ML18236A507

NRC Accident Tolerant Fuel Project Plan Final Draft (August 2018)

ML18236A507
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Issue date:	08/24/2018
From:	Proffitt J NRC/NRR/DLP/PLPB
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Proffitt J
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OFFICIAL USE ONLY - SENSITIVE INTERNAL INFORMATION PROJECT PLAN TO PREPARE THE U.S. NUCLEAR REGULATORY COMMISSION FOR EFFICIENT AND EFFECTIVE LICENSING OF ACCIDENT TOLERANT FUEL Version 1.0 September 2018

PROJECT PLAN TO PREPARE THE U.S. NUCLEAR REGULATORY COMMISSION FOR EFFICIENT AND EFFECTIVE LICENSING OF ACCIDENT TOLERANT FUEL 1 INTRODUCTION ................................................................................................................. 3 2 BACKGROUND ................................................................................................................... 4 3 ACCIDENT TOLERANT FUEL LICENSING PROCESS ..................................................... 5 3.1 Milestone Schedule ........................................................................................................ 6 3.2 Assumptions ................................................................................................................... 7 3.3 New Fuel Technical Basis Development Process .......................................................... 7 3.4 Project Plan Paradigm.................................................................................................... 9 3.4.1 Old Paradigm .......................................................................................................... 9 3.4.2 New Paradigm ......................................................................................................... 9 3.4.3 Phenomena Identification and Ranking Table Exercises ...................................... 10 3.4.4 Concept-Specific Licensing Roadmaps ................................................................. 14 3.4.5 Additional Efficiencies ........................................................................................... 15 3.5 NRC Approach to Confirmatory Analysis ..................................................................... 15 4 STAKEHOLDER INTERACTIONS .................................................................................... 16 4.1 Meetings, Stakeholder Interactions, and Critical Skill Development ............................ 16 5 INITIATING STAFF ACTIVITIES ....................................................................................... 18 6 PREPARATORY ACTIVITIES ........................................................................................... 18 6.1 Task 1: Regulatory Framework, In-Reactor Performance ........................................... 18 6.2 Task 2: Fuel Cycle, Transportation, and Storage Regulatory Framework .................. 19 6.3 Task 3: Probabilistic Risk Analysis Activities............................................................... 19 6.4 Task 4: Developing Independent Confirmatory Calculation Capabilities..................... 19 6.4.1 Advanced Modeling and Simulation ...................................................................... 20 7 TASK 1: 10 CFR PART 50, 10 CFR PART 52, AND 10 CFR PART 100 REGULATORY FRAMEWORK, IN-REACTOR PERFORMANCE ............................................................. 22 7.1 Additional Considerations ............................................................................................ 24 7.2 Lead Test Assemblies .................................................................................................. 25 7.3 Initiating Activity ........................................................................................................... 25 7.4 Deliverables ................................................................................................................. 25 8 TASK 2: REGULATORY FRAMEWORK FUEL FACILITIES, TRANSPORTATION, AND STORAGE ......................................................................................................................... 26 1

8.1 Regulatory Infrastructure Analysis ............................................................................... 26 8.2 Facility, Transportation, and Storage Reviews ............................................................. 27 8.2.1 Fuel Fabrication Facility Reviews .......................................................................... 27 8.2.2 Unirradiated Fuel Transportation Package Reviews ............................................. 27 8.2.3 Irradiated Fuel Transportation Package and Storage Cask Reviews .................... 28 8.2.4 Potential Challenges ............................................................................................. 28 8.2.5 Lead Test Assemblies ........................................................................................... 30 8.2.6 Initiating Activity ..................................................................................................... 30 8.2.7 Anticipated Regulatory Actions ............................................................................. 30 9 TASK 3: PROBABILISTIC RISK-ASSESSMENT ACTIVITIES ........................................ 31 10 TASK 4: DEVELOPING INDEPENDENT CONFIRMATORY CALCULATION CAPABILITIES .................................................................................................................. 36 11 PATH FORWARD ............................................................................................................. 38 Appendix A NRC PLANS TO DEVELOP ANALYSIS CAPABILITY ...................................... 39 Task 4.a: Fuel Performance ................................................................................................... 39 Task 4.b: Thermal-Hydraulics ................................................................................................ 43 Task 4.c: Neutronics ............................................................................................................... 46 Task 4.d: Source Term ........................................................................................................... 50 2

1 INTRODUCTION The U.S. Nuclear Regulatory Commission (NRC) is committed to enabling the safe use of new technologies, especially those that can increase the safety of NRC-regulated facilities. The U.S.

nuclear industry, with the assistance of the U.S. Department of Energy (DOE), plans to deploy batch loads of some accident tolerant fuel (ATF) designs in the operating fleet on an aggressive timeline (by the early to mid-2020s). The NRC is optimistic that its preparation strategy and new paradigm of fuel licensing outlined in this project plan will support that schedule while still providing reasonable assurance of public health and safety at U.S. nuclear power plants. The NRC understands that it may face challenges in its preparations and technical and licensing reviews, but it is committed to working through such challenges in a thoughtful and deliberative manner.

In an attempt to increase regulatory stability and certainty along with enhancing and optimizing NRC review, the staff has developed this plan, which includes a vision for a new paradigm for ATF licensing. The staff believes that adherence to this strategy will benefit all the agencys stakeholders in the planned deployment of ATF designs.

The NRC staff has extensively engaged with its stakeholders in the development and finalization of this project plan, consistent with the NRCs principles of good regulation and statutory requirements. The staff has held numerous public meetings with external stakeholders, including licensees, nuclear fuel vendors, industry groups, nongovernmental organizations, and international counterparts. The staff has found these interactions invaluable and has considered the views and comments of the NRCs stakeholders in finalizing this version of the ATF Project Plan. A separate, companion document to this plan compiles the staffs resolution of the feedback received during the official public comment period and is available at ADAMS MLxx.

The project plan presents the high-level strategy that the staff will follow to ensure that it is ready to receive ATF topical reports (TRs) or licensing actions for review. At this point, the strategy is concept and technology independent. Concepts are defined as a family of ATF designs with largely similar characteristics. Examples include chromium (Cr)-coated zirconium (Zr) alloy claddings, steel claddings, silicon carbide (SiC) claddings, or metallic fuels. Individual vendors may implement variations within each concept as specific technologies. For example, the different methods that are used to apply Cr coatings would be identified as separate technologies.

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2 BACKGROUND In a coordinated effort under the direction of the NRCs ATF steering committee, the Office of Nuclear Reactor Regulation (NRR), Office of New Reactors (NRO), Office of Nuclear Material Safety and Safeguards (NMSS), and Office of Nuclear Regulatory Research (RES) are preparing for the licensing and use of ATF in U.S. commercial power reactors.

Figure 2.1 The NRCs ATF steering committee In coordination with DOE, several fuel vendors have announced plans to develop and seek approval for various fuel designs with enhanced accident tolerance (i.e., fuels with longer coping times during loss of cooling conditions). The designs considered in the development of this plan, both within and outside of the DOE program, include coated Zr claddings, doped uranium dioxide (UO2) pellets, iron-chrome-aluminum-based (FeCrAl) cladding, SiC cladding, uranium silicide (U3Si2) pellets, and metallic fuels (e.g., Lightbridge). For these ATF designs, the industrys stated schedules for the initial irradiation of lead test assemblies (LTAs) and the review of TR and license amendment requests (LARs) were used as a basis for the timelines discussed in this plan.

This project plan covers the complete fuel cycle, including consideration for the front and back ends (e.g., fabrication, transportation, and storage), and outlines the strategy for preparing the NRC to license ATF designs with a focus on the preparation, review, and approval of TRs. It also identifies the lead organization for each planned activity. The plan only briefly touches on existing licensing activities, such as the TR process, the implementation of LTA programs, and the LAR process, as such activities follow existing processes that have well-established schedules and regulatory approaches or are being clarified through NRC initiatives outside of the ATF steering committee and working group.

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In preparing the agency to conduct meaningful and timely reviews of these advanced fuel designs, the NRC is reviewing the existing regulatory infrastructure and identifying needs for additional analysis capabilities and developing unique critical skillsets within the staff. The NRC has entered a memorandum of understanding (MOU) with DOE to coordinate on the nuclear safety research of ATFs that will make the appropriate data available for regulatory decision-making processes. In addition, the NRC has established an MOU with the Electric Power Research Institute (EPRI) to facilitate data sharing and coordination on expert elicitation.

For the purpose of developing this plan, ATF concepts are broadly categorized as near term and longer term. The plan considers near-term ATF concepts as those for which the agency can largely rely on existing data, models, and methods for its safety evaluations (SEs). Coated Zr cladding, FeCrAl cladding, and doped UO2 pellets are a few examples of near-term ATF concepts. In general, the industry is pursuing these near-term concepts for deployment by the early to mid-2020s. Longer term ATF concepts are those for which substantial new data, models, and methods need to be acquired or developed to support the agencys SEs. U3Si2 fuel, metallic fuel, and SiC-based cladding are a few examples of longer term ATF concepts.

Note that near term and longer term are terms of convenience used to indicate the current expected deployment timeframe for the ATF concept.

Regulatory requirements do not vary between near-term and longer-term concepts, and the NRC will evaluate all designs based on their individual technical basis. The timeline for licensing will be commensurate with the deviation of the ATF technology from the current state of practice and the number and complexity of issues related to phenomena identified during an expert elicitation process (e.g., a phenomena identification and ranking table (PIRT)).

The agency is focusing its current ATF licensing preparation on the use of ATF in light-water reactors (LWRs) in the operating fleet. Some overlap may occur between LWR ATF fuel development and fuel safety qualification of some types of non-LWR fuels for advanced reactor designs. As appropriate, the NRC will leverage previous experience to help optimize licensing efficiency and effectiveness.

This project plan will be a living document that may evolve as (1) ATF concepts are more clearly defined, (2) schedules are refined, (3) the knowledge level of specific concepts increases as experimental testing programs are completed, and (4) potential extensions to the current operating envelope of fuel are identified.

3 ACCIDENT TOLERANT FUEL LICENSING PROCESS This project plan focuses on the NRCs preparations to conduct efficient and effective reviews of TRs for ATF designs on a schedule consistent with published industry timelines. TRs provide the generic safety basis for a fuel design and do not, by themselves, grant approval for operating plants to begin loading ATF. These reviews for new fuel designs have historically taken between 2 and 3 years to complete. Based on past experience, vendors should also anticipate that the NRCs Advisory Committee on Reactor Safeguards may request to review ATF TRs and should include time for such reviews in their planning and schedules.

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In addition, the licensee would then need to submit an LAR to modify its license to allow for the use of a new fuel design. LARs address all plant-specific aspects of implementing the ATF concept and are typically completed on a 1-year review schedule.

Upon final approval of the plant-specific LAR, a licensee would be authorized to load and irradiate batch quantities of the specific ATF design in accordance with its license.

3.1 Milestone Schedule Table 3.1 outlines some of the high-level milestones and associated dates related to implementation of the ATF project plan. These dates are based on the staffs current state of knowledge at the time of publishing.

Table 3.1 ATF Milestone Schedule Milestone/Activity Schedule Complete ATF project plan. Summer 2018 Issue final LTA guidance. Early 2019 Conduct PIRTs and refine review infrastructure for near-term concepts. 2019-2020 Identify and implement adjustments to the regulatory infrastructure, if necessary, to enable the full potential of ATF (e.g., increase 2019-2025 enrichment, increase burnup).

