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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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Defining Used Fuel Performance Margins April 23, 2019

White Paper Concept Defining Used Thermal Fuel NEI White Paper Margin Source Term Margin

Current Understanding Performance

Methods Radiological Margin

Risk Insights Margin Risk-appropriate Regulatory

Guidance

Schedule / Prioritization Confinement Framework for Used Fuel Margin Storage and Transportation Criticality NRC Margin Alignment Roadmap*

in 2019 Moisture Margin Vendor Amendments

&/or Topicals NRC Approval PIRTs (as applicable) 2020 +

NRC Involvement EPRI Topicals 2019-20 NRC Approval 2020 +

Not all elements of the roadmap will be addressed in the same manner or at the same pace ©2018 Nuclear Energy Institute 2

White Paper Overview High 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. Conclusion

Introduction (1 of 2)

Background:

brief overview of the history that has led us to a highly conservative regime Initial DFS was going to be for <20 years so easy to be conservative (its only temporary)

Some limits were based on very conservative values (we know its 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 fees Also results in unnecessary requirements and restrictions on licensees

Introduction (2 of 2)

Objective: 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 real conditions and actual safety 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 safety

Summary of Performance Margins and Current Methods Margins Source term development conservatisms vs. real conditions Modeling approaches that lead to conservative results Allowable values (limits) that include significant conservatisms to real, scientifically based limits or thresholds

Risk Insights Use risk insights to inform the process Identify those areas that are more significant vs. those that are less significant to safety

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 things

Source Terms Review the conservatisms in how source terms are generated Inputs to source term codes How much accuracy in the fuel data (burnup, enrichment detail, power history) is adequate without being too conservative Realistic vs. bounding

Thermal: Modeling 1st step: Focus the effort Identify the inputs, modeling approaches/techniques that have large impact on the results, Use results to identify areas that dont have a large impact and hence dont 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

Thermal: Acceptance Criteria for PCT Work to develop a peak cladding temperature (PCT) limit that is based on more scientific information Currently in the US we are treating 400ºC as a knife edge limit This is not the case; for example it is reported in Europe a limit of 450ºC has been used

Radiological (1 of 3)

Modeling and Computation 1st step: define the parameters on which the effort should be focused

Identify the inputs, modeling approaches/techniques that have large impact on the results,

Use results to identify areas that dont have a large impact and hence dont 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 (2 of 3)

Acceptance Criteria for Fuel Qualification Work to develop Criteria for Fuel Qualification for radiological shielding

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 (3 of 3)

Criteria for Dose Rate Measurements / Compliance Develop Critical Parameters for Dose Rates

Average dose rates adequate for Off-Site Dose compliance

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

Criticality Control (1 of 3)

Analysis Methods PWR/BWR - Having to assume worst case geometric location of materials in conjunction with worst case tolerances on all components. In particular on tolerancing of 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 effort

Criticality Control (2 of 3)

Safety/Administrative Margin A 5% margin required is excessive when all normal/off-normal/accident worst case conditions have already been considered, including code uncertainty and bias.

Transport Moderator 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 (3 of 3)

Damaged Fuel In 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.

Materials The 75% or 90% credit on neutron absorber panels seems excessive given current fabrication controls and testing methods.

Confinement (1 of 2)

Understand design margin basis, material susceptibility, and aging management concerns Implement aging management programs that include inspections and repair/mitigation processes that preserve/restore margin Analytical assessment informs aging management programs to preserve confinement integrity Recognize time to address any identified degradation Understand potential operational challenges to confinement integrity Analytical assessment informs reasonable limits to prevent loss of integrity

Confinement (2 of 2)

NRC has already made progress in the area of containment thru issuance of several ISGs:

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

Moisture Current limit (0.43 gram mole in How dry do we need to be to NUREG-1536) derived from a avoid any issues?

test geared to understand how CNWRA report states dry a cask could get

5-55 moles (0.1-1.0 L) of PNL-6365 drying report water are insufficient to be a

3 of 4 tests loaded the cask corrosion concern with dry fuel (not from a

17.4 moles of water may be SFP) sufficient to reach

Wet-loaded canister took at flammability criteria (after >72 least 5 attempts and 1.5-3 years of radiolysis) days to dry HBU Demo cask measured 5.4 moles (maximum) of water after the drying process, or <0.1 liters of water

Applications (1 of 2)

Better understanding of source term margin Minimize impact on spent fuel management

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

Use it where its needed Improve off-site dose calculations

Reduce need for unnecessary loading restrictions

Reduce frequency of 72.212 updates for off-site dose More efficient use of resources for thermal licensing calculations

Applicant and regulator have more emphasis on safety by focusing on more impactful areas

Applications (2 of 2)

Better understanding of criticality margin Reduce B-10 requirements

Simplifies validation testing

Significant cost reduction Eliminate need to cycle SFP boron concentration

Current higher boron concentrations required for cask loading that needs to be diluted back down for plant operation

Schedule NEI White Paper

Vision

Conclusion Industry looks forward to NRCs feedback and continued dialogue in this effort to enhance used fuel safety by focusing resources on safety significant areas

ML19109A134

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