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Reviewing Accident Tolerant Fuel: In-Reactor Performance | Fuel Cycle, Transportation, and Storage | Probabilistic Risk Assessment | Independent Confirmatory Calculation

How the NRC Reviews In-Reactor Performance of Accident Tolerant Fuels

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Fuels licensing process

For many decades, power reactor fuels have consisted of uranium dioxide (UO2) pellets surrounded by zirconium (Zr)-alloy cladding. Accident tolerant fuel (ATF) technologies currently under development change the fuel design and/or materials and will generally require review and approval by the NRC to assure that public health and safety are maintained.

All fuel designs, including ATF, must meet the stringent requirements detailed in Part 50 of Title 10 of the Code of Federal Regulations. In general, the following major steps may be necessary to design, qualify, and license a new fuel design and/or new fuel materials for unrestricted use in batch loads1:

  1. Development and Testing

    1. Fuel vendors conduct research to fully characterize the material, mechanical, chemical, thermal, and nuclear properties and how these properties might change under irradiation.

    2. Fuel vendors conduct separate-effects2 and integral3 testing to fully characterize the performance of the new designs/materials under the wide range of anticipated operational occurrences4 (AOOs) and accident conditions required by regulations. This testing identifies degradation mechanisms, establishes performance objectives, and defines design requirements and analytical limits that ensure acceptable fuel performance.

    3. Fuel vendors also conduct separate-effects and integral testing to fully characterize fission product release (e.g., chemical forms and release kinetics) during normal steady-state operation and under design basis accident conditions. The results of these tests may inform analyses that show that plants are in compliance with dose limits to the public and their workers.

  2. Determination of General Requirements. Based upon the testing described in Step 1 above, fuel vendors identify any existing regulatory requirements that are not satisfied or where new design-specific regulatory goals and requirements are necessary to support the unique design and performance features.

  3. Analytical Models. Fuel Vendors and licensees develop, calibrate, and validate analytical models which simulate the performance of the new design features under normal and accident conditions, quantify uncertainties, and define how these models will be applied.

  4. New Reactor-specific Requirements. For their specific reactor, licensees define Technical Specification Safety Limits, Limiting Safety System Setpoints (LSSS), and Limiting Conditions of Operation (LCO) which ensure acceptable performance under normal operation, AOOs, and postulated accidents.

  5. Justification for Use. Licensees may demonstrate that all design and regulatory requirements are satisfied for their specific reactor5.

    1. During normal operations, the fuel should maintain geometric stability, integrity, and compatibility with reactor internals, co-resident fuel, and handling equipment.

    2. During AOOs, the fuel should maintain geometry, integrity, and ability to perform intended safety functions.

    3. During postulated accidents including safe shutdown earthquake, the fuel should maintain geometry and integrity to the extent required to perform intended safety functions, including:

      1. The ability to insert control rods

      2. The ability to achieve safe shutdown

      3. Maintaining known, coolable geometry (includes minimizing fuel fragmentation, dispersal, and melting)

      4. Limiting fuel damage to satisfy allowable limits for on-site and off-site radiological consequences

The justification for use described above should be detailed in a topical report or license amendment request submitted to the NRC for review. The NRC staff may use applicable regulations, guidance, and the vendor's data to determine if the new fuel design meets the NRC's regulations. The following is a non-exhaustive list of the factors that may be in the NRC's review:

  • Review of past precedent topical reports and license amendments
  • Applicable NRC regulations and guidance
    • For example, one of the major guidance documents is NUREG-0800 that establishes the criteria the NRC staff uses in evaluating whether an applicant/licensee meets the NRC's regulations and Chapter 4 details the criteria for the reactor and reactor systems, of which fuel design is a component.
  • Environmental concerns
  • Responses to requests for additional information asked of the fuel vendor or licensee during the topical report or license amendment process
  • Independent Confirmatory Calculations performed using applicable fuel performance (or other) codes, if necessary
  • Determination of any limitations and conditions for the use of the topical report or license conditions for an amendment, if necessary

When the NRC completes its review, it issues a safety evaluation stating if the new fuel design being implemented meets the NRC's regulations and maintains the health and safety of the public, and any limitations or conditions on that evaluation.

Additionally, the NRC's Advisory Committee on Reactor Safeguards may review the topical report or license amendment request and associated NRC safety evaluation.

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Uniqueness for Accident Tolerant Fuels

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 NRC 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.

The NRC is ready to review near-term ATF technologies and has developed a project plan to prepare for the forthcoming ATF applications. This potentially transformational technology that industry is pursuing has led the NRC to reflect on the fuel licensing process and determine where improvements to efficiency and effectiveness can be gained.  

The ATF Project Plan outlines a plan on enhancing regulatory stability and efficiency through the following actions:

  • The early performance of phenomena identification and ranking table exercises. These exercises are defined as a process in which experts in the subject field identify and rank new phenomena important to safety introduced by an ATF concept.
  • Data sharing. Vendors are encouraged (but not required) to send their materials and test fuel rod performance data to the NRC prior to submitting the topical report.  This allows the NRC staff to complete the prework for the independent confirmatory calculations and to prepare for conducting the most efficient