Conduct TR reviews for near-term concepts. 2020-2022 Conduct LAR reviews for near-term concepts. 2022-2023 TBD (in accordance with Conduct LTA/TR/LAR reviews for longer term concepts.

industry schedules) 6

3.2 Assumptions Given the current uncertainty related to the development and deployment of ATF designs, the NRC staff made the following major assumptions to help in its development of this plan:

• The NRC will not need to perform independent confirmatory testing for specific ATF designs. The NRC expects that the applicant, DOE, or other organizations will provide the agency with all data needed to support the safety basis for a concept. Additionally, the NRC expects that all reactor and test-generated fuel behavior data will be provided to the agency in a timely manner so that it can assess NRC analysis capabilities.

• Interaction with DOE, EPRI, vendors, and other organizations involved in ATF-related experimental programs will take place in real-time and, whenever possible, in advance of experiments being conducted.

• The NRCs interactions with external stakeholders will keep the staff and stakeholders informed about both technical and programmatic developments that are affecting activities identified in this project plan.

3.3 New Fuel Technical Basis Development Process The development of the technical basis necessary to qualify a new fuel design is an extensive process, both in terms of resources and time. Figure 3.1 depicts the basic steps applicants take toward obtaining the appropriate data and experience needed to license a new fuel design for batch loading (typically one-third of the fuel assemblies in the reactor core per cycle) at a U.S. commercial nuclear power plant.

Figure 3.1 New fuel design technical basis development process Figure 3.1 depicts the relative order of testing needed to develop the technical basis for a new fuel design, including ATF. The first box in orange (unirradiated materials testing) represents the testing necessary to characterize the material, mechanical, chemical, and thermal properties of a new design. The second box in pink (test reactor irradiations and testing) represents the vendors characterization of the evolution of those properties obtained in the first box with irradiation and time spent in the reactor. The third box in yellow (LTA irradiations and testing) provides the integral testing to fully characterize the fuel in prototypical operating conditions and donor material for the next step. The fourth box in green (transient irradiations and testing) is focused on the use of fuel segments harvested from LTAs to perform tests that mimic transient and accident conditions. Such tests are key to ensuring safety. The fifth box in light blue (source term and other nonfuel performance testing) happens concurrently with these four 7

tasks. It includes testing to characterize fission product release, core melt progression, core relocation, and mechanical and chemical interactions. Finally, the sixth box in dark blue (updates to analyses of record) involves the development, calibration, verification, and validation of analytical models to simulate the performance of the new fuel design under normal and accident conditions. This also requires the quantification of uncertainties and the definition of an application methodology.

Even under ideal circumstances, the time and effort required to fully develop the technical basis are substantial largely because of the irradiated testing needed to fully understand and characterize how a design or material acts under steady-state, transient, and accident conditions. Understanding these characteristics and being able to model them appropriately represents the critical path to the licensing of ATF.

Advancements in instrumentation and examination equipment used to collect data from irradiated fuel may allow for information to be gathered either more quickly following irradiation or at a higher fidelity than in the past. Progress in this area could expedite the timeline required to document the experimental and testing data required to fully support the technical basis for an ATF concept. Staff will maintain engagement with industry progress in this area to ensure preparation for new or novel applications.

The NRC understands that vendors and organizations participating in the development of the technical basis for ATF designs could possibly leverage advanced modeling and simulation capabilities to expedite the development timeline. Insights gained from mechanistic computational tools could improve testing programs and thus limit or eliminate failed experiments. This could allow for data that support the technical basis and ultimately an TR to be gathered more quickly.

The staff is currently aware of the latest developments in modeling and simulation credibility, as reflected in the literature and in standards on the topic, which echo the need to perform experimental testing. The staff does recognize the need to maintain engagement and continues to monitor state-of-the-art advances that could potentially contribute to substantial timeline constrictions, especially for longer term concepts.

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3.4 Project Plan Paradigm This project plan envisions an improved fuel licensing paradigm, depicted in Figure 3.2, that can increase the efficiency and effectiveness of the NRCs review of ATF designs.

Figure 3.2 ATF project plan new paradigm 3.4.1 Old Paradigm In the old paradigm, the regulatory infrastructure was essentially developed before, or soon after, commencement of technical basis development activities, such as experimental testing programs, because, over the past several decades, most LWR new fuel designs and characteristics have been relatively minor changes within the UO2-Zr alloy fuel matrix.

Licensing activities would then commence upon completion of the development of the technical basis, with little or no interaction with the NRC staff along the way. This lack of interaction has produced several issues, including cases in which additional testing that required irradiation was necessary before NRC approval, which caused extensive delays.

3.4.2 New Paradigm Recent initiatives related to ATF are leading to the largest potential departures from the basic UO2-Zr alloy fuel matrix, which has been used in U.S. and foreign reactors over the past 50 years. This potentially transformational technology that industry is pursuing has led the staff to reflect on the NRCs fuel licensing process and determine where improvements to efficiency and effectiveness can be gained. The goal of this new paradigm is to enhance regulatory stability and add efficiency to the timeline required for licensing activities following the completion of the technical basis to support an ATF design.

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As illustrated in Figure 3.2, the schedule efficiency enabled by the new paradigm requires the staff to conduct thorough and meaningful PIRT exercises for each ATF concept and maintain the results of the PIRT as the collective state of knowledge evolves. The outcome of the ATF PIRT process, elaborated on below, will allow the staff to refine the regulatory infrastructure, as needed, for each concept and facilitate the development of concept-specific licensing roadmaps. These activities will proceed in parallel with the continued development of the full technical basis by the vendor.

Refinement of the regulatory infrastructure will be done in real-time and with significant communication with agency stakeholders to maintain transparency and clearly communicate regulatory expectations to the vendors as early as possible in the process. In addition, the new paradigm allows for licensing activities in the form of TRs to proceed in parallel with the completion of the technical basis for a specific concept. Data sharing and close engagement with the vendor during this time will be critical in gaining the efficiency depicted in Figure 3.2.

The staff will need to perform significant amounts of prework to prepare for and conduct the most efficient reviews of ATF LTRs.

The overall goal of this strategy is to develop and communicate the NRCs expectations for the technical basis of specific ATF concepts in real-time to minimize the lag between the completion of the technical basis and the licensing of ATF designs to their full potential.

3.4.3 Phenomena Identification and Ranking Table Exercises As stated above, the success of the strategy outlined in the project plan requires the staff to conduct thorough and meaningful PIRT exercises for each concept and maintain the results of the PIRT as the collective state of knowledge for each concept is advanced. For the purpose of this project plan, the term PIRT is defined as an expert elicitation process in which panelists will identify and rank new phenomena important to safety introduced by an ATF concept. The staff imagines that these exercises will vary greatly in scope and depth based on the departure of the concept from the current state of practice and the maturity of the concept. The experts selected for the PIRT panel must consider the full intended use of the concept to ensure that the PIRT results are meaningful even if initial licensing applications do not intend to seek credit for the enhanced capabilities of the concept. Lack of consideration of the full operating envelope in the initial PIRT exercise could lead to uncertainty further along in the process when a vendor or licensee does seek to credit those capabilities.

The NRC staff relies on the agencys significant expertise in the Zr-clad UO2 fuel system during the review of current fuel licensing submittals. However, the staff does not necessarily have this same level of knowledge for all the ATF concepts that industry is currently pursuing. The NRC staff is monitoring the literature and experimental testing programs conducted in the public domain and is participating in industry and DOE update meetings on ATF concept development.

However, more in-depth expertise may be needed to support the efficient and effective review of ATF licensing submittals. PIRT exercises will allow the staff to benefit from external expertise to identify phenomenon important to safety for each concept and, therefore, to refine the regulatory 10

framework that is necessary for a concept ahead of licensing submittals and that will serve as baseline guidance for the NRCs technical review.

In addition to concept-specific PIRTs, discipline-specific PIRTs may be useful in some cases.

Examples considered to date include PIRTs in the areas of severe accidents, storage and transportation, burnup above 65 gigawatt-days per metric ton of uranium and enrichment above 5 weight percent. The experts necessary to identify and evaluate new phenomena important to safety in these areas should be the same or similar experts for all or many of the ATF concepts under development. Therefore, the NRC staff believes that it would be more efficient to conduct these PIRTs in a discipline-specific manner instead of as part of the concept-specific exercises.

For each concept, a portion of the information needed to examine the phenomena important to safety is generic to the concept and publicly available, whereas a portion of the information is proprietary and specific to a fuel vendors technology. Figure 3.3 illustrates this relationship.

Figure 3.3 ATF concept information map For concept-specific PIRTs, the series of events will generally follow the sequence depicted in Figure 3.4, whereby an expert panel collects, synthesizes, and reviews publicly available information pertinent to the concept. In this model, information specific to a vendors technology is reviewed separately after the results of the PIRT exercise have been documented and as far ahead of the submittal of TRs as possible.

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Figure 3.4 Generic ATF PIRT exercise timeline The NRC intends to begin a pilot PIRT exercise on Cr-coated cladding in 2018. Because of the maturity of the Cr-coated cladding concept and its relatively minor departure from currently licensed fuel systems, the NRC staff envisions a targeted-scope expert elicitation process that focuses on new phenomena introduced by applying a thin coating to a known substrate (e.g., potential delamination effects, potential dissimilar metal effects, and potential phase changes in the materials). In addition, the NRC acknowledges that an industry effort to produce a white paper on Cr-coated cladding will be an important part of the information collected and synthesized in the first phase of the PIRT process and may aid in expediting the process. Given these factors, Figure 3.5 depicts the planned timeline for a Cr-coated cladding PIRT exercise.

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Figure 3.5 Planned Cr-coated cladding PIRT exercise timeline The NRC will develop timelines for subsequent ATF PIRT exercises and additional implementation details through coordination with its external stakeholders. The NRC will provide details of this information in an update to this project plan or in an appended document as it completes the pilot exercise for Cr-coated cladding.

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3.4.4 Concept-Specific Licensing Roadmaps Given that this project plan is high level and concept independent, it will be augmented by concept-specific licensing roadmaps. The staff will use the outcome of the expert elicitation process completed for each ATF concept and the strategy outlined in this plan to create these roadmaps, which will be made publicly available as addenda to this plan. The roadmap for each concept will outline a pathway for vendors to ensure that the technical basis, which they will submit to the NRC in a TR, will meet the staffs expectations. These roadmaps will identify any gaps or deficiencies in the regulatory framework for an ATF concept and will clearly note where the current framework applies and is sufficient. The NRC will develop the roadmaps in a transparent manner with opportunities for stakeholder feedback before their finalization.

The vendor will then take these roadmaps and develop, or modify, their strategy for licensing each specific ATF technology. Figure 3.6 shows this concept.

Figure 3.6 ATF concept-specific licensing roadmap The staff will assess the results of the concept-specific expert elicitation process against the current licensing and regulatory framework (e.g., Title 10 of the Code of Federal Regulations (10 CFR); NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition (SRP); regulatory guides (RGs); and NUREG reports) to determine which regulations and guidance remain applicable and to identify (1) the areas for which an applicant would need to propose new acceptance criteria or (2) an alternative means 14

for meeting the regulations. The NRC will develop these roadmaps within 6 to 12 months of completion of the concept-specific PIRT.

The staff plans to complete these roadmaps sufficiently ahead of TR submittals so that vendors are able to ensure that their planned experimental testing programs will adequately address the NRCs needs for approving the design.

3.4.5 Additional Efficiencies The staff is exploring additional innovative and transformative ideas and solutions to further improve the agencys approach. A few examples that the NRC is currently pursuing include the following:

• development of a standard TR change process

• use of DOE and advanced computational capabilities

• expedited issuance of NRC guidance In addition, the staff welcomes any additional suggestions from stakeholders of other potential efficiencies that could aid NRC in increasing the efficiency and effectiveness of its preparation and reviews of ATF designs.

