Frequently Asked Questions

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General Questions

How will the NRC know if ATF is safe?

The NRC regulations contained in Title 10 of the Code of Federal Regulations provide reasonable assurance of adequate protection of public health and safety. All vendors and licensees must meet all applicable regulations, and the new technologies being developed as ATF are no exception. When ATF topical reports and other ATF-related licensing actions are received by the NRC, they will be reviewed, and NRC staff will make a safety finding as part of this review process.

Why does the NRC need to review ATF?

NRC-granted licenses are generally explicit regarding what materials can be used in fuels inserted into power reactors. Additionally, the NRC approves the fuel analysis methods for normal, abnormal, and accident scenarios. ATF would likely alter both materials and analysis methods; therefore, licensees would need to request approval to make those changes, with supporting evidence verifying that they will maintain reactor safety.

Does the NRC have to make any changes to the regulations to allow licensees to use ATF?

As with any new technology being submitted for NRC review, the NRC may establish new and/or refine existing regulatory requirements in a timely manner if concept-specific accident tolerant fuel (ATF) features warrant it.

What other U.S. Government organizations is the NRC coordinating with?

The U.S. Department of Energy (DOE) promotes the United States' ATF program and issues grants or loans to vendors that are developing the ATF concepts. Regular NRC/DOE meetings discuss progress and provide feedback on the activities conducted for enabling ATF.

The NRC staff is also examining DOE-funded National Laboratories' roles and abilities for ATF. The NRC staff has visited Oak Ridge National Laboratory and Idaho National Laboratory to observe tests and to learn about the labs' experimental capabilities for irradiated and un-irradiated fuel and cladding.

The NRC staff will coordinate with other U.S. Government agencies when the need arises.

See the webpage on DOE interactions for more information.

Other countries are pursuing ATF concepts. How is NRC staff interacting with international counterparts?

The NRC frequently interacts with international counterparts through Nuclear Energy Agency (NEA) working groups – most notably the Working Group on Fuel Safety – and through international cooperative research programs – such as the Studsvik Cladding Integrity Project (SCIP) in Sweden and the QUENCH program at the Karlsruhe Institute of Technology in Germany.

See the webpage on international interaction for more information.

The ATF Project Plan assumes that no confirmatory testing will be needed for the current ATF development timelines. Are there conditions under which NRC would pursue independent testing?

As stated in the ATF Project Plan, in some cases of large safety significance and large uncertainty [in confirmatory calculations], the NRC has pursued independent confirmatory testing before reaching a determination on an application. The NRC staff actively coordinate with DOE and applicants during their testing campaigns.

Who are the main fuel vendors involved in ATF?

As of July 2020, there are three main fuel vendors developing ATF: Westinghouse Electric Company, Framatome, and Global Nuclear Fuels. All three fuel vendors each have multiple types of ATF under development.

Increased Enrichment and Higher Burnup Questions

What is increased enrichment?

Enrichment refers to the percentage of Uranium-235 (U-235) isotope contained within a mass of Uranium, with the rest being Uranium-238 (U-238). When natural Uranium is mined from the earth, approximately 0.7% is U-235 and 99.3% is U-238. Historically, U.S. commercial nuclear plants have used fuel within the range of around 3.5% to 5% U-235. For implementation of the near-term ATF concepts, industry has expressed interest to increased enrichment above the current licensed enrichment limit of 5% to up to 10% U-235 by weight.

See the webpage for increased enrichment for more information.

How will the NRC determine if ATF with increased enrichment is safe?

The NRC regulations provide reasonable assurance of adequate protection of public health and safety. Vendors and licensees must meet the applicable regulations, and fuel with increased enrichment is no exception. When increased enrichment topical reports and other related licensing actions are received by the NRC, they will be reviewed, and the NRC staff will make a safety finding as part of this review process.

What are the regulatory and technical challenges of increased enrichment?

