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NEI - Spent Fuel Management: Safety and Efficiency Improvements Through Technology and Performance Margins - NRC PIRTs Public Meeting Rev. 8.15.2024 Final
	ML24229A001
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Site:	Nuclear Energy Institute
Issue date:	08/19/2024
From:	Richter M; Nuclear Energy Institute
To:	; Office of Nuclear Material Safety and Safeguards
	Daneira Melendez
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Spent Fuel Management :

Safety and Efficiency Improvements Through Technology and Performance Margins

NEI-NRC Public Meeting

Mark Richter Technical Advisor Nuclear Energy Institute

August 19, 2024 Nuclear Energy: National Importance Inflation Reduction Act (IRA)

Congress sent a clear signal that nuclear is essential to the transition to a clean energy economy

Backbone of stable electric grid

Meeting growing demand for more clean energy

ADVANCE Act

Maintains U.S. global leadership

Enables testing, demonstration and deployment of accident tolerant fuels and advanced nuclear reactor fuel concepts

Supports modernization of licensing and oversight of operating reactors

Enables efficiencies necessary for dry storage capabilities to keep pace with nuclear growth

Consistent with other notable improvements (e.g. license renewal)

Embodies NRC Principles of Good Regulation

Advanced fuels may increase source term and decay heat Load canisters more efficiently with shielding while reducing dose, handling and improving spent fuel pool management Ultra conservative assumptions reduce safety and efficiency Actionable definitions of gross rupture, performance-based peak clad temperature and thermal margin enable efficiencies and flexibility in analysis and operations-enhance safety and efficiency Use of performance-based results provides a good example of how NRC can use risk insights and allow us to live up to our collective safety imperative

Pathway to Implementation NRC Public Meeting

Keith Waldrop Senior Technical Leader

August 19, 2024

Background

DEFINING SPENT FUEL PERFORMANCE MARGINS by NEI Spent Fuel Margins White Paper Development Task Force November 8, 2019

Recommendations IV-1-5 Goals: Update thermal Most efficient regulatory modeling methods for implementation vehicle improved focus on safety pending further discussions:

including a revised thermal EPRI Topical Report and/or modeling metric and a NRC Standard Review Plan graded approach for thermal update modeling to expand margins and operational flexibilities with increased safety and cost savings

Background

Fuel Performance PIRT

- The onset of fuel failure is not abrupt, but rather a continuum no cliff-edge effect is associated with the current NRC recommended 400 °C limit (ISG-11 Rev. 3)

Decay Heat PIRT

- Regulatory guidance (RG. 3.54 Rev. 1) significantly overestimates decay heat over wide range of fuel burnups.

Thermal PIRT

- Excess conservatism in thermal calculations leads to negative impact on worker doseand fuel performance

Background

Gross Rupture PIRT

- RCS chemistry records from operation may be used to establish if fuel cladding gross rupture failure occurred to determine storage requirements Alternate Fuel Performance Metrics PIRT

- Increasing PCT limit to 450oC or 500oC are potentially acceptable metrics to pursue

Other metrics considered but not recommended: hoop stress, average cladding temperature

Excessive Layers of Conser vatisms More doseo theorkers Lessegulatory efficiency

Objectives of PIRTs Implementation

Define pathways to leverage the benefits of all the PIRTs for

Gross Rupture

PCT and Graded Approach

Gross Rupture

Gross Rupture Topical Report - Background

Unclear guidance in NUREG-2215

- p xxviii: Gross Breach. A breach in the spent fuel cladding that is larger than either a pinhole leak or a hairline crack and allows the release of particulate matter from the spent fuel rod. Current definition of gross

- p8-35 SFAs with any of the following characteristics are rupture non-actionable expected to be classified as damaged: due to:

Reactor operating records or fuel classification records

Lack of criteria indicate that the SFA contains fuel rods with gross regarding use of reactor breaches. operating records

- p8-36: gross breaches should be considered to be any

Demonstration that cladding breach greater than 1 mm (0.04 in.). 1mm criteria is met And in NUREG-2216 difficult to achieve

- p7-49: The staff considers that adequate reactor operating records that identify only gaseous or volatile decay products (no heavy metals) in the reactor coolant system are acceptable evidence that cladding breaches are no larger than a pinhole leak or hairline crack.

Gross Rupture Topical Report

Scope

- Implement new definition to use chemistry records in lieu of fuel inspections (e.g. sipping) to demonstrate cladding integrity Safety benefits of New Gross Rupture Definition

- Better comply with ALARA principles Reduce dose to the workers

- Reduce risk Avoid unnecessary handling by minimizing need for sipping campaigns and pool-side exams

New definition of gross rupture FSARs and CoC do not preclude to clarify NRC guidance and allow the use of the new gross stakeholders to use consistent rupture definition and actionable metrics Clarity Efficiency Reliability

Gross Rupture Topical Report

Topical report

- Technical basis for new definition of gross rupture based Input from Gross Rupture PIRT Guidance regarding:

- Applicability criteria (e.g., quality of chemistry records)

- Key parameters to survey (e.g., increase in fuel products concentration during cycle and vs.

previous cycle)

Impacts on SRP and other NRC guidance documents

- Generic Directly referenceable by licensees Expected Industry Actions Expected NRC Actions

CoC amendments may not needed Review / approve TR (SER)

Most CoCs already have wording that allows using Revise guidance (SRP storage and transportation) the new definition industry does not need to wait to start Underlying documents for fuel qualification may implementing once SER is issued need to be changed + criteria used in site Revise inspection criteria procedures

