ML20274A192
| ML20274A192 | |
| Person / Time | |
|---|---|
| Issue date: | 09/24/2021 |
| From: | Paul Clifford NRC/NRR/DSS |
| To: | Joseph Donoghue NRC/NRR/DSS |
| Clifford,P., NRR/DSS, 301-415-4043 | |
| Shared Package | |
| ML20274A189 | List: |
| References | |
| Download: ML20274A192 (10) | |
Text
September 24, 2021 MEMORANDUM TO: Joseph Donoghue, Director Division of Safety Systems Office of Nuclear Reactor Regulation FROM: Paul M. Clifford, Senior Technical Advisor /RA/
Division of Safety Systems Office of Nuclear Reactor Regulation
SUBJECT:
50.46C EMERGENCY CORE COOLING SYSTEM SAFETY ASSESSMENT - 2019 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 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:
As stated CONTACT: Paul M. Clifford, NRR/DSS (301) 415-4043
Pkg ML20274A189 *via email NRR-106 OFFICE NRR/DSS*
NAME PClifford DATE 9/ 24 /2020 2019 50.46c ECCS Performance Safety Assessment Scope and Purpose 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 as well as 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.
In response to the research findings in Research Information Letter 0801, Technical Basis for Revision of Embrittlement Criteria in 10 CFR 50.46, (Agencywide Documents Access and Management System (ADAMS) Accession number, ML081350225), the staff performed a preliminary safety assessment of currently operating reactors (ADAMS Accession number ML081620302 Proprietary, ML090340073 Non-Proprietary). This assessment found that, due to measured cladding performance under LOCA conditions, realistic fuel rod power history, and current analytical conservatisms, sufficient safety margin exists for operating reactors.
Therefore, the U.S. Nuclear Regulatory Commission staff determined that immediate regulatory action was not required, and that changes to the 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 Pressurized Water Reactor Owners Group (PWROG) and Boiling Water Reactor Owners Group (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 PWROG (ADAMS Accession number ML11139A3090) and BWROG (ADAMS Accession number ML1119501390) 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, Division of Safety Systems (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 Equivalent Cladding Reacted (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
2019 Annual Update:
The ECCS Margin Database was updated to capture all 50.46(a)(3)(iii) reports and relevant LARs over the period from June 2018 to June 2019. In summary, the following impacts were reported:
Boiling Water Reactors (32 total reactors, Oyster Creek shut down)
- 23 plants reported no change in PCT.
- 2 plants reported a reduction in PCT.
- 4 plants reported an increase in PCT.
Pressurized Water Reactors (66 total reactors, TMI Unit 1 and Fort Calhoun shut down, Vogtle Units 3 and 4 added)
LBLOCA:
- 64 plants reported no change in PCT.
- 0 plants reported a reduction in PCT.
- 2 plants reported an increase in PCT.
SBLOCA:
- 62 plants reported no change in PCT.
- 0 plants reported a reduction in PCT.
- 2 plants 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_2019_r0.pdf Examination of the 2019 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 2 BWRs performed new LOCA calculations which credit COLR Thermal-Mechanical Operating Limits (TMOL) reduced rod power at higher burnup to satisfy new analytical limits.
o Reduction from 3 plants in 2018 margin assessment.
o Remaining 7 PWRs credit rod peaking factor burn down.
o Reduction from 11 plants in 2018 margin assessment.
o Remaining 32 PWRs credit either new LOCA calculations (including rebaselined PCTs) or identified credits to satisfy new analytical limits.
o No change from 2018 margin assessment.
New Breakaway Oxidation Analytical Limit:
- All 98 plants needed no adjustments or new calculations.
o Only 1 plant had time-at-temperature duration above 1000 seconds.
o Oyster Creek (2019 shutdown) was the only plant with minimal margin.
Figure 2-1: Estimated Change in ECR Margin
Table 2-1: 50.46c ECCS Margin Database Record of Revision Revision Date Database Description 0 9/2011 ECCS_Margin_Database Initial release. Data collected from PWROG Report OG-11-143, BWROG Report TP-11-010, and via vendor audits.
1 9/2012 ECCS_Margin_Database_ First annual revision. PWR LBLOCA and Sept2012 SBLOCA assesments separated.
Revisions to AOR and error reports captured. ECR tables added to assess impact of PCT change on CP-ECR.
1a 10/2012 ECCS_Margin_Database_ Revise dECR/dT calculations with Sept2012_r1 corrected cladding thickness. Added ECR estimates for SBLOCA based on bounding 1000 second time-at-temperature.
2 9/2013 ECCS_Margin_Database_ 2nd annual revision. Revisions to AOR and Sept2013_r0 50.46 change/error reports captured.
dECR calculated based upon dPCT and AOR time-at-temperature profile.
2a 1/2014 ECCS_Margin_Database_ Capture revision to PWROG margin Sept2013_r1 assessment (PA-ASC-1094).
Westinghouse evaluated inpact of TCD and past rack-ups, documented new credits, and re-grouped plants.
3 1/2015 ECCS_Margin_Database_ 3rd annual revision. Revisions to AOR and 2014_r0 50.46 change/error reports captured.
Added fuel type and accession numbers.
4 1/2016 ECCS_Margin_Database_ 4th annual revision. Revisions to AOR and 2015_r0 50.46 change/error reports captured.
5 1/2017 ECCS_Margin_Database_ 5th annual revision. Revisions to AOR and 2016_r0 50.46 change/error reports 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. Both SBLOCA and LBLOCA assessed.
6 1/2018 ECCS_Margin_Database_ 6th annual revision. Revisions to AOR and 2017_r0 50.46 change/error reports captured.
7 10/2019 ECCS_Margin_Database_ 7th annual revision. Revisions to AOR and 2018_r0 50.46 change/error reports captured.
8 9/2020 ECCS_Margin_Database_ 8th annual revision. Revisions to AOR and 2019_r0 50.46 change/error reports captured.
- 3. Conclusion The staffs ECCS performance safety assessment represents a snapshot of the available 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 abnormal occurrence reports. The revised database is available in ADAMS.
ECCS_Margin_Database_2019_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.