The regulation in 10 CFR 50.68, "Criticality accident requirements," limits new fuel assemblies to 5% enrichment, but provides an exception for those licensees maintaining a criticality monitoring system per 10 CFR 70.24, "Criticality accident requirements". Similarly, the regulation in 10 CFR 71.55, "General requirements for fissile material packages," provides an exception for licensees from evaluating water intrusion in packages for the transport of enriched uranium hexafluoride to 5% enrichment. For Part 50 licensees to increase above this 5% limit, either an exemption or a change in the rule will be required, and the need for rulemaking to amend these regulations will be reviewed regularly by the NRC. For Part 71 licensees, a water intrusion evaluation is required by regulation, or an applicant may request an exemption.

The changes to the vendors' facilities necessary to accommodate small increases above 5% enrichment appear to be minor. For larger deviations from the current 5% limit, there are greater challenges related to criticality benchmarks, development of new codes, and licensing of transportation packages and storage systems.

Will approval of ATF for increased enrichment require a rulemaking?

Approval of increased enrichment does not require a rulemaking. Under the current regulations, licensees are able to request exemptions from 10 CFR 50.68 and 10 CFR 71.55 that, if approved, may allow them to increase their fuel enrichment to the requested levels.

How will the NRC ensure increased enrichment processes prevent proliferation?

The regulations in 10 CFR Part 73, "Physical protection of plants and materials," proscribes requirements for the establishment and maintenance of a physical protection system which will have capabilities for the protection of special nuclear material. Licensees must follow the security requirements of 10 CFR Part 73. For increased enrichment, vendors and licensees must make appropriate changes, if any, to their security plans, equipment, and forces to maintain the same level of physical protection as prior to increased enrichment.

What is higher burnup?

Burnup refers to the amount of power produced by a given mass of nuclear fuel, and has units of gigawatt-days per metric ton of Uranium, abbreviated GWd/MTU. Limits are placed on burnup because fuel and cladding properties evolve as burnup increases, and the fuel properties need to be well understood within their range of operation. Generally, the NRC currently limits burnup to 62 GWd/MTU on a fuel rod average basis, any burnup limits higher than 62 GWd/MTU rod-average is considered "higher burnup". Vendors and licensees are exploring burnup levels as high as 75 or 80 GWd/MTU rod-average for ATF.

See the webpage for higher burnup for more information.

What are the regulatory and technical challenges of high burnup?

The NRC staff do not anticipate any significant regulatory challenges because the licensees will still have to meet the current fuel integrity requirements. Burnup for individual Part 50 (power reactor) licensees is limited through their approved fuel analysis methods listed in their Technical Specifications. To increase burnup, licensees will need a license amendment to modify their fuel analysis methods.

A potential technical challenge associated with high burnup is fuel fragmentation, relocation, and dispersal (FFRD). FFRD is the observation that under certain postulated accident conditions fuel that has exceeded the current burnup limits may fragment into small pieces, relocate within the fuel rod, and potentially disperse into the reactor coolant if a rod were to rupture. The NRC, fuel vendors, and licensees continue to participate in international research programs aimed at furthering understanding of FFRD and recognize the importance of these burnup-sensitive phenomena. How FFRD will be addressed for higher burnup fuels is still under discussion with industry stakeholders.

For Part 71 licensees (spent fuel storage and transportation), higher burnup presents some challenges that are similar to unirradiated material transport. For example, benchmarking criticality safety is still an issue for transportation and storage of irradiated (spent) fuels for increased enrichments (between 5 and 8 wt % U-235) that are necessary to obtain higher burnups. Additional challenges may exist depending on the licensing or certification strategy. Other areas where challenges may exist include performance of the cladding material during vacuum drying, aging while in dry cask storage, fatigue data for transportation, and benchmarking the isotopic depletion analyses for use in the shielding. More details on the challenges can be found in the ATF Project Plan Appendix A.

Source terms for nuclear facilities using ATF or fuels designed for higher burnup and increased enrichment could be impacted. What is being done to consider these possible impacts?

Implementation of ATF or fuels designed for high burnup and increased enrichments, and associated facility modification by nuclear power plant licensees, should be supported by licensee evaluations of significant radiological and nonradiological impacts of the proposed actions. Such evaluations should consider the impact of the proposed changes on the facility’s compliance with the regulations, licensee commitments relevant to accident source terms, and any other facility-specific requirements. The NRC staff does not expect a complete recalculation of all facility radiological analyses but does expect licensees to evaluate impacts of the proposed changes and update affected analyses and the design documents appropriately.

The NRC is currently in the process of updating Regulatory Guide 1.1831, 2, "Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors". This update will account for near-term ATF concepts, fuel burnup levels up to approximately 68 GWd/MTU peak rod-average, and U-235 enrichments up to 8%. Also, the NRC is currently working to determine the generic source term for a burnup level of approximately 75 gigawatt days per metric ton of uranium peak rod average using the MELCOR severe accident code and incorporating insights from the NRC-sponsored severe accident phenomena identification and ranking table exercise.

1 Regulatory Guide 1.183 Revision Public Meeting Slides (11/19/2020)
2 Final Regulatory Guide (RG) 1.183 Revision Public Meeting #2 Slides

Transportation and Storage

How will irradiated ATF be stored?

The NRC has engaged the fuel cycle industry on their plans for storage of irradiated ATF. The NRC has not received any plans from industry for the storage of spent ATF. The NRC anticipates that industry will develop storage systems for irradiated ATF that are variations on existing concepts; this can likely be done within the existing regulatory framework, particularly for the near-term concepts.

Is irradiated ATF more dangerous than normal irradiated fuel?

The NRC staff is reviewing and evaluating the performance of systems containing irradiated ATF. For the ATF lead test assemblies inserted in power reactors, the staff has not identified any characteristics that suggest the irradiated ATF designs present a greater hazard to the public. The NRC is applying the same performance requirements to irradiated ATF fuel as conventional zirconium alloy cladded uranium dioxide fuel.

Please see the Spent Fuel Storage in Pools and Dry Casks Key Points and Questions & Answers for more information about spent fuel transportation and storage.

Codes and Modeling

Does the NRC use simulation and modeling to support its reviews? If so, how?

The NRC uses a range of tools to verify the safety case made by an applicant. In some instances, the NRC can reach a safety determination by drawing on previous knowledge, accumulated expertise, and the information presented by the applicant. In other cases, NRC staff performs calculations with extensively validated modeling and simulation computer codes specifically tailored to evaluate (1) if the submittal meets regulatory requirements and (2) the phenomena important to safety. Using modeling and simulation generally provides increased confidence in the applicant's results and allows for a more effective and efficient review.

See the webpage for independent confirmatory calculation for more information.

Can "advanced modeling and simulation" be used to accelerate the licensing process?

Advanced modeling and simulation can inform experimental programs and identify testing priorities. However, today's advanced modeling and simulation tools may not be mature enough to substitute modeling for experiments. Fuel performance has always been an area where extrapolating data is extremely difficult.

Phenomena Identification and Ranking Table (PIRT) exercises

What is a PIRT? Why is it important for ATF?

PIRT stands for phenomena identification and ranking table. PIRTs are used to systematically identify phenomena that are of both high importance and high uncertainty, and thus of primary interest for further studies. To develop a PIRT, experts in the specific technical area are gathered together to identify and rank the potential technical issues based on published literature and their own knowledge of the subject. This ranking is used to develop a report, which is then used to inform the NRC staff as to the technical issues the experts might foresee for an ATF technology.

See the webpage for PIRTs for more information.

What are the staff's plans for conducting PIRT exercises?

The NRC has completed a PIRT on the in-reactor performance of chromium-coated zirconium-alloy cladding. The PIRT panel meeting was conducted in April 2019, and the final report was released in June 2019. For more information on upcoming PIRT exercises, please see the PIRT webpage.

Page Last Reviewed/Updated Monday, May 03, 2021