Benefits for the NRC

Clarification of NRC guidance Reduce risk inconsistencies - Eliminate unnecessary fuel moves

- Definition of gross rupture currently non-Better compliance with ALARA actionable by industry principles

- Inconsistency between storage and - Eliminate unnecessary dose-consuming transport SRPs activities

- Comply with principles of good regulation Generic implementation of criteria Reduce undue burdens change for broad adoption - Reduce number of damage fuel containers

- Reduce regulatory burden and review Recommendations to update guidance

- RCS chemistry applicability criteria

- Technical justification of key parameters

PCT and Graded Approach

PCT and Graded Approach Topical Report

Scope

- Implement increased PCT limit of 450oC Include a graded approach for storage and transportation safety analyses Safety benefits

- Shorter loading times Compliance with ALARA principles, such as:

- Fewer canisters loaded

Public: Less transportation events (precursor)

- Regulation efficiency increased

Wo r ke r s : Increased shielding, Fewer/shorter

- Risk-informed analyses leveraged loadings Operational Flexibility

- More time for certain operations,

- Eliminate supplemental cooling system,

- Add shielding

Peak Cladding Temperature - Graded Approach

Performance Margin - Fuel rod has additional capacity to go to higher temperature without failure Methods Margin - Conservatisms in the modeling Graded approach would require increased analytical rigor to tighten uncertainties as you approach 450° C limit PCT (° C)Uncertainties reduction through analytical rigor

450

425

400

Analytical Rigor Objective: Develop a risk-informed and graded approach and avoid making licensing more complicated with additional analyses where more margin exists

NAC-STC High-Burnup Fuel Amendment

Excessive analytical level of detail (academic exercise) required for a licensed system resulting in no impact to the safety of the design NAC initially provided thermal analysis, based on the existing NRC approved thermal methods/models for the NAC-STC to support reasonable assurance the cask would perform as required

No basket design changes

Change in individual fuel thermal loads and loading pattern NRCs thermal reviewers suggested that NAC needed to develop and qualify new, best estimate models for assessing HBU fuel performance, and included directions to use NUREG -

2152 (authored by the thermal reviewers) for the development and qualification of thermal models

18 Why the industry believes the PIRT work and applying Risk Informed criteria is Valuable

In 2019-20, NAC submitted an amendment for a content change using the license basis MOE.

Ultimately, regulatory direction, via RAI, was for NAC to develop 3 brand new discrete thermal models to support a methodical discretization allowing performance of a Grid Convergence Index (GCI).

Why? Because NACs reported PCT was within 25C of the 400C limit If the PCT limit is really a safety limit and Risk Informed Regulation determines that reporting temperatures within some percent triggers the need to address uncertainties (i.e. GCI performance) then this should be clear to the licensee through regulation and not invoked through the RAI process.

After a long RAI process ultimately involving the GCI methodology, the associated costs to the project and delays in licensing resulted in a little over one degree F lower PCT for the three additional models.

19 NAC-STC High-Burnup Fuel Amendment (Contd)

NAC was had no choice but to develop 3 brand new discrete thermal models (utilizing 900K, 3000K, 7200K elements including mesh refinement in axial direction) to support a methodical discretization allowing development of a Grid Convergence Index (GCI)

Model ID Number of PCT (F) T (F)

Hexahedral Elements

Model No. 1 7,168,000 633 151 Model No. 2 3,024,000 632 151 Model No. 3 896,000 629 151 Base Model 95,672 638 153

Performance of this work required over 2000 additional man -hours, demonstrated no significant change in PCT and resulted in no changes to the design or loading configuration(s)

Proximity (~25F) of our calculated PCT, with no credit given for conservatisms in the model, to the PCT limit was used as the basis for requiring the GCI be performed

20 Benefits for the NRC

Improved clarity, efficiency, and reliability

- Graded approach to apply resources more efficiently and effectively Generic implementation of criteria change for broad adoption

- Reduce regulatory burden and review Recommendations to implement updated metrics into future NRC guidance

- Clarify limits and uncertainties

- Reflect better understanding of thermal margins into NRC guidance Risk-informed application of operational margin to be leveraged to:

- Decrease dose to the workers

- Decrease operational risks by eliminating unnecessary actions

Implementation

Potential Pathway

Fee waiver Gross Rupture TR SER Next: request NRC Review SRP / Guidance Gross Rupture Plant Procedure Up d ates

TR Up d ates

Future: PCT TR

Fee Case Study SER waiver Gap Data PCT TR NRC Review request Vendor CoC Amendment NRC Review Amendments Plant document updates

Timeline

September 2024: Fee Waiver Request Letter for If fee waiver for Gross Rupture TR GR approved Q1 2025: Gross Rupture TR Duration NRC review?

TBD: Fee Waiver Request Letter for PCT and Graded Approach TRIf fee waiver

for PCT approved TBD: PCT and Graded Approach TR

Summary

- The 5 PIRTs provide strong basis to improve regulatory efficiency, clarity and reliability for all stakeholders.

- Objective is to improve safety and efficiency:

Decrease dose to the workers Reduce accident precursor events (transportation)

Improve realism to optimize safety and efficiency

- Near term: Gross Rupture TR Fee waiver request Directly applicable recommendations to improve regulatory guidance

- Longer term: PCT and Graded Approach TR Increased PCT limit Graded approach Operational impacts

TOGETHERSHAPING THE FUTURE OF ENERGY

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