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Presentation Slides: Defining Used Fuel Performance Margins, NEI
	ML19109A134
Person / Time
Site:	Nuclear Energy Institute
Issue date:	04/19/2019
From:	Richter M; Nuclear Energy Institute
To:	Christian Jacobs; Spent Fuel Licensing Branch
	Jacobs C
References
	Download: ML19109A134 (24)
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Current Understanding

Methods*Risk Insights

Guidance*Schedule / Prioritization Thermal MarginRadiological MarginCriticality MarginConfinement MarginVendor Amendments &/or TopicalsEPRI TopicalsRoadmap*PIRTs (as applicable)

NRCAlignment in 2019 NRCInvolvement 2019-20 NRCApproval 2020 +NRCApproval 2020 +Risk-appropriate Regulatory Framework for Used Fuel Storage and Transportation*Not all elements of the roadmap will be addressed in the same manner or at the same paceDefining Used Fuel Performance MarginSource Term MarginMoisture Margin

©2019 Nuclear Energy Institute 3High Level TOC I.Introduction II.Summary of Performance Margins and Current Methods Margins III.Risk Insights IV.Guidance for Further Advancing the Definition of Performance Margin for Source Terms V.Guidance for Further Advancing the Definition of Performance Margin for Thermal VI.Guidance for Further Advancing the Definition of Performance Margin for Radiological VII.Guidance for Further Advancing the Definition of Performance Margin for Criticality VIII.Guidance for Further Advancing the Definition of Performance Margin for Confinement IX.Guidance for Further Advancing the Definition of Performance Margin for Moisture X.Applications XI.ConclusionWhite Paper Overview

Background:

brief overview of the history that has led us to a highly conservative regimeInitial DFS was going to be for <20 years so easy to be conservative ("it's only temporary")Some "limits" were based on very conservative values ("we know it's not a problem")Increasingly more information and detail being requested by NRC staff reviewers Result is overly conservative designs, and ever increasing costs for application preparation and NRC review feesAlso results in unnecessary requirements and restrictions on licenseesIntroduction (1 of 2)

©2019 Nuclear Energy Institute 5Objective: improve safety by focusing efforts on the most safety significant parameters, phenomenon, etc. thereby improving regulatory efficiency develop more accurate analytical models and analysis approaches, as well as more realistic, scientifically based limits, for source terms and thermal, radiological, criticality, and confinement safety analyses. accomplished through understanding and identifying the existing margin between realconditions and actualsafety limits and criteria, and then applying that knowledge in a risk informed manner Meet the regulatory requirement to provide reasonable assurance of adequate protection of public health and safetyIntroduction (2 of 2)

©2019 Nuclear Energy Institute 6Source term development conservatisms vs. real conditionsModeling approaches that lead to conservative resultsAllowable values (limits) that include significant conservatisms to real, scientifically based limits or thresholdsSummary of Performance Margins and Current Methods Margins

Focus our efforts and resources on the more significant issues

Less focus on those that have little safety significance

PIRT(s) may be utilized, as applicable, to identify the inputs and modeling approaches/techniques that have large impact on the results, More or less focus does not mean less safety, it means enhanced safety because we are focusing on the right thingsRisk Insights

©2019 Nuclear Energy Institute 8Review the conservatisms in how source terms are generatedInputs to source term codes How much accuracy in the fuel data (burnup, enrichment detail, power history) is adequate without being too conservativeRealistic vs. boundingSource Terms

©2019 Nuclear Energy Institute 91st step: Focus the effortIdentify the inputs, modeling approaches/techniques that have large impact on the results, Use results to identify areas that don't have a large impact and hence don't require scrutiny (i.e. a reasonable value can be assumed

)Identify areas for modeling simplificationDevelop a consensus

-based modeling approachConsider a best practices guide Thermal: Modeling

©2019 Nuclear Energy Institute 10Work to develop a peak cladding temperature (PCT) limit that is based on more scientific informationCurrently in the US we are treating 400ºC as a "knife edge" limitThis is not the case; for example it is reported in Europe a limit of 450ºC has been usedThermal: Acceptance Criteria for PCT

