ML25267A015

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Radiological Consequences Analyses Proposed License Amendment Request for Increased Fuel Enrichment and Burnup to Support 24-month Cycle Operation - NRC Pre-Submittal Meeting September 25, 2025
ML25267A015
Person / Time
Site: Millstone Dominion icon.png
Issue date: 09/25/2025
From:
Dominion Energy
To:
Office of Nuclear Reactor Regulation
References
EPID L-2025-LRM-0108
Download: ML25267A015 (1)
v • d • e


Text

Millstone Power Station, Unit 3 Radiological Consequences Analyses Proposed License Amendment Request for Increased Fuel Enrichment and Burnup to Support 24-month Cycle Operation NRC Pre-Submittal Meeting September 25, 2025

Agenda

  • Purpose
  • Schedule Overview
  • Introduction/Background
  • Discussion of Technical Positions
  • Planned Presentation of Radiological Analyses
  • Schedule Review 2

Purpose & Note Purpose Present and discuss key analysis considerations and approaches for radiological consequences analyses to create a higher quality submittal and a more efficient NRC review.

Focus on technical assumptions and justifications, especially for new topics arising from LEU+ and HBU fuel and the application of Draft Regulatory Guide (DG)-1425 positions [Proposed Revision 2 for RG 1.183].

Inform NRC of potential exemption request to 10 CFR 50.67 (and GDC 19) acceptance criterion.

Note Proposed positions in this presentation are currently anticipated but are subject to change.

3

Schedule Overview 4

Introduction

  • Dominion Energy intends to license and operate Millstone Power Station Unit 3 (MPS3) with 24-month fuel cycles beginning in spring of 2028

- Requires core designs with higher than currently licensed fuel enrichment and burnup

- Desired upper limits of 6.5 wt% U-235 enrichment and 75 GWD/MTU peak rod average burnup

  • This pre-submittal meeting only focuses on technical aspects of DBA radiological consequences.

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Introduction

- Limits of applicability are 5 wt% enrichment and 62_GWD/MTU peak rod average burnup

  • Regulatory Guide 1.183, Rev. 1 limits of applicability are 8 wt% enrichment and 68 GWD/MTU peak rod average burnup
  • DG-1425 limits of applicability are 8 wt% enrichment and 80 GWD/MTU peak rod average burnup

- Currently in development 6

=

Background===

  • DG-1425

- Regulatory Position 1.5: Submittal Requirements and Information

  • The amendment may be issued only if the applicants analysis demonstrates with reasonable assurance that the identified dose criteria are met.
  • Should describe the licensees analyses of radiological and non-radiological impacts of the proposed modification in sufficient detail to support review by the NRC Staff.

- DE intends to reference supporting technical documentation for DG-1425 positions 7

DG-1425 Technical Positions

  • Technical Positions within DG-1425 to be presented and discussed:

- MHA release fractions and timing

- Fuel Fragmentation, Relocation, and Dispersal (FFRD)

- Non-LOCA Fission Product Release Fractions

  • Including Fuel Pellet Fragmentation

- 10 CFR 50.46 Large Break LOCA Dose Assessment

- Natural Deposition Removal

- Fuel Handling Accident (FHA) Rod Internal Pressure (RIP)

- Environmental Equipment Qualification (EEQ)

- Acceptance Criteria 8

DG-1425 Technical Positions

  • Maximum Hypothetical Accident (MHA)

- Draft Regulatory Positions (RPs) 3.2, 3.3, and 3.4 present MHA modeling

  • Release Fractions (Table 2 for PWRs)
  • Phase timing and durations (Table 5)
  • Radionuclide groups (Table 6)
  • SAND2023-01313 report is the basis for RPs in DG-1425 9

DG-1425 Technical Positions

  • Maximum Hypothetical Accident (MHA)

- DE approach:

  • SAND2023-01313 as the basis of release fractions, timing durations, and radionuclide groups

- Referenced directly

  • Address minor differences between DG-1425 and SAND2023-01313

- Release fractions for Noble Metals, Cerium, and Lanthanide groups with release fractions <1.0E-6 truncated to 0.0

- Gap phase conservatively set to start of event consistent with current modeling

- No iodine chemical form updates are being considered 10

DG-1425 Technical Positions

  • Maximum Hypothetical Accident (MHA) 11 Table 2 of DG-1425: PWR Core Inventory Fraction Released into Containment Atmosphere

DG-1425 Technical Positions

  • Fuel Fragmentation, Relocation, and Dispersal (FFRD)