3.5 NRC Approach to Confirmatory Analysis The staffs approach to independent confirmatory analysis of TRs and LARs follows a graded approach. This approach varies based on the complexity of the application, the safety significance of the issues presented, and the uncertainty of the key phenomena involved.

The NRC uses a range of tools to verify the safety case made and presented by an applicant.

In some instances, the staff can perform its confirmatory analysis and reach a safety determination by drawing on previous knowledge, accumulated expertise, and the information presented by the applicant. In other cases, confirmatory calculations performed by the staff can allow for a more effective and efficient review. The staff typically performs independent confirmatory calculations to assist in reaching a safety finding in reviews where the uncertainties are large, or the margin is small. In some cases of large safety significance and large uncertainty, the NRC has pursued independent confirmatory testing before reaching a determination on an application. Ultimately, the staff bases its safety finding on the technical basis and safety case provided by the applicant. Confirmatory analyses performed by the staff provide increased confidence in the applicants results.

The NRCs approach for most ATF designs will likely follow the middle approach of performing independent confirmatory calculations to assist the staff in reaching a safety determination. In support of this, the staff is actively enhancing the NRCs suite of computational analysis tools and engaging DOE to understand the capabilities of its codes and methods.

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4 STAKEHOLDER INTERACTIONS Table 4.1 outlines key meetings and interactions scheduled during the development and review of ATF designs. The primary risks to timely licensing of ATF relate to current uncertainties in the schedules for necessary experimental programs. The staff intends to remain closely engaged with the organizations and entities acquiring data and adjust the plan as new information becomes available. The staff is closely following ongoing efforts to identify alternatives for testing that the shutdown of the Halden Research Reactor in Norway will affect.

Another potentially significant risk to the successful implementation of ATF is a delayed recognition that changes to the regulations or regulatory guidance are required. The staff has initiated dialogue with stakeholders to communicate timelines required for various modifications to the regulatory infrastructure and to solicit input for changes that may be necessary for the different ATF concepts being explored.

4.1 Meetings, Stakeholder Interactions, and Critical Skill Development The NRC is committed to engaging in industry project update meetings and supporting staff participation in experimental program discussions to maintain awareness of industry and DOE efforts and to prepare for regulatory reviews. The NRC will develop staff and contractors with critical skills required to support projected applications of high to moderate certainty. All stakeholder interactions will occur in accordance with the NRCs public meeting policy.

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Table 4.1 Meetings and Stakeholder Interactions Meeting Frequency Desired Outcome EPRI/DOE/Idaho National Biannually Assess the technical progress of ATF Laboratory (INL) update research and development (R&D).

meetings Obtain information necessary for developing analytical capabilities and licensing strategies.

TOPFUEL (rotates between Annually Assess the technical progress of ATF R&D.

the United States, Europe, Obtain information necessary for developing and Asia) analytical capabilities and licensing strategies.

ATF standards and guidance Annually Discuss licensing approach with international development activities with counterparts.

the Organisation for Economic Co-operation and Development /Nuclear Energy Agency, International Atomic Energy Agency, and international counterparts Fuel vendor update meetings Annually Assess the technical progress of ATF R&D.

(rotates from NRC (per vendor)

Obtain information necessary for developing Headquarters to the vendors analytical capabilities and licensing strategies headquarters)

(in addition to a number of other non-ATF outcomes).

ATR/TREAT test planning Annually Develop an understanding of testing that will and test observation characterize the performance characteristics meetings of ATF designs.

ATF fuel fabrication facilities As needed Develop an understanding of manufacturing tour and audit processes and obtain information for developing licensing strategies.

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5 INITIATING STAFF ACTIVITIES Because of design-specific aspects and schedules, the NRCs activities are linked to the industrys progress and plans to deploy ATF. For this reason, the agency must have a mechanism for communicating schedules and resource needs in advance of licensing activities.

The staff is currently exploring the issuance of a generic communication (i.e., a regulatory issue summary (RIS)) to solicit information from industry on schedules and plans for ATF designs.

The NRC expects the ATF RIS to be very similar to previously issued agency RISs on advanced reactors (Agencywide Documents Access and Management System (ADAMS) Accession No. ML17262B022) and molybdenum-99 production facilities (ADAMS Accession No. ML13078A385).

This plan provides estimated lead times for each activity associated with preparing the agency to conduct an effective and efficient licensing reviews of ATF LTRs. As the NRC staff gains more experience with these reviews, it will adjust lead times to account for difficulties or efficiencies, as necessary. These lead times dictate the amount of time ahead of licensing activities that data should be provided and a formal communication of intent should be made through a response to the RIS, presubmittal meetings, or other formal interaction with the staff.

6 PREPARATORY ACTIVITIES The staff has grouped its preparatory activities into four tasks. The highlights of each task are briefly described below; subsequent sections describe these tasks in full detail.

6.1 Task 1: Regulatory Framework, In-Reactor Performance

• Participate in coordinated PIRT exercises on in-reactor degradation mechanisms and failure modes under a wide array of accident conditions, performance-based metrics, and analytical criteria to ensure acceptable performance.

• Perform a scoping study to (1) evaluate the applicability of existing regulations and guidance for each ATF design, (2) identify changes to, or the need for, new regulations and guidance, and (3) identify any key policy issues.

• Identify consensus standards that need to be updated for ATF and participate in the update process where appropriate.

• Determine and clarify the regulatory criteria that need to be satisfied for partial or full core use of ATF and the regulatory options available to applicants and vendors.

• If needed, resolve policy issues and initiate rulemaking and guidance development activities.

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6.2 Task 2: Fuel Cycle, Transportation, and Storage Regulatory Framework

• 10 CFR Part 70, Domestic Licensing of Special Nuclear Materials; 10 CFR Part 71, Packaging and Transportation of Radioactive Material; and 10 CFR Part 72, Licensing Requirements for the independent Storage of Spent Nuclear Fuel and High-Level Radioactive Waste, and Reactor-Related Greater Than Class C Waste, are largely performance based; therefore, the staff does not anticipate identification of gaps or deficiencies in these regulations.

• Gaps in the review guidance may develop as the fuel cycle industry develops plans for manufacturing, transporting, and storing ATF. The NRC will monitor the fuel cycle industrys plans and identify and develop any necessary regulatory guidance in a timely manner.

• The NRC has not identified any fuel cycle facility licensing activities for near-term ATF concepts.

• The NRC has identified two reviews of LTA transportation casks for near-term ATF concepts.

6.3 Task 3: Probabilistic Risk Analysis Activities

• The staff will evaluate how industry batch loading of ATF may affect the current risk-informed programs such as risk-informed technical specification initiatives 4b and 5b.

• The NRCs risk-informed oversight activities (e.g., the significance determination process) depend on standardized plan analysis risk (SPAR) models, which may need to be updated to reflect the batch loading of ATF.

6.4 Task 4: Developing Independent Confirmatory Calculation Capabilities The NRCs independent confirmatory analysis of TRs and LARs follows a graded approach, which varies based on the complexity of the application, the safety significance of the issues presented, and the uncertainty of the key phenomena involved. Further, independent confirmatory analysis does not always require independent confirmatory calculations using NRC-developed tools. In some cases, the staff can perform its confirmatory analysis and reach a safety determination by drawing on previous knowledge, accumulated expertise, and the information presented by the applicant. For many incremental changes in fuel design, independent confirmatory calculations using NRC-developed tools were not necessary. The NRC typically performs independent confirmatory calculations to review cases in which adequate margin is essential to safety.

For initial ATF licensing for which limited data will be available to formulate and validate models, independent confirmatory calculations will likely be needed. In these instances, the staff that performs the confirmatory calculations must have a clear understanding of the assumptions and limitations of the analytical tools that it uses. The staff has experience using both NRC-developed and non-NRC codes for confirmatory calculations. In either case, to use a 19

code for confirmatory calculations, the staff must understand the range of conditions for which the code has been validated and understand the nature of the validation database.

The staff will take the most efficient and effective approach to verify the safety case presented for each ATF concept. When confirmatory calculations are warranted, the staff will select an approach that weighs several factors. These factors include the level of effort necessary to modify and validate existing NRC codes and the level of effort needed to understand and validate a non-NRC code. For example, for ATF concepts that use coatings on a Zr-based alloy, little effort is required to modify existing codes to model coating performance, whereas a significant effort is required to adopt a new code. In addition, the selected approach will consider the review schedule and the required models for each ATF concept. The concept-specific licensing roadmaps will elaborate on the selected approach for each ATF concept.

Based on the information available to date, the staff believes it will be more efficient and effective to pursue relatively minor modifications to existing NRC codes to model near-term ATF fuel concepts. The NRC has specifically tailored its codes to evaluate regulatory requirements and phenomena important to safety and has extensively validated them. These features make them easy for the staff to use and give the staff high confidence in the results that they provide.

At this time, the NRC plans to modify those codes that are developed to analyze fuel performance, thermal hydraulics, neutronics, and severe accidents and source terms. In addition, the agency is considering modifying existing NRC codes to model more long-term ATF fuel concepts in cases that require minimal effort.

Where possible, the NRC will coordinate with DOE to reduce duplication of effort in accordance with the DOE-NRC MOU.1 6.4.1 Advanced Modeling and Simulation NRC staff maintain an awareness of the advancements in modeling and simulation for nuclear applications. Staff have participated in training sessions for a number of DOEs advanced modeling and simulation tools and is continuing to learn about the capabilities of these tools. A recent targeted effort to couple the NRCs TRACE thermal-hydraulic code with DOEs BISON fuel performance code through MOOSE, an independent solver, has given the staff a significantly greater understanding of DOEs codes. The staff has also successfully combined elements of DOEs codes developed under the Consortium for the Advanced Simulation of Light-Water Reactors program into the NRCs neutronics code, SCALE. These efforts have demonstrated that specific opportunities exist to leverage elements of DOEs codes to improve the NRCs analysis capabilities. The staff expects to continue to follow DOEs development efforts in the area of advanced modeling and simulation and to search for opportunities to leverage their capabilities.

1 The 2017 ATF addendum to the NRC-DOE MOU appears in the NRC Library under Document Collections and Memorandum of Understanding at https://www.nrc.gov/reading-rm/doc-collections/memo-understanding/2017/.

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The staff is aware of efforts to use advanced modeling and simulation in a variety of applications or families of codes: mechanistic codes, steady-state codes, and transient codes. Although advanced modeling and simulation in mechanistic codes can inform experimental programs, improve upon highly empirical correlations, and identify testing priorities, current advanced modeling and simulation tools do not appear to be mature enough to substitute modeling for experiments because of the complex nature of fuel and reactor behavior. Further, the state of knowledge in many areas still only permits semi-empirical modeling of key phenomena.

Validation of these tools against relevant data will be essential to demonstrate their potential to support licensing activities. The staff will continue to coordinate with DOE and the national laboratories to better understand the capabilities of the DOE codes to potentially reduce the number of time-consuming and costly experiments and demonstrations.

Figure 6.1 Example applications and use of code families in the area of fuel performance At this time, the staff has had no indication from fuel vendors that they intend to use advanced modeling and simulation with atomic-scale, mechanistic modeling to support license applications. However, these vendors have indicated interest in using these tools to design better experiments and inform model development in higher level codes used for licensing.

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7 TASK 1: 10 CFR PART 50, 10 CFR PART 52, AND 10 CFR PART 100 REGULATORY FRAMEWORK, IN-REACTOR PERFORMANCE ATF presents new and unique technical issues that current guidance, review plans, and regulatory criteria for UO2-Zr-based nuclear fuel may not readily address. To prepare the agency to conduct meaningful and timely licensing reviews of ATF designs, well-developed and vetted positions on potential policy issues that may arise during ATF licensing are needed.

These positions must be communicated to stakeholders clearly and early.

This plan contemplates two distinct ATF activities that may require changes to the regulatory framework: (1) batch loading of ATF into NRC regulated power plants and (2) crediting the safety enhancements of ATF in the licensing basis of NRC regulated power plants. The regulatory framework changes that may be necessary for each of these activities are likely to be different, and the staff anticipates that such changes will need to be made to address batch loading before making changes needed to credit the safety enhancements of ATF in the licensing basis.

The degree to which existing regulations and guidance are affected and in need of revision, or new regulatory requirements established and new guidance developed, depends on the level of departure from existing fuel designs. The regulations at Appendix A, General Design Criteria for Nuclear Power Plants, to 10 CFR Part 50, Domestic Licensing of Production and Utilization Facilities, provide principle design and performance requirements. The general design criteria (GDC) listed in Table 7.1 relate to fuel design and overall fuel performance under normal and accident conditions. Additional GDC may be affected if ATF performance becomes more challenging for the control or protection systems that ensure acceptable consequences under accident conditions. For each ATF design, the staff plans to map the hazards and failure mechanisms to the design and performance criteria of the GDC to determine the appropriate applicability and potential need for additional criteria.

Note that loading an ATF design in a specific plant will ultimately need to meet relevant plant specific criteria. This is especially important for those reactors in the United States that were licensed before the issuance of the GDC (about 40 percent of the operating plants).

Table 7.1 Potentially Affected GDC GDC No. Title 1 Quality Standards and Records 2 Design Bases for Protection against Natural Phenomena 10 Reactor Design 11 Reactor Inherent Protection 12 Suppression of Reactor Power Oscillations 13 Instrumentation and Control 20 Protection System Functions 22

GDC No. Title 25 Protection System Requirements for Reactivity Control Malfunctions 26 Reactivity Control System Redundancy and Capability 27 Combined Reactivity Control Systems Capability 28 Reactivity Limits 34 Residual Heat Removal 35 Emergency Core Cooling 61 Fuel Storage and Handling and Radioactivity Control 62 Prevention of Criticality in Fuel Storage and Handling Even if a particular ATF design is unable to demonstrate verbatim compliance, the intent of these principle design and performance requirements must be satisfied or new requirements developed.

In addition to the GDC, the use of ATF may affect the regulations related to fuel design and performance listed in Table 7.2. For each ATF design, the staff plans to map the hazards and failure mechanisms to these requirements to determine whether any changes are necessary.

Table 7.2 Potentially Affected Regulations Regulation Title (10 CFR) 20 Standards for Protection against Radiation 50.34 Contents of Applications; Technical Information 50.46 Acceptance Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors 50.67 Accident Source Term 50.68 Criticality Accident Requirements Part 50, Quality Assurance Criteria for Nuclear Power Plants and Fuel Appendix B Reprocessing Plants Part 50, ECCS Evaluation Models Appendix K Part 50, Earthquake Engineering Criteria for Nuclear Power Plants Appendix S Part 100 Reactor Site Criteria The regulatory guidance documents listed in Table 7.3 contain fuel-related information. For each ATF design, the staff plans to map the hazards and failure mechanisms to the guidance documents to determine what, if any, changes are necessary.

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Table 7.3 Potentially Affected Guidance Guidance Title Document NUREG-0630 Cladding Swelling and Rupture Models for LOCA Analysis NUREG-0800 Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition RG 1.157 Best-Estimate Calculations of Emergency Core Cooling System Performance RG 1.183 Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors RG 1.195 Methods and Assumptions for Evaluating Radiological Consequences of Design Basis Accidents at Light-Water Nuclear Power Reactors RG 1.203 Transient and Accident Analysis Methods 7.1 Additional Considerations Aspects of ATF designs or implementation strategy such as the following could expand the scope, level of complexity, and schedule of the staffs review:

• an increase in uranium-235 enrichment, uranium-235 density, or fuel burnup beyond current limits for batch loading of ATF

• characterization of fission product release (e.g., chemical forms and release kinetics),

core melt progression, core relocation, and mechanical and chemical interactions under severe accidents for non-UO2 ceramic pellet fuel designs for the batch loading of ATF The staff has recognized through heightened stakeholder interactions that requests for increased fuel burnups, beyond the current licensed limits, are very likely to be included along with near-term ATF applications. Therefore, the staff is proactively undertaking an initiative to begin assessing the current knowledge and experimental data base associated with high burnup fuels beginning with NUREG/CR-6744, Phenomenon Identification and Ranking Tables (PIRTs) for Loss-of-Coolant Accidents in Pressurized and Boiling Water Reactors Containing High Burnup Fuel (ADAMS Accession No. ML013540584). Continued engagement with industry and the fuel vendors on this topic will inform the staff as to whether this plan needs to be amended to include the staffs complete strategy for addressing increases fuel burnup limits or if that activity can proceed in parallel with the plan.

Staff expects that industry decisions on targeted maximum burnups will direct plans with regards to an associated increase in enrichment to efficiently achieve the desired burnup. So along with the NRCs work associated with increased fuel burnups, the staff is also beginning an assessment of what enrichment increase the current knowledge and database could support.

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7.2 Lead Test Assemblies LTA programs provide pool-side, post-irradiation examination data collection; irradiated material for subsequent hot-cell examination and research; and demonstration of in-reactor performance.

This characterization of irradiated material properties and performance is essential for qualifying analytical codes and methods and developing the safety design bases for new design features or new fuel designs.

The NRC has recently published a draft letter to the Nuclear Energy Institute (ADAMS Accession No. ML18100A045) that, upon its finalization, will document the agencys position and should be reviewed for guidance because it pertains to the irradiation of ATF LTAs.

7.3 Initiating Activity The staffs expenditures associated with developing regulatory strategies and the framework for design-independent ATF licensing began in fiscal year (FY) 2017 and will continue as long as DOE and industry actively pursue ATF development. The staffs expenditures to support design-specific regulatory hurdles will begin upon formal notification from a vendor of its intent to pursue licensing of a specific design.

7.4 Deliverables Table 7.4 Anticipated In-Reactor Deliverables*

Title Due Date (near term/longer term)

Map of hazards and failure mechanism to GDC, regulations, 6-12 months from and guidance documents. completion of the PIRT exercise Develop rulemaking to address any identified necessary 24-48/36-60 months from changes. completion of the PIRT exercise Develop or revise guidance to address any identified 24-48/36-60 months from necessary changes. completion of the PIRT exercise

• The technical lead is the NRR Division of Safety Systems, Nuclear Performance and Code Review Branch.

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8 TASK 2: REGULATORY FRAMEWORK FUEL FACILITIES, TRANSPORTATION, AND STORAGE 8.1 Regulatory Infrastructure Analysis The NRC gives the regulations for fuel cycle activities of fuel fabrication, radioactive material transportation, and spent fuel storage in 10 CFR Part 70, 10 CFR Part 71, and 10 CFR Part 72, respectively. The regulations identify general performance requirements and have been used for licensing a broad spectrum of fuel fabrication facilities and for the certification of a broad spectrum of transportation and storage packages. The NRC does not expect these regulations to need modification to accommodate the fabrication, transportation, or storage of ATF.

Table 8.1.1 identifies the current guidance documents for the review of fuel facility licensing, transportation packages, and spent fuel storage designs.

Table 8.1 NRC Fuel Cycle Review Guidance Review Guidance Title Document NUREG-1609 Standard Review Plan for Transportation Packages for Radioactive Material NUREG-1617 Standard Review Plan for Transportation Packages for Spent Nuclear Fuel NUREG-1520 Standard Review Plan for Fuel Cycle Facilities License Applications NUREG-2215 Standard Review Plan for Spent Fuel Dry Storage Systems and Facilities Interim staff https://www.nrc.gov/reading-rm/doc-collections/isg/spent-guidance fuel.html These review guidance documents draw on industry experience in the fabrication, transportation, and storage of Zr-clad UO2 fuel with up to 5-percent enrichment. The NRC may need to supplement some of the guidance to address safety-related issues that could arise from ATF designs that involve different fuel or clad materials, higher enrichments, or changes in the processes and systems used to produce or manage the ATF. Potential areas for which review guidance may be expanded include criticality safety for systems in which the enrichment is greater than 5 percent, fuel or cladding material properties that are used in the analysis of transportation or storage packages, and failure mechanisms that must be considered for irradiated fuel other than Zr-clad UO2. Two specific examples for which guidance may be developed are material properties for FeCrAl alloys and SiC materials that are used as ATF cladding.

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The NRC staff will continue to monitor industry plans for fabricating and transporting unirradiated ATF fuel designs and for managing irradiated ATF. When the staff believes that supplemental information or guidance would facilitate the preparation and review of applications involving the fabrication, transportation, and storage of ATF designs, it will discuss this with stakeholders and take actions where practical.

8.2 Facility, Transportation, and Storage Reviews The regulatory reviews to support the development and deployment of ATF will occur in several fuel cycle areas over the near term to support irradiation of LTAs and over the longer term to support the batch deployment of ATF. The sections below discuss these various reviews.

8.2.1 Fuel Fabrication Facility Reviews The ATF fabrication operations for UO2-based fuel are expected to involve operations that are similar to currently licensed operations. Licensees will use the regulations at 10 CFR 70.72, Facility Change and Change Process, to determine whether NRC approval is required before implementing a change for the production of ATF.

ATF fabrication operations that are substantially different from those used for the fabrication of Zr-clad UO2 fuel (e.g., production of metal ATF, production or use of fuel material with enrichments greater than 5 percent) will likely require a license amendment. The NRC expects that licensees will submit such amendment requests at a later date, beyond the current planning horizon. Future updates of this plan will address such amendment requests as the industrys plans become more certain.

8.2.2 Unirradiated Fuel Transportation Package Reviews In the near term, the staff expects vendors that are developing ATF to request approval of packages for transporting LTAs from the fabrication facilities to reactors for test irradiation.

Currently, two transportation package reviews for LTAs are planned as noted in Table 8.3. The staff will review these requests against the requirements of 10 CFR Part 71 and will use NUREG-1609 and pertinent interim staff guidance for the safety review.

As industry prepares for the batch loading of ATF, the staff expects to receive requests for the approval of transportation packages that allow large-scale (i.e., batch) shipment of unirradiated ATF assemblies. The staff expects that any such requests will be made after 2020 and that future updates of this project plan will address such activities more specifically as the industrys plans become more certain.

The NRC staff will support PIRT efforts that focus on the identification and evaluation of material properties used in the safety analyses of transportation packages with ATF contents. These PIRT efforts are expected to help the staff develop additional regulatory guidance for ATF transportation, if required.

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8.2.3 Irradiated Fuel Transportation Package and Storage Cask Reviews The agency expects any shipments of irradiated ATF LTAs or rods from ATF LTAs to be made in NRC-approved transportation packages. Requests could be made under 10 CFR Part 71 (i.e., letters of special authorization) for a limited number of shipments of irradiated LTAs over a limited timeframe similar to that expected for unirradiated LTAs.

For the batch loading of ATF, the staff expects to receive requests for the approval of transportation packages under 10 CFR Part 71, which allows large-scale shipment of irradiated ATF assemblies. The staff expects that any such requests will be made after FY 2023, and future updates of this project plan will address such activities as the industrys plans become more certain. The NRC will review these requests against the requirements of 10 CFR Part 71, and the staff will use NUREG-1617 and pertinent interim staff guidance for the safety review.

If NRC-licensed reactors use ATF assemblies, such plants will need storage systems for irradiated ATF that are licensed (or certified) under 10 CFR Part 72. The NRC expects the need for irradiated ATF storage systems to develop after 2023, and future updates of this project plan will address such systems as the industrys plans become more certain.

The NRC staff will support PIRT efforts that focus on the identification and evaluation of material properties and fuel degradation mechanisms that the review of transportation packages or storage systems for irradiated ATF should consider. These PIRT efforts should help the staff develop additional regulatory guidance for irradiated ATF, if required.

8.2.4 Potential Challenges Certain aspects of ATF designs or fuel cycle implementation strategies could affect the scope, level of complexity, and schedule of the staffs review.

The major fuel cycle changes that are possible as a result of ATF development include (1) higher enriched uranium (e.g., greater than 5-percent enrichment), (2) different fuel material (e.g., Cr-doped UO2, U3Si2, or metallic fuel material), and (3) different cladding (e.g., FeCrAl, SiC, or coated Zr cladding). The number and nature of changes in these areas affect the effort required to review proposed fuel cycle changes. Table 8.2 identifies potential regulatory actions for the fuel cycle facilities and operations that might be required for these potential fuel cycle changes.

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Table 8.2 Potential ATF Fuel Cycle Action and Associated Regulatory Actions Potential Potential Regulatory Actions at Affected Facilities/Operations ATF Fuel Enrichment Fuel Fabrication Transportation Irradiated Fuel Cycle Facility Facility Operations Storage Higher License License Application for Applications for enrichment amendment to amendment to amendment to a amendments to a produce manufacture transportation transportation higher higher enriched certificate or new certificate or new enrichment fuel transportation spent fuel material packages (fuel storage systems material) (e.g., are expected uranium regardless of hexafluoride ATF enrichment package) (see box below)

Different Facility changes Application for Applications for fuel that do not meet amendment to, or amendments material the criteria of new, transportation to, or new, 10 CFR 70.72(c) packages spent fuel will require NRC (unirradiated fuel, storage approval irradiated fuel) systems with ATF-specific license conditions Different Application for Applications for fuel amendment to, or amendments cladding new, transportation to, or new, packages spent fuel (unirradiated fuel, storage irradiated fuel) systems with ATF-specific license conditions The greater the differences between an ATF design and Zr-clad UO2, the more likely supplemental review guidance will be required and the more likely the review will require greater staff effort. As an example, one potential ATF fuel material, U3Si2, is more susceptible to chemical reactions (e.g., water, air) than UO2. This hazard needs to be considered in the design and operation of a facility that produces or stores this material, and the NRC staff will need to review such facility designs and safety controls as part of the licensing process.

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8.2.5 Lead Test Assemblies Limited near-term regulatory activities are expected for fuel cycle activities associated with the fabrication and transportation of LTAs that involve coated Zr-clad UO2 fuel. Some regulatory actions may be necessary for the certification of transportation packages for LTAs that rely on structural performance of non-Zr cladding material.

8.2.6 Initiating Activity The staffs expenditures associated with developing regulatory strategies and the framework for design-independent ATF fuel cycle licensing began in FY 2017 and will continue as long as DOE and industry are actively pursuing ATF development. The staffs expenditures to support design-specific fuel cycle regulatory hurdles will begin when responses to the ATF RIS identify future actions, when an applicant briefs the staff on its proposed submittal, or when the staff receives an application that presents details that it must review.

8.2.7 Anticipated Regulatory Actions Near-term regulatory actions are all associated with the review of transportation packages for unirradiated LTAs, as identified in the Table 8.3. Other regulatory actions are expected and will be identified in future revisions of the plan after industry actions become clearer.

Table 8.3 Anticipated Fuel Cycle Regulatory Actions*

Anticipated Action Assumed Submittal Due Review of unirradiated LTA 1 transportation package Summer 2018 Review of unirradiated LTA 2 transportation package Fall 2018

• The technical leads are the (1) NMSS Division of Fuel Cycle Safety, Safeguards, and Environmental Reviews, Fuel Manufacturing Branch (fuel facilities), and (2) NMSS Division of Spent Fuel Management, Renewals and Materials Branch (transportation and storage).

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9 TASK 3: PROBABILISTIC RISK-ASSESSMENT ACTIVITIES The NRC uses probabilistic risk assessments (PRAs) to estimate risk to investigate what can go wrong, how likely it is, and what the consequences could be. The results of PRAs provide the NRC with insights into the strengths and weaknesses of the design and operation of a nuclear power plant. PRAs cover a wide range of NRC regulatory activities, including many risk-informed licensing and oversight activities (e.g., risk-informed technical specification initiatives, the significance determination process portion of the Reactor Oversight Process).

These activities make use of both plant-specific licensee PRA models and plant-specific NRC PRA models. The NRC uses the former models predominantly for licensing and operational activities and the latter models predominantly for oversight activities. A key tenet of risk-informed decision-making is that these models reflect the as-designed, as-operated plant.

For this reason, these models must be updated to reflect significant plant modifications. The introduction of significantly different fuel into the reactor core has the potential to affect these models, particularly once the reactor core composition significantly influences the plants response to a postulated accident (e.g., time to fuel heat up and degradation, amount of total hydrogen generation).

Activities associated with the development of capabilities to support risk-informed regulatory activities following the implementation of ATFs could be significant, and information about the industrys intended approach is needed to create a meaningful plan. Early interactions within the PRA community on ATF activities, including early preapplication meetings, have been used to encourage industry to ensure that the approach being pursued is consistent with the related regulatory requirements and staff guidance. This plan recognizes that the staffs PRA-related preparatory work involves two separate, but closely related, aspects:

(1) The staff must prepare for, and review, PRA-related information submitted as part of the licensing process for both the batch loading of ATF and incorporation of the safety enhancements of ATF into the licensing basis.

(2) The staff must develop PRA-related capabilities that allow it to do the following effectively:

• Review risk-informed licensing applications and ensure that applicants are using acceptable PRA models once ATF is implemented.

• Perform risk-informed oversight evaluations (e.g., significance determination process) once ATF is implemented.

The nature of item 1 is highly dependent on the approach taken by each vendor or licensee, or both, in its licensing application. However, item 2 is somewhat independent of the licensing approach for the batch loading of ATF; therefore, this plan currently focuses more attention on item 2.

As illustrated by the above categorization, PRA is more broadly relevant to ATF than simply the incorporation of ATF safety enhancements into the licensing basis. Again, this stems from the 31

fact that the NRC uses a risk-informed licensing and oversight approach that relies on plant-specific PRAs that represent the as-built and as-operated plant. Near-term ATF designs may have a limited impact on PRA modeling, whereas longer term ATF designs may have a more significant impact on PRA modeling. In general, the PRA modeling changes in question include the following:

• selection of core damage surrogates used in defining PRA end states (e.g., peak nodal clad temperature of 1,204 degrees Celsius, water level at two-thirds active fuel height)

• accident sequence modeling assumptions used to create event tree models that define the high-level successes and failures that can prevent core damage (e.g., late containment venting is required for avoiding core damage)

• system success criteria used in fault trees for defining the minimum hardware needed to fulfill specific mitigation functions (e.g., two relief valves are needed to prevent injection pump deadhead when feed and bleed cooling is used for a transient with no feedwater)

• sequence timing assumptions used in accident sequence modeling, success criteria determinations, and human reliability analysis to establish relevant time windows (e.g., feed and bleed cooling must be initiated within 20 minutes of low-low steam generator water level).

The staff will need to ensure that licensees PRAs continue to use acceptable models and assumptions as part of the implementation of ATF and update the NRCs models (as necessary) to reflect the ATF plant modifications.

Much of the needed underlying deterministic knowledge to address these points can leverage the work covered elsewhere in this plan, particularly the fuel performance, thermal hydraulics, and severe accident calculation capability development. It is envisioned that much of the analytical investigation needed to assess PRA-related impacts and support PRA-related changes in the agencys SPAR models can use the MELCOR modeling and analysis discussed elsewhere in this plan. If needed, additional confirmatory analysis could also be pursued using MELCOR plant models developed for other NRC initiatives (such as those documented in NUREG-1953, Confirmatory Thermal-Hydraulic Analysis to Support Specific Success Criteria in the Standardized Plant Analysis Risk ModelsSurry and Peach Bottom, issued September 2011, and NUREG-2187, Confirmatory Thermal-Hydraulic Analysis to Support Specific Success Criteria in the Standardized Plant Analysis Risk ModelsByron Unit 1, issued January 2016). This leveraging of resources between severe accident analysis tools and PRAs is routine.

In the nearer term, PRA-related impacts can be assessed using the general knowledge being developed in these other ATF project plan areas in conjunction with one or more pilot efforts using the existing SPAR models. Such pilots would help gain risk insights, assess the potential changes in core damage frequency (CDF) and large early release frequency (LERF),2 and 2 Differences in LERFs could occur because of (1) differing fuel heatup and degradation time windows, (2) the generation of differing amounts of in- vessel hydrogen, (3) changes to the fission product release rates, and 32

highlight areas where existing guidance3 or methods may require refinement to address the implementation of ATF.

As a final introductory point, engagement on PRA-related topics both within the staff and with external stakeholders is important at all stages. Effective interaction will foster a common understanding of the acceptability of PRA methods used to model plant modifications and the impact that will ultimately be realized when these modifications are integrated into PRAs and risk-informed processes. Effective interaction can also ensure that information required to develop PRA modeling assumptions related to plant modifications is properly coordinated with the deterministic review. In this case, PRA relevance has been identified early in the process, and time is available to address the PRA-related needs in a thoughtful and symbiotic manner.

For the purpose of identifying the PRA-related milestones, the following key assumptions are necessary (some restate assumptions made elsewhere in this plan):

• The timing of PRA-related efforts will be cross-coordinated with those of the previously identified partner areas (e.g., severe accident analysis) to allow the leveraging of deterministic work to make the PRA-related efforts efficient. A different approach might be needed if there is a strong desire to assess the industrys early perspective on the potential risk significance of ATF designs as they relate to future submittals aimed at leveraging ATF to reduce regulatory requirements.

• For all designs in question, the earliest TR/LAR review would start in 2020, with longer term ATF design licensing reviews occurring no earlier than 2023.

• This plan does not account for new regulatory initiatives that might be requested to maximize the operational or economic benefit of ATF, such as the following:

modifications to the categorization process in 10 CFR 50.69, Risk-Informed Categorization and Treatment of Structures, Systems and Components for Nuclear Power Reactors, associated with the use of relative (as opposed to absolute) CDF/LERF criteria4 reduction of requirements associated with security and emergency preparedness programs (4) shifts in the balance of challenges to other vessel and connected piping system components stemming from higher in-core temperatures before the relocation of debris.

3 This guidance encompasses the guidance used in risk-informed licensing and oversight (e.g., the SRP; relevant RGs; Inspection Manual Chapter (IMC) 0609, Significant Determination Process, dated April 29, 2015; the risk-assessment standardization process manual). In reality, most of this guidance would not require revisions because the concepts and processes would continue to apply. However, some aspects could require modifications, such as those involving the LERF multipliers used in IMC 0609, Appendix H, Containment Integrity Significance Determination Process, dated May 6, 2004, whereas some guidance may benefit from additional discussion of ATF impacts.

4 This initiative has been mentioned as a potential limitation in the degree of benefit that would be gained in risk-informed licensing space, and it contrasts to the use of absolute risk measures in other relevant risk-informed licensing activities such as risk-informed technical specification initiatives.

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Table 9.1 PRA ActivitiesMilestones Milestone Input Needed Lead Needed By Time/

Duration Participate in internal and external N/A Ongoing N/A discussions and knowledge 1 development related to ATF (e.g., internal working group meetings, public meetings)

Complete licensing reviews, including More information TBD TBD potential TRs or industry guidance, regarding the specific 2 related to the risk-informed aspects of licensing approach ATF licensing Complete a SPAR pilot of a near-term Deterministic 6 months 1 year before ATF design for a boiling-water reactor knowledge base being the first (BWR) and pressurized-water reactor developed under other near-term ATF 3 (PWR) subject plant to assess tasks (e.g., MELCOR core load1 CDF/LERF impacts, gain risk insights, analysis) and identify potential improvements to guidance Complete a SPAR pilot of a longer-term Deterministic 6 months2 1 year before ATF design for a BWR and PWR knowledge base being the first longer 4 subject plant to assess CDF/LERF developed under other term ATF core impacts, gain risk insights, and identify tasks (e.g., MELCOR load1 potential improvements to guidance analysis)

Update guidance (as necessary) to Completion of the 1 year Before the ATF support licensing and oversight items above core load1 5 functions for plants making ATF-related modifications Update agency PRA models to reflect Details of the plant 1 year3 As needed to ATF-related changes to the as-built, modifications support the 6 as-operated plant for relevant agencys risk plants/models evaluations 1 Here, core load means the replacement of a large proportion (e.g., 50 percent or more) of the core with ATF assemblies, assuming that non-ATF fuel will be generally more limiting to PRA impacts if a mixed core exists.

2 This task should be performed sequentially after the equivalent task for near-term ATF designs as long as both near-term and longer term designs are of regulatory interest.

3 This would occur after approval of the associated licensing action.

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Table 9.2 PRA ActivitiesDeliverables*

Title Lead Time Safety Evaluation contributions for TRs and LARs related to TBD ATF Report that documents results and recommendations from a 1 year before the first near-term near-term ATF SPAR pilot study ATF core load Report documenting results and recommendations from a 1 year before the first longer term longer-term ATF SPAR pilot study ATF core load Updated guidance (e.g., risk-assessment standardization project Varies depending on the guidance changes) to support licensing and oversight functions documents that require for plants making ATF-related modifications modifications Updated agency PRA models to reflect ATF-related changes to As needed to support the agencys the as-built, as-operated plant for relevant plants/models risk evaluations

• The technical lead is the NRR Division of Risk Analysis, Probabilistic Risk Assessment Licensing Branch.

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10 TASK 4: DEVELOPING INDEPENDENT CONFIRMATORY CALCULATION CAPABILITIES Independent confirmatory calculations are one of the tools that the staff can use in its safety review of TRs and LARs. Confirmatory calculations provide the staff insight on the phenomenology and potential consequences of transient and accident scenarios. In addition, sensitivity studies help to identify risk significant contributors to the safety analyses and assist in focusing the staffs review. RG 1.70, Standard Format and Content of Safety Analysis Reports for Nuclear Power Plants (LWR Edition), identifies the standard format and content of safety analysis reports for nuclear power plants, and the SRP identifies the criteria that the staff should use to review licensee safety analyses. The NRC plans to continue to develop independent confirmatory analysis tools that support robust SEs and provide insights into safety significant factors for each ATF design. Vendor codes used for ATF modeling capabilities will likely be based on smaller data sets than those of the current Zr-UO2 models. This will result in greater uncertainty in the results of the safety analyses and the margins to the specified acceptable fuel design limits. For these reasons, confirmatory calculation capabilities will be critical for generating confidence in the safety assessment of ATF against all applicable regulatory requirements (see Section 7 for more details). A confirmatory code can be used to independently quantify the impact of modeling uncertainties and support more efficient reviews with the potential for fewer requests for additional information. Finally, the experience and insights gained by developing an in-house code can be leveraged in reviews of externally developed models and methods, thus making reviews more efficient and effective.

The staff identified four technical disciplines needing calculation capability development to support TR/LAR safety reviews: (1) fuel performance, (2) thermal hydraulics, (3) neutronics, and (4) severe accidents. The NRC has developed a suite of codes to analyze these disciplines, and they have been used successfully to support regulatory decision-making. Further development of these codes is appropriate to ensure that the NRC has the capability to analyze ATF designs. Having tools that the staff can use to analyze ATF will be particularly important because applicants will use computational tools to demonstrate that they have met fuel safety acceptance criteria and because, in some cases, the ATF properties and models within the computational tools will be based on limited experimental data.

The development of calculation capabilities will proceed with similar activities in each area, as follows:

• PIRT exercises help ensure that all new phenomena important to safety have been identified and considered in the planning phases. PIRT results will be used to inform code development efforts.

• Scoping studies will be performed to identify the architecture and model updates needed to model various ATF concepts.

• Where necessary, code architecture modifications will be made (e.g., to remove Zr/UO2 hard wired properties and assumptions or to solve the governing equations for non-cylindrical geometry).

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• Material properties will be added, and new models will be developed, where necessary.

• Integral assessment of the updated codes will be completed and documented. It is likely that results from integral assessments and uncertainty studies performed using updated codes will be used to revisit and maintain PIRT products.

Figure 10.1 depicts a generic schematic of tasks associated with developing calculation capabilities for near-term ATF designs, whether such capabilities are developed by the applicant, DOE, or the NRC. This figure defines the lead time and duration concepts for calculation capability. Lead time refers to the time required to complete the process for developing the calculation capability, and durations refer to the time required for the conduct of discrete tasks within the process.

Figure 10.1 Development process for near-term ATF calculation capability Figure 10.1 shows that code development requires testing and data to feed model development and validation. Developing codes to demonstrate that ATF can be used safely includes updating codes with ATF material properties and models and then validating the updated codes against relevant experimental data. The validation exercise ensures that a code appropriately models key phenomena and accurately predicts the parameters of safety importance. The datasets used to develop models often come from separate effects testing, whereas code assessment and validation often use data generated in integral test programs. The lead time to develop a calculation capability is intrinsically linked to the production and availability of data from ongoing testing programs. The DOE and EPRI MOUs establish mechanisms that the NRC staff can use to communicate data needs discovered through model development and code assessment efforts.

The diagram is relevant for all ATF concepts; however, the NRC recognizes that some concepts have limited new phenomena and therefore the duration, and breadth, of each element will vary with each ATF concept.

The calculation capability development process for longer-term ATF designs will likely be more iterative than the process for near-term ATF designs and some of the tasks may proceed in parallel. This is because it is expected that some code architecture updates and new model needs will only become evident as more data for longer-term ATF designs becomes available and as codes are assessed. As with near-term ATF designs, the property and model development and the code assessment and validation tasks require data. Therefore, the 37

duration of these tasks are intrinsically linked to the production and availability of data from ongoing testing programs.

For each discipline, the level of effort to complete these activities will vary based on the characteristics of the ATF design and the availability of information on the properties and phenomenological behaviors of the fuel, which will be addressed for each discipline in separate reference material. The estimated lead times to develop the codes necessary to analyze all currently proposed fuel/cladding types range from 3 to 6 years. The lead times include all code development activities and consider the time required to generate new data and new models for code development and integral assessment. The lead times vary by discipline and vary for near-term and longer-term ATF designs. Generally, longer lead times are estimated for longer term designs with the expectation that new phenomenological models will need to be developed and validated. The lead times are not independent between various ATF designs because it is anticipated that code architecture updates made for the first design can be leveraged for other ATF designs.

Although this plan addresses calculation capability development in four different disciplines, technical overlap between disciplines exists, including the introduction of new material properties. To reduce duplication of effort, the analysis tools will be coupled to allow codes to send and receive information between each other. For example, neutronics codes can be used to provide fuel performance codes with pellet radial power distribution information as a function of burnup, and fuel performance codes can provide neutronics codes with fuel temperature and deformation calculations. Thus, coupling the codes leverages information sharing to improve the overall analysis capabilities and ensures consistency across codes. The NRC will update its graphical user interface, Symbolic Nuclear Analysis Package, as needed to make it the interfacing tool between the NRCs suite of analysis tools. Where possible, the NRC will coordinate with DOE to reduce duplication of effort in calculation capability development.

Appendix A describes the NRCs plans to develop analysis capabilities in the areas of fuel performance, neutronics, thermal hydraulics, and severe accidents and source terms.

11 PATH FORWARD This project plan represents the high-level strategy to prepare the NRC for conducting efficient and effective reviews of ATF designs. The plan is intended to be a living document that may evolve as industry plans are refined and the state of knowledge for ATF concepts advances.

The plan will be augmented with concept-specific licensing roadmaps following completion of the PIRT, or expert elicitation, exercise for each concept to clearly identify the regulatory criteria which must be satisfied for approval.

The staffs priority, now that this plan has been finalized, is to: 1) engage directly with the nuclear fuel vendors pursuing near-term ATF concepts with the objective of understanding the nexus between the phenomena identified as important to safety and their testing plans, and 2) understand the areas of margin recovery or operational flexibility that licensees plan to seek such that staff can begin to proactively refine the regulatory framework where necessary.

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Appendix A NRC PLANS TO DEVELOP ANALYSIS CAPABILITY Task 4.a: Fuel Performance The Nuclear Regulatory Commissions (NRCs) long-standing fuel performance codes, FRAPCON and FRAPTRAN, have been merged into a single, modern code called FAST (Fuel Analysis under Steady-state and Transients), which will need to be updated to conduct analysis of accident tolerant fuel (ATF) fuel performance and support licensing reviews. Fuel performance codes are needed to support licensing review because they are used to demonstrate that specified acceptable fuel design limits (SAFDLs) are maintained and to provide initial conditions for design-basis accident analysis. Additionally, fuel performance codes are used to support the safety limits for loading and storing spent nuclear fuel in dry casks.

Fuel performance code updates needed to model some ATF designs, including FeCrAl cladding and coated zirconium-based alloy claddings, will require minimal changes to FAST, with the work focused on new material properties, code assessment and benchmarking. Updates needed to model SiC tubing, non-UO2 fuel, and fuel designs in non-cylindrical fuel forms, would be more extensive. FAST development activities include the following tasks:

Scoping Study - For FAST, this will be a low resource, straight forward activity because the code development needs are largely understood.

Code Architecture Updates - For FAST, this includes modifying the code to be more modular.

The staff will remove any computational assumptions embedded in the code for the Zirconium/UO2 system and ensure that the heat transfer, solid mechanics, and diffusion solutions are generic while calling a separate set of libraries (MatLib) containing the relevant material properties. The staff will modify FAST to allow for modeling non-cylindrical fuel forms which entails re-solving the physics modeled by the code (e.g., heat conduction, diffusion, solid mechanics, etc.) with different geometrical conditions and possibly modeling in multi-dimension.

Finally, infrastructure development will include changes necessary to allow for interaction with other NRC tools via the SNAP interface. Additionally, FAST is currently under development to support non-LWR licensing activities, including developing multi-dimensional solvers, which is expected to reduce the level of effort in the code architecture updates for longer-term ATF concepts.

Property and Model Development - Material properties (non-irradiated and irradiated) for ATF claddings and fuels will be added to MatLib (the material properties library in FAST), as well as models to address new phenomenon and failure modes presented by ATF. There are a series of thermal, mechanical, and irradiation-induced properties that need to be updated for each new fuel and cladding material. For new fuels, additional considerations include fission gas release and fuel creep. For claddings, additional correlations are needed to model hydrogen pickup, steady-state and transient corrosion, thermal and irradiation creep, high-temperature deformation, and new failure models. As experience is gained through use of ATF fuel and cladding in-reactor, it is expected that significant data will be available to the NRC to develop models that capture how the properties evolve as a function of burnup (e.g., thermal 39

conductivity degradation). It is expected that additional properties will be needed to model new materials that are yet unknown, such as the diffusion of oxygen or volatiles, and fuel/cladding/coolant interaction, which may require more extensive modifications to both MatLib and FAST. As the phenomena becomes known, models related to long-term spent fuel handling, storage, and transportation will be updated.

Code Assessment and Validation - Integral performance data from Advanced Test Reactor (ATR), Halden, Transient Reactor Test Facility (TREAT) and LTA programs, as well as other available sources, will be used to confirm that the material properties and models added to FAST fully account for the integral behavior of ATF. This will be the most time-consuming task for several reasons. The focus of code assessment will need to be determined based on licensing requirements for each ATF design. For example, the current assessment of FAST for steady-state calculations looks at fission gas release, fuel centerline temperature, cladding strain, oxidation, and rod internal pressure, all of which are part of the SAFDLs outlined in NUREG-0800, Section 4.2, Fuel System Design. The code assessment, also referred to as the Integral Assessment, analyzes the integral effects of all of the models and correlations working together to analyze the thermal-mechanical behavior of the fuel rod under typical light water reactor conditions. A proper assessment requires numerous cases that cover the breadth of boundary conditions and operating regimes that the fuel design will experience under normal operation, anticipated operational occurrences (AOO), and design-basis accident (DBA) conditions. For example, the FAST integral assessment currently consists of > 200 non-proprietary cases for the uranium dioxide/zirconium system and numerous proprietary cases and data sets that the code (and its predecessor codes) has been assessed against over the last several decades. The integral assessment is the key to identify phenomena that may not be properly modeled. If discrepancies are shown between the code and the data, then the codes models and correlations will be re-examined and updated as necessary to achieve reasonable agreement. This will require an iteration between architecture updates (if new physics are determined to occur), material properties updates, and re-running of the integral assessment. In addition, the more detailed the integral assessment is, the more knowledge is gained on understanding the uncertainties of the code. The amount of data that is available to create the assessment database greatly affects the uncertainty of the results of analyses from the fuel performance codes. As more data is included in the assessment database, the confidence in the results of the analyses increases. As the size of the assessment database increases topical report reviews will become more efficient because there will be less uncertainty in the results of the fuel performance codes.

The milestones are listed in the tables below with their related trigger or needed input, lead time, and schedule driver 40

Table 4.a.1 Near-term ATF Concepts Data Needs Activity Duration Needed By and Inputs Scoping Study Low level of resources needed; short duration task Code Architecture Updates - 1 year -

Material Property and Model Separate 2 years -

Development Effects Data Code Assessment and Integral Effects Fuel Topical 1 year Validation Data Report Submittal Lead Time for Fuel Performance Calculation 2-4 years capability for Near-term ATF 41

Table 4.a.2 Longer-term ATF Concepts Data Needs and Activity Duration Needed By Inputs Scoping Study Low level of resources needed; short duration task Code Architecture Updates (Remove Zr and UO2 hard-wired - 1 year1 -

properties)

Code Architecture Updates Completion of (Remove assumptions related to previous 2 years2 -

fuel geometry and Zr-UO2 milestone interaction)

Material Property and Model Separate Effects 2-4 years2 -

Development Data available Integral Effects Fuel Topical Code Assessment and Validation 2-3 years Data Report Submittal Lead Time for Fuel Performance Calculation capability for Longer- 3-5 years term ATF 1

Task will not be required if the near-term activities are completed 2

Tasks can be worked in parallel Technical Lead: RES/DSA/FSCB 42

Task 4.b: Thermal-Hydraulics To support confirmatory analyses for licensing reviews, the Nuclear Regulatory Commissions (NRCs) TRAC/RELAP Advanced Computational Engine (TRACE) system safety thermal-hydraulic code will need to be updated to analyze accident tolerant fuel (ATF) fuel performance.

TRACE will be used to perform design-basis transient and design-basis accident (DBA) analyses and sensitivity studies. TRACE uncertainty quantification tools can be used to assess the impact that uncertainties in material properties have on fuel performance in DBAs. The existing TRACE fuel rod model assumes that the fuel is oxide fuel and the cladding is a Zirconium alloy. ATF designs may have different fuel or cladding or both. Therefore, updates to the TRACE fuel rod models will be needed to model the ATF designs. Small changes to TRACE should be able to accommodate cylindrical fuel with metallic cladding, including FeCrAl cladding and coated zirconium-based alloy claddings. Updates needed to model SiC tubing, any non-UO2 fuel and metallic fuel in non-cylindrical fuel forms, would be more extensive.

TRACE may also need to be coupled to external fuel rod models such as Fuel Analysis under Steady-state and Transients (FAST) or Department of Energy (DOE) BISON to model some aspects of ATF. The expected limiting factor on completion is getting adequate data for materials and testing from the DOE and/or fuel vendors.

Scoping Study - A scoping study will be conducted to determine what changes need to be made to allow TRACE to perform plant accident and transient calculations with ATF. A set of sample plant calculations will be selected to demonstrate the changes in plant response due to ATF.

Code Architecture Updates - TRACE will need updates to the fuel and cladding mechanical and thermal material properties for the new or different materials proposed for use in ATF designs. Additionally, the cladding oxidation and rupture models in TRACE are based on empirical data for zirconium alloy cladding and will need to be modified to model clad oxidation and rupture for the other cladding materials proposed for use in ATF designs. In the case of non-cylindrical solid metallic fuel rods, a method will be needed to analyze the non-cylindrical geometry and the impact it has on conduction and convective heat transfer and fluid flow.

TRACE may also need to be coupled to external fuel rod models such as the FAST or the BISON fuel analysis models through the TRACE Exterior Communications Interface (ECI) to model some aspects of ATF.

Property and Model Development - The physical models that need to be updated are fuel and cladding mechanical and thermal properties, cladding oxidation kinetics models, clad rupture models, fuel boiling and convective heat transfer models including minimum stable film boiling temperature models for each cladding, and critical heat flux (CHF) models. It is expected that most of this information will be provide by the industry or DOE.

Code Assessment and Validation - The updated code will be validated against steady state and transient data collected by DOE and the industry. Demonstration calculations of reactor accidents will be performed to examine the impact of the new fuel on the evolution of the accident and safety criteria.

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The milestones are listed in the table below with their related trigger or needed input, lead time, and schedule driver.

Table 4.b.1 Near-term ATF Concepts Activity Data Needs and Duration Needed By Inputs Scoping Study Low level of resources needed; short duration task assuming models maintain same qualitative form as current models.

Code Architecture Updates Assumes models 2 years 3 years before maintain same safety analysis qualitative form as topical report current models. (TR) submittal Need conceptual design information (geometry, materials)

Data available from 2 2 years before Material Property and Model literature, fuel years safety analysis Development vendor, and DOE TR submittal test programs, etc.

Code Assessment and Reactor physics 1.5 years Validation and Sample Plant capability to Safety analysis Calculations generate cross TR submittal.

sections/point kinetics parameters for sample plant calculations. Data available from fuel vendor and DOE test programs.

Assessment data needed 1.5 years before TR submittal Lead Time for T/H Calculation 3 years capability for Near-term ATF 44

Table 4.b.2 Longer-term ATF Concepts1 Data Needs Activity Duration Needed By and Inputs Scoping Study Low level of resources needed; short duration task assuming models maintain same qualitative form as current models.

Code Architecture Updates Conceptual design 1 year2 -

(Remove Zr and UO2 hard-wired information for ATF properties) designs (geometry, materials)

Code Architecture Updates Completion of previous 1 years3 -

(Remove assumptions related to milestone fuel geometry and Zr-UO2 interaction)

Material Property and Model Separate Effects Data 1 years3 -

Development Code Assessment and Validation Integral Effects Data. 1.5 years3 Safety Assessment data analysis TR needed 1.5 years submittal.

before TR submittal Lead Time for T/H Calculation 2 years capability for Longer-term ATF 1 Table assumes near-term fuel work has been completed.

2 Task will not be required if the near-term activities are completed and new models have the same qualitative form as the current models.

3 Tasks can be worked on in parallel assuming that the new models have the same qualitative form as the current models.

Technical Lead: RES/DSA/CRAB 45

Task 4.c: Neutronics Neutronics calculations are an integral part of the confirmatory review process, as exampled by Chapter 4 and Chapter 15 of NUREG-0800, because they provide decay heat rates, core power, and reactivity values used by thermal hydraulic and fuel performance codes. Neutronics analysis is also required for the quantification of nuclide inventory for severe-accident/consequence analyses required by Title 10 of the Code of Federal Regulations (10 CFR) Part 100 and for evaluations supporting 10 CFR 50.68, Criticality Accident Requirements, 10 CFR Part 71, Packaging and Transportation of Radioactive Material, and 10 CFR Part 72, Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater than Class C Waste. Neutronics analysis is also performed to support decisions for spent fuel pool (SFP) loading, for confirming fluence calculations necessary to quantify vessel embrittlement, shielding analyses to support as low as reasonably achievable (ALARA) objectives, calculations of SFP decay heat rates and dose for human reliability analyses, core power/reactivity for transient calculations, assembly decay heat rates for cask loading, etc.

The Nuclear Regulatory Commissions (NRCs) main neutronics codes are SCALE which provides a computational capability to evaluate nuclear systems, and GenPMAXS/PARCS to evaluate time dependent core performance. SCALE is used by the NRC staff to support licensing reviews by performing criticality safety evaluations of enrichment and fuel fabrication facilities, developing lattice physics parameters for reactor operations, performing safety evaluations for transport and storage, as well as use in spent fuel pools, severe accidents and input into probabilistic risk assessment (PRA). PARCS is used as a core simulator that supports thermal hydraulic reviews under design basis scenarios. GenPMAXS reads the lattice physics parameters from SCALE (nodal averaged cross sections and kinetic parameters) and converts the nuclear data into the format form that is required by PARCS.

Scoping Study - A review of the current NRC neutronics packages, SCALE and GenPMAXS/PARCS, will be necessary to understand the modifications, if any, that will be required to characterize accident tolerant fuel (ATF) for the whole fuel cycle. These codes need to be reviewed against the unique features of the ATF fuel designs (e.g., coated-Zirconium, doped-UO2, FeCrAl, U3Si2, SiC), and for enrichments of higher than 5 weight percent U-235 (up to 20 weight percent). The SCALE code suite covers the following functional areas: nuclear data and methods, Monte-Carlo methods, isotope decay, depletion and activation methods, reactor physics methods, and sensitivity and uncertainty methods. The GenPMAXS/PARCS code covers reactor operations and transient performance.

Consideration of the needs for each of these functional areas will be included in the scoping study. The scoping study will necessarily involve a review of the fuel cycle and the associated impact of ATFs designs.

Code Architecture Updates - The required infrastructure development activities for SCALE and GenPMAXS/PARCS involve any code modifications and/or enhancements identified in the scoping study for implementation.

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Expected development activities include:

• Coupling of SCALE and PARCS with Fuel Analysis under Steady-state and Transients (FAST) in order to receive detailed fuel data (such as temperatures and geometry) while providing intra-pin radial power profiles and axial rod power profile to FAST (to eliminate the need for FAST to develop new correlations)

• SCALE geometry enhancements to support non-cylindrical fuel for both 3D Monte Carlo analysis and 2D lattice physics calculations

• SCALE input and modeling investigations and enhancements to model coated cladding, where coat thickness is the less than 100 microns

• Updates to the energy group structures (both the fine multi-group structure that is used in SCALE and the collapsed broad group structure that is used by PARCS for core calculations), updates to the nuclear data library, nuclear methods development (at the lattice and nodal level) to enable pin power reconstruction for non-cylindrical fuel, and input interfaces Model Development - The required development activities for SCALE and GenPMAX/PARCS involve any code modifications and/or enhancements identified in the scoping study for implementation. Expected development activities include:

• Evaluation of the depletion and activation effects for ATF fuel and cladding compositions

• Evaluation of the impact of thermal hydraulic and fuel performance effects that may be more or less important for ATF such as thermal expansion, heat capacity, thermal conductivity, swelling and gap closure, etc.

• Evaluation of a different cross section parameterization methodology and the development of different fuel temperature averaging techniques in order to better characterize Doppler feedback for the different carbide, ceramic, and metallic fuels under consideration

• Evaluation of the methodology for thermal-hydraulic calculations with steady-state PARCS calculations (PARCS includes PATHS, a simplified drift flux formulation, for thermal-hydraulic feedback). This will include the accommodation for more extreme axial discontinuities and heterogeneities, the evaluation of the coupling of the fuel and wall temperature to the determination of bulk fluid temperature, the evaluation of the applicability of the current constitutive relationships (void fraction quality models, subcooled quality, wall friction factor, and two-phase flow friction factors)

• Sensitivity and uncertainty assessment, including assessment of modelling techniques, for ATF candidates

• Evaluation of tritium release through advanced cladding materials such as SiC cladding (Note: parts of these activities can be started before the receipt of necessary data) 47

Code Assessment and Validation - Code verification and validation are important elements of the SCALE and GenPMAXS/PARCS software quality assurance (SQA) program. All new updates, modification, enhancements, etc., must be assessed against test data. These would include test data from post irradiation examinations (e.g., destructive assay of fuel and/or clad to validate depletion), but would be finalized by the gap analysis.

Within the code assessment and validation task, testing of the combined code sequence for a particular application will also be required. Within reactor operations space, the following computer codes will have to be tested together, SCALE-GenPMAXS/PARCS-TRACE for example.

The milestones are listed in the table below with their related trigger or needed input, lead time, and schedule driver.

Table 4.c.1 Near-term ATF Concepts Data Needs Activity Duration Needed By and Inputs Scoping Study Short duration task Code Architecture Updates Completion of 1 year -

previous milestone Model Development Data as 1 year -

required from scoping study Code Assessment and Data as 1 year Safety Analysis Validation required from Submittal scoping study Lead Time for Neutronics 1-2 years Calculation capability for Near-term ATF 48

Table 4.c.2 Longer-term ATF Concepts Activity Data Needs and Duration Needed By Inputs Scoping Study Short duration task Code Architecture Updates Completion of 2 years1 -

previous milestone Model Development Data as required 2 years1 -

from scoping study Code Assessment and Validation Data as required 2 years1 Safety Analysis from scoping Submittal study Lead Time for Neutronics 2-3 years Calculation capability for Longer-term ATF 1 Tasks can be worked on in parallel Technical Lead: RES/DSA/FSCB 49

Task 4.d: Source Term Recent efforts by several fuel vendors, in coordination with the DOE have announced plans to develop and seek approval for various fuel designs with enhanced accident tolerance. Certain ATF designs could lead to a departure from the current regulatory source terms used by the staff concerning fission product release during an accident, anticipated operational occurrences, or normal operation which are based upon insights derived from current generation light-water reactors using typical zirconium-alloy fuel. Prompted by these efforts, the staff undertook an examination of the technical bases of the various regulatory source terms to assess the potential impacts to the current assumptions to the regulatory process.

Regulatory source terms are deeply embedded in the regulatory policy and practices of the NRC where the current licensing process has evolved over the past 50 years. It is based upon the concept of defense-in-depth in which power plant design, operation, siting, and emergency planning comprise independent layers of nuclear safety. This approach encourages nuclear plant designers to incorporate several lines of defense in order to maintain the effectiveness of physical barriers between radiation sources and materials from workers, members of the public and environment in operational states and, for some barriers, in accident conditions. The approach centers on the concept of design basis accidents (DBAs) which aims to determine the effectiveness of each line of defense. The DBAs establish and confirm the design basis of the nuclear facility, including its safety-related structures, systems and components, and items important to safety, ensuring that the plant design meets the safety and numerical radiological criteria set forth in regulation and subsequent guidance. From this foundation, specific safety requirements have evolved through a number of criteria, procedures and evaluations, as reflected in regulations, regulatory guides, standard review plans, technical specifications and license conditions and TID, WASH and NUREG documents.

The various regulatory source terms, used in conjunction with the DBAs, establish and confirm the design basis of the nuclear facility including items important to safety; ensuring that the plant design meets the safety and numerical radiological criteria set forth in the Code of Federal Regulations (CFR) (e.g., 10 CFR 100.11, 10 CFR 50.67, 10 CFR 50.34(a)(1)(iv), GDC-19), and subsequent staff guidance. Current regulatory requirements did not envision the ATF designs that are being considered. As a result, a number of regulatory requirements may no longer be appropriate because of substantive changes in the assumptions of the various regulatory source terms. Potentially impacted regulatory requirements include:

1- Regulations (10 CFR Parts 50, 52, and 100);

2- Regulatory guides; 3- Technical specifications; 4- Emergency preparedness procedures; and, 5- Evaluation methods for assessing the environmental impact of the accident.

Specific examples of the various regulatory source terms can be found in NUREG-0800, the Standard Review Plan (SRP) which provides information on the staffs regulatory guides.

Examples of the various regulatory source terms discussed in the SRP include the:

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1- Accident source term - based on DBAs to establish and confirm the design basis of the nuclear facility and items important to safety while ensuring that the plant design meets the safety and numerical radiological criteria set forth in the CFR (e.g., 10 CFR 100.11, 10 CFR 50.67, 10 CFR 50.34(a)(1)(iv), GDC-19, and subsequent staff guidance).

Addressed in SRP Chapter 15, Transient and Accident Analysis.

2- Equipment qualification source term - to assess dose and dose rates to equipment.

Further guidance is addressed in SRP Sections 3.11 and 12.2, RG 1.89, or RG 1.183 App I.

3- Post-accident shielding source term - to assess vital area access, including work in the area. Further guidance is addressed in SRP Section 12.2, NUREG-0737 Item II.B.2, RG 1.89, or RG 1.183.

4- Design basis source term - based on 0.25 - 1 percent fuel defects to determine the adequacy of shielding and ventilation design features. Further guidance is addressed in SRP Section 12.2.

5- Anticipated operational occurrences source term - based on the technical specifications, or the design basis source term, whichever is more limiting, to determine the effects of events like primary to secondary leakage and reactor steam source term. Further guidance is addressed in SRP Section 11.1 for reactor coolant (primary and secondary) and reactor steam design details.

6- Normal operational source term - based on operational reactor experience, as described in American National Standards Institute/American National Standard N18.1. Further guidance is addressed in SRP Section 11.1 and 11.2 for reactor coolant (primary and secondary) and reactor steam design details, and SRP Section 11.3 for system design features used to process and treat liquid and gaseous effluents before being released or recycled.

It is concluded that the appropriate method for incorporating new information concerning ATF-specific accident source terms, would be an on-going process. An applicant may propose changes in source term parameters (timing, release magnitude, and chemical form) from those contained in the applicable guidance, based upon and justified by design specific features.

Attributes of an acceptable alternative source term are provided in Regulatory Position 2 of Regulatory Guide 1.183.

Scoping Study - A review of the current capabilities of the MELCOR code is needed to better understand the necessary code modifications in the code packages for the simulation of accident progression (i.e., core heat up and degradation, combustible gas generation, and fission product release and transport to the containment).

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Code Architecture Updates - The required infrastructure development activities for MELCOR involve code modifications identified in the scoping study for implementation of new and/or improved models. MELCOR contains various models for the modeling of the core components (e.g., fuel, cladding, and channel boxes), hydrogen and carbon monoxide generation and combustion, and fission product release from the core components. Some models and correlations (for fission product release and core degradation) in the code need to be modified for application to new fuel designs (e.g., through access to sensitivity coefficients and control functions or through generalized models). Examples include fuel rod collapse and eutectic interactions (e.g., impact of Zr-Cr intermetallic reactions), and the oxidation kinetics of cladding based on experimental data (at high temperature and pressure). The material properties for new designs also need to be added to the code database.

Property and Model Development - It is expected that clad coatings would affect oxidation behavior/combustible gas generation rate, and that fuel composition would affect fission product release rates and potentially chemical speciation. There could also exist other potential effects such as chemical reactions between fuel and clad that accelerate or retard core degradation or between fission products and clad or fuel that could either enhance or diminish the release of fission products. The code should be able to account for fission product speciation under various conditions, and information from experiments on fission product release (for non-UO2 fuel only) are needed for the development of the source term and code assessment.

The MELCOR code is well suited for developing a regulatory source term with flexible models to support evaluation of differences between current standard fuel and ATF concepts. The models existing in MELCOR are general in nature and can be adjusted to reflect differing properties of advanced fuels. Depending on results, the ATF models would likely be similar to those for standard fuel but with different parameters. Containment combustible gas control would need to be based on the expected release from the fuel/cladding combination based on the clad type.

For Lightbridge (or other future designs using metal fuel), the contribution of the fuel itself to gas generation may need to be considered.

Code Assessment and Validation - Code verification and validation are important elements of the MELCOR software quality assurance program. All new code models need to be assessed against available test data. Experiments characterizing clad oxidation and combustible gas generation rates, fission product release magnitudes, and chemical forms and rates are required to ensure that the release models for ATF are representative and that such effects are accounted for, if significant.

Source Term Development - The process for developing radiological source terms involves:

gathering of this experimental data; development and implementation of applicable models in the MELCOR code; simulating a series of accidents representing most of the core damage frequency for both boiling water reactors (BWRs) and pressurized water reactors (PWRs) to obtain fission-product-group-specific release behavior (i.e., gap and early in-vessel fission products release initiation time and duration, core release fraction and chemical forms); and finally collapsing the data into a simplified representative set of release fractions and timings for rapid use in simplified codes used for siting evaluation. As an example, the source term 52

analysis for high-burnup and mixed-oxide fuel5 was time consuming and involved about 30 detailed code calculations for BWRs and PWRs representing various accident scenarios (e.g., station blackouts, small and large break loss-of-coolant accidents).

The milestones are listed in the tables below with their related trigger or needed input, lead time, and schedule driver.

Table 4.d.1 Near-term ATF Concepts Data Needs Activity Duration Needed By and Inputs Scoping Study Short duration task Code Architecture Updates Completion of 1 year1 -

previous milestone Model Development Separate 1 year1 -

Effects Data Code Assessment and Integral Effects 1 year1 -

Validation Data Lead Time for MELCOR 1 year Calculation capability for Near-term ATF Accident Progression MELCOR code 1 year2 Safety Analysis Calculations with MELCOR Submittal Lead Time for Source Term 2 years3 Development for Near-term ATF 1 Tasks can be worked on in parallel (involves different phenomenology) 2 Task can be worked on after completion of MELCOR calculation capability 3 Includes the time for MELCOR capability and calculations 5

SAND 2011-0128 - Accident source terms for light-water nuclear power plants using high-burnup or MOX fuel 53

Table 4.d.2 Longer-term ATF Concepts Data Needs Activity Duration Needed By and Inputs Scoping Study Short duration task Code Architecture Updates Completion of 1 years1 -

previous milestone Model Development Separate Effects 2 years1 -

Data Code Assessment and Integral Effects 2 years1 -

Validation Data Lead Time for MELCOR 2-3 years Calculation capability for Longer-term ATF Accident Progression MELCOR code 2-3 years2 Safety Analysis Calculations with MELCOR Submittal Lead Time for Source Term 4-6 years3 Development for Longer-term ATF 1 Tasks can be worked on in parallel (benefits from work done for near-term designs) 2 Task can be worked on after completion of MELCOR calculation capability 3 Includes the time for MELCOR capability and calculations Technical Lead: RES/DSA/FSCB 54