Identify the inputs, modeling approaches/techniques that have large impact on the results, *Use results to identify areas that don't have a large impact and hence don't require scrutiny (i.e. a reasonable value can be assumed)Identify areas for modeling simplification

Develop a consensus

-based modeling approach

Consider a best practices guide including reporting of results Radiological (1 of 3)

Currently in the US, Fuel Qualification for shielding is complex and requires demonstration by calculation

Dose Rate measurements / benchmarks can be employed to inform criteria. Radiological (2 of 3)

Average dose rates adequate for Off

-Site Dose compliance

Maximum dose rate / locations identified adequately and controlled (shielding) during operational evolutions Radiological (3 of 3)

©2019 Nuclear Energy Institute 14Analysis MethodsPWR/BWR -Having to assume worst case geometric location of materials in conjunction with worst case tolerances on all components.In particular on tolerancingof components a statistical treatment is justified and should be permissible.BWR -Fresh fuel is evaluated.Guidance should be available for BWR burnup credit.While NUREGs have been issued on this subject they contain a significant caution in that the quantity of information is limited and that further work is needed.This does not provide a clear path to a successful licensing effortCriticality Control (1 of 3)

-normal/accident worst case conditions have already been considered, including code uncertainty and bias.TransportModerator in the TSC under normal conditions.NRC interpretation of 71.55(b) unless exception is taking under 71.55(c).Packages are constructed and designed to not allow significant leakage (dispersal requirements under 71.51 limit the amount of material).Criticality Control (2 of 3)

©2019 Nuclear Energy Institute 16Damaged FuelIn the context of both storage and transportation system the assumption of "floating" fuel and optimum size/spacing introduces a significant conservatism.EPRI analysis have shown that a more "realistic" impact of fuel configuration as a result of damage is small

.MaterialsThe 75% or 90% credit on neutron absorber panels seems excessive given current fabrication controls and testing methods.Criticality Control (3 of 3)

©2019 Nuclear Energy Institute 17Understand design margin basis, material susceptibility, and aging management concernsImplement aging management programs that include inspections and repair/mitigation processes that preserve/restore marginAnalytical assessment informs aging management programs to preserve confinement integrityRecognize time to address any identified degradationUnderstand potential operational challenges to confinement integrityAnalytical assessment informs reasonable limits to prevent loss of integrityConfinement (1 of 2)

ISG-5, ISG-15 (supersedes ISG-4), ISG-18, ISG-19Specifically, the removal of helium leak testing of the closure welds (ISG-18) provided significant dose reductions during loadingAlso, interim pathway for licensing moderator exclusion in ISG

-19Confinement (2 of 2)

3 of 4 tests loaded the cask with dry fuel (not from a SFP)*Wet-loaded canister took at least 5 attempts and 1.5

-3 days to dryHow dry do we need to be to avoid any issues?CNWRA report states

5-55 moles (0.1

-1.0 L) of water are insufficient to be a corrosion concern

17.4 moles of water may be sufficient to reach flammability criteria (after >72 years of radiolysis) HBU Demo cask measured 5.4 moles (maximum) of water after the drying process, or <0.1 liters of water

Avoids unnecessarily delaying loading due to site dose budget More effective ALARA planning

Use it where it's neededImprove off

-site dose calculations

Reduce need for unnecessary loading restrictions

Reduce frequency of 72.212 updates for off

-site doseMore efficient use of resources for thermal licensing calculations

Applicant and regulator have more emphasis on safety by focusing on more impactful areasApplications (1 of 2)

Simplifies validation testing

Significant cost reductionEliminate need to cycle SFP boron concentration

Current higher boron concentrations required for cask loading that needs to be diluted back down for plant operationApplications (2 of 2)

ML19109A134

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