- RP 3.2 presents a position on FFRD for the MHA event

  • Not considered for the burnup and enrichment applicability in SAND2023-01313

- NRC Internal Memorandum dated July 20, 2021 (Reference 44 of DG-1425)

  • MHA Assessed FFRD and determined the significant fission product release was likely bounding 12

DG-1425 Technical Positions

  • Fuel Fragmentation, Relocation, and Dispersal (FFRD)

- DE Approach

  • MHA analysis considers significant radionuclide release as specified in SAND2023-01313
  • Reference the NRC Internal Memorandum to confirm additional FFRD releases not considered for MHA 13

DG-1425 Technical Positions

  • Non-LOCA Maximum Steady-State Fission Product Release Fractions

- RP 3.2 presents non-LOCA gap release fractions (Table 4 for PWRs)

  • PWR 14x14 and 17x17 fuel array release fractions
  • Figure 1 presents an applicability curve of rod average power versus rod average burnup
  • NRC-calculated values using methods for RG 1.183, Rev. 1 basis

- NRC internal memorandum, Technical Basis for Draft RG 1.183, Revision 1 (2021) Non-LOCA Fission Product Release Fractions, July 28, 2021 (ML21209A524) 14

DG-1425 Technical Positions

  • Non-LOCA Maximum Steady-State Fission Product Release Fractions

- DE Approach

  • Apply the 17x17 fuel array non-LOCA gas fractions from Table 4 of DG-1425 without adjustment

- MPS3 utilizes a 17x17 fuel array

  • DG-1425 inclusion will be justification for acceptability
  • Rod average power versus burnup applicability will be confirmed on a cycle basis 15

DG-1425 Technical Positions 16 Table 4 of DG-1425: PWR Steady-State Fission Product Release Fractions Residing in the Fuel Plenum and Gap

DG-1425 Technical Positions 17 Figure 1 of DG-1425: Maximum Allowable Power Operating Envelope for PWR Steady-State Release Fractions

DG-1425 Technical Positions

  • Non-LOCA Fuel Pellet Fragmentation

- RP 3.2 identifies high-temperature non-LOCA DBAs predicting fuel rod cladding failure may be susceptible to additional fission product release due to fuel pellet fragmentation

  • Identifies PWR RCP locked rotor (LR) and main steamline break (MSLB)
  • Acceptable methods for addressing

- Prevent balloon and burst through design and analysis OR

- Estimate a bounding source term contribution (possibly using RIL 2021-13 insights) 18

DG-1425 Technical Positions

  • Non-LOCA Fuel Pellet Fragmentation

- DE Approach

  • Calculate no fuel failures (cladding failures through DNB assessment) occur for RCP LR or MSLB
  • Will be confirmed through thermal-hydraulic analyses following approved DE Methods
  • Locked rotor dose analysis may continue to model a bounding amount of cladding failure

- Represents a bounding surrogate

- Fuel assemblies susceptible to DNB failure are higher powered and at lower burnup conditions (<62 GWD/MTU rod average) 19

DG-1425 Technical Positions

- RP 3.7 presents assessment of radiological consequences due to FFRD of a 50.46 Large-break LOCA

  • Acceptance criteria assumed to be consistent with other non-MHA LOCA events (i.e., 6.3 rem TEDE offsite)
  • Basis of source term

- Fraction of MHA using RIL 2021-13 OR

- Best-estimate FFRD-induced source term

  • Release timing of FFRD source term is instantaneous at time of burst
  • Appendix A provides LOCA modeling assumptions acceptable to the Staff 20

DG-1425 Technical Positions

- DE Approach

  • Analysis scenario not part of current licensing basis
  • Scenario does not confirm 50.46 compliance
  • Probability of occurrence significantly lower than other non-MHA events

- Acceptance criteria should be aligned with MHA

  • MHA provides bounding LOCA dose scenario 21

DG-1425 Technical Positions

  • Natural Deposition Removal

- RP A-2.2 states reduction in airborne radioactivity in containment due to natural deposition within the containment may be credited

  • DG-1425 states the NUREG/CR-6189 methods may be credited if adjusted to incorporate revised MHA source term 22

DG-1425 Technical Positions

  • Natural Deposition Removal

- Current Design Basis

  • Powers Model is incorporated into RADTRAD-NAI utilizing NUREG/CR-6189
  • MPS3 MHA analysis credits Powers Model natural deposition removal in unsprayed containment compartment

- Open discussion based on September 2025 NRC workshop 23

DG-1425 Technical Positions

  • Fuel Handling Accident (FHA) Rod Internal Pressure (RIP)

- Model introduced in RG 1.183 Rev. 1

- Appendix B formulas relate RIP to bubble diameter, bubble rise time, and decontamination factor (DF) 24

