ML25223A299
| ML25223A299 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 08/11/2025 |
| From: | Southern Nuclear Operating Co |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| EPID L-2025-LRM-0109 | |
| Download: ML25223A299 (16) | |
Text
Planned Vogtle Electric Generating Plant Unit 4 Emergency License Amendment Request Emergency LAR: TS SR 3.5.2.4 CMT Boron Concentration Discussion Meeting: August 11, 2025
Agenda
Background
Technical Specification (TS) SR 3.5.2.4 Planned Changes Technical Evaluation Summary Schedule Discussion
Passive Core Cooling System - Core Makeup Tank System Design and Operation
- Passive Core Cooling System (PXS) primary function is to provide emergency core cooling following postulated design basis events
- PXS includes two redundant Core Makeup Tanks (CMTs) connected to the direct vessel injection (DVI) lines
- Contain borated water at >3400 ppm to assure reactivity control
- Inject into RCS on loss of inventory
- Provide emergency boration and core makeup when CVS is unavailable and to mitigate consequences of any DBA requiring safety injection of borated water 3
Passive Core Cooling System - Core Makeup Tank 4
TS SR 3.5.2.4 CMT Boron Concentration Planned Changes Current Technical Specification SR 3.5.2.4
- If one CMT is inoperable due to water temperature or boron concentration not within limits, the plant is allowed to continue operation for up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
- If the water temperature and boron concentration are not restored to within limits within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, the plant must be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
- TS SR 3.5.2.4 requires verifying that the boron concentration in each CMT is 3400 ppm and 4500 ppm at a frequency of every 31 days.
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TS SR 3.5.2.4 CMT Boron Concentration Planned Changes Reason for the Proposed Change
- Vogtle U4 is at end-of-cycle (<1 month to refueling outage)
- CMT boron concentration 3400 ppm per SR 3.5.2.4
- Large concentration gradient creates challenges for maintaining CMTs in higher range of band
- Multiple borations in recent months to maintain boron concentration for remainder of cycle 6
TS SR 3.5.2.4 CMT Boron Concentration Planned Changes Description of the Proposed Change
- Temporary change to TS SR 3.5.2.4 CMT Boron Concentration 7
TS SR 3.5.2.4 CMT Boron Concentration Planned Changes Basis for Emergency Processing
- Vogtle Unit 4 is experiencing challenges maintaining CMT minimum boron concentration due to large concentration gradient between TS 3.5.2 and end-of-cycle RCS limits
- CMT A valve leakage is allowing RCS (low boron concentrations) into CMT, further challenging Operators
- Repair of CMT A valve internals planned during Unit 4 refueling outage in <30 days
- Unit 4 shutdown required if CMT boron concentration not restored in 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> 8
Technical Evaluation Deterministic Evaluation
- No adverse impact on the Loss of Normal Feedwater (LONF) event
- CMT boration is not credited in the Steam Generator Tube Rupture (SGTR) event
- No adverse impact on shutdown margin for Small Break Loss of Coolant Accidents (SBLOCA)
- Large Break Loss of Coolant Accidents (LBLOCA)
- CMT injection with 3100 ppm boron concentration
- > 800 ppm shutdown margin Double Ended Direct Vessel Injection (DEDVI) is limiting 9
Technical Evaluation Deterministic Evaluation (continued)
Anticipated Transient Without a SCRAM (ATWS)
- Minimum allowable average boron concentration of 3400 ppm is offset by conservatisms Conservatisms in the ATWS and SSD analyses would more than offset the 100ppm reduction (3500 ppm - 3400 ppm) in combined CMT boron concentration End of life/cycle (EOL) moderator temperature coefficient (MTC) is more negative than the beginning of cycle and the ability of the EOL MTC to mitigate an ATWS or SSD event is increased significantly. Use of the EOL MTC would also more than offset the 100 ppm difference in CMT boron concentration for the ATWS and SSD events.
Allowing the Unit 4 CMT A boron concentration to be less than the required limit would not adversely impact the capability of the passive core cooling system to perform its design function as discussed in FSAR Chapter 6 and 15.
- The margins in these analyses are sufficient to bound the impacts of a CMT A boron concentration as low as 3100 ppm provided the boron concentration from CMT A and B combined are 3400 ppm 10
Technical Evaluation There is adequate duration to ensure CMT A remains above minimum boron concentration of 3100 ppm until Mode 5 is reached during entry into 4R01 on 9/7/2025 and meets all design safety functions to maintain shutdown margin.
Utilizing the most recent sample result of 3479 ppm on 8/9/25 at (13:57) and the calculated leakage of 46.9 gpd into the Core Makeup Tank, CMT A will remain 3400 ppm for a period of 8.8 days, from the time of the sample.
If the CMT A leakage increases to 66 gal/day, the final concentration at the end of 6 days will be 3403 ppm and at 28 days will be 3138.5 ppm using infinite dilution equation.
Using the Volume of Bleed and Feed to Change Concentration the CMT A will be 3403 ppm in 6 days and 3140.5 ppm in 28 days.
RCS monitoring will be ongoing to confirm the concentration in CMT A remains above TS limit of 3100 ppm until 4R01.
11 KL1
Regulatory Evaluation
- Applicable Regulatory Requirements
- Amendment request is in accordance with 10 CFR 50.90 & 50.91(a)(5)
- 10 CFR 50, Appendix A, General Design Criteria (GDC) met:
- GDC 14, Reactor coolant pressure boundary
- GDC 26, Reactivity control system redundancy and capability
- GDC 27, Combined reactivity control systems capability
- GDC 35, Emergency core cooling
- GDC 37, Testing of emergency core cooling system
- No Significant Hazards Consideration involved 12
Emergency LAR: TS SR 3.5.2.4 CMT Boron Concentration Schedule
- Target Transmittal to NRC:
August 11, 2025
- SNC Request NRC Issuance of Amendment:
August 16, 2025 - to prevent unnecessary end-of-core life shutdown 13
Discussion 14
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Backup Slides 16