revision. Revisions to AOR 3 1/2015 ECCS_Margin_Database_ and 50.46 change/error reports 2014 _r0 captured. Added fuel type and accession numbers.
4th annual revision. Revisions to AOR 4 1/2016 ECCS_Margin_Database_ and 50.46 change/error reports 2015_r0 captured.
5th annual revision. Revisions to AOR 5 1/2017 ECCS_Margin_Database_ and 50.46 change/error reports 2016 _r0 captured. Incorporated revised PWROG margin assessment, PWROG-16057-NP, including information gathered during audits. Revised PWROG report evaluated past rack-up, identified new credits, and re-grouped plants.
6th annual revision. Revisions to AOR 6 1/2018 ECCS_Margin_Database_ and 50.46 change/error reports 2017 _r0 captured.
7th annual revision. Revisions to AOR 7 10/2019 ECCS_Margin_Database_ and 50.46 change/error reports 2018_r0 captured.
8th annual revision. Revisions to AOR 8 9/2020 ECCS_Margin_Database_ and 50.46 change/error reports 2019_r0 captured.
9th annual revision. Revisions to AOR 9 12/2021 ECCS_Margin_Database_ and 50.46 change/error reports 2020_2021_r0 captured.
10 1/2023 ECCS_Margin_Database_ 10th annual revision. Revisions to AOR 2021_2022_r0 and 50.46 change/error reports captured
11 7/2024 ECCS_Margin_Database_ 11th annual revision. Revisions to AOR 2022_2023_r0 and 50.46 change/error reports captured
- 4. Conclusion
The staffs ECCS performance safety assessment represents a snapshot of the available post quench ductility (PQD) and breakaway oxidation margin at the time the plant specific information was compiled. Changes to and errors discovered in ECCS models, as well as planned license amendment requests (e.g., power uprates, fuel transitions), challenge the continued applicability of the ECCS performance safety assessment. To ensure continued safe operation until the proposed 10 CFR 50.46c requirements are implemented, DSS committed to perform annual updates.
The ECCS Margin Database has been updated to capture reported changes and error as well as any new LOCA AORs. The revised database is available in ADAMS.
ECCS_Margin_Database_2022 _2023_r0.pdf
Section 2.0 summarizes the impact of these changes on available ECCS performance margin.
All plants show positive margin to the proposed 50.46c analytical limits. As such, a coolable core geometry would have been preserved in the event of a postulated LOCA.
In conclusion, the staff has updated the 50.46c ECCS performance safety assessment, captured the latest results and changes, and confirmed safe operation of all nuclear power plants with respect to the new, proposed requirements.