DG-1425 Technical Positions

  • Fuel Handling Accident (FHA) Rod Internal Pressure (RIP)

- DE Approach

  • Bounding value of 1500 psig will be modeled

- NRC Technical Basis evaluated 1400 psig

  • Calculated combined DFs are 264 (@1400 psig) and 256

(@1500 psig)

- Anticipated <3% difference in total dose based on iodine release from the pool 25

DG-1425 Technical Positions

  • Environmental Equipment Qualification (EEQ)

- RP 1.3.5 instructs the licensee to update EQ analyses

  • Appendix D of RG 1.89, Revision 2 presents qualification in the radiation environment

- References RG 1.183 (not revision specific) for guidance on accident radiological source terms and modeling assumptions 26

DG-1425 Technical Positions

  • Environmental Equipment Qualification (EEQ)

- Current Design Basis

- Open discussion based on September 2025 NRC workshop 27

DG-1425 Technical Positions

  • Acceptance Criteria

- RP 4.4 and Table 7 present the regulatory acceptance criteria for offsite (EAB, LPZ) and control room (CR) dose locations

  • Offsite acceptance criteria unchanged from RG 1.183, Revision 0
  • CR acceptance criteria increased for various events

- 5 rem TEDE to 10 rem TEDE

- Associated with Increased Enrichment Rulemaking

DG-1425 Technical Positions

  • Acceptance Criteria

- DE Approach

- 10 CFR 50.12

  • Clarification of TSC acceptance criterion also needed 29

Planned Presentation of Radiological Analyses

  • Proposed Contents of LAR

- Description of event analyses

- Input tables

  • Current Input
  • New Analysis Value
  • Reason for change (if applicable)

- RADTRAD-NAI run files 30

Schedule Summary

  • Proposed Schedule:

- Submit LAR - September 2026

- NRC Review Complete - September 2027

  • 12-month requested review and approval

- Cycle Startup - Early Q2 2028 (spring outage) 31

QUESTIONS?

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Acronyms & Definitions AST - Alternate Source Term CFR - Code of Federal Regulations CR - Control Room DBA - Design Basis Accident DE - Dominion Energy DF - Decontamination Factor DG - Draft Regulatory Guide DNB - Departure from Nuclear Boiling EAB - Exclusion Area Boundary EEQ - Environment Equipment Qualification FFRD - Fuel Fragmentation, Relocation, and Dispersal FHA - Fuel Handling Accident FSAR - Final Safety Analysis Report GDC - General Design Criteria GWD/MTU - Gigawatt-Day per Metric Ton of Uranium HBU - High Burnup Fuel IE - Increased Enrichment LAR - License Amendment Request LCO - Limiting Condition for Operation LEU+ - Fuel enriched between 5 - 10 wt%

LOCA - Loss of Coolant Accident LPZ - Low Population Zone LR - Locked Rotor MHA - Maximum Hypothetical Accident MPS3 - Millstone Unit 3 MSLB - Main Steamline Break MWt - Megawatt thermal NRC - Nuclear Regulatory Commission PWR - Pressurized Water Reactor RCP - Reactor Coolant Pump RG - Regulatory Guide RIL - Research Information Letter RIP - Rod Internal Pressure RP - Regulatory Position SPU - Stretch Power Uprate TEDE - Total Effective Dose Equivalent TID - Technical Information Document TS - Technical Specifications TSC - Technical Support Center wt% - U-235 weight percent enrichment 33

References Regulatory Guide 1.183, Rev. 0, Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors, July 2000 (ML003716792)

Regulatory Guide 1.183, Rev. 1, Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors, October 2023 (ML23082A305)

Draft Regulatory Guide DG-1425, Alternative Radiological Source Terms for Evaluating Design-Basis Accidents at Nuclear Power Reactors, Pre-Decisional Draft (ML24304A864)

SAND2023-01313, High Burnup Fuel Source Term Accident Sequence Analysis, April 2023 (ML23097A087)

NRC Internal Memorandum Letter Report on Evaluation of the Impact of Fuel Fragmentation, Relocation, and Dispersal for the Radiological Design Basis Accidents in Regulatory Guide 1.183, July 20, 2021 (ML21197A067)

NRC internal memorandum, Technical Basis for Draft RG 1.183, Revision 1 (2021)

Non-LOCA Fission Product Release Fractions, July 28, 2021 (ML21209A524)

NRC Research Information Letter (RIL) 2021-13, Interpretation of Research on Fuel Fragmentation, Relocation, and Dispersal at High Burnup, December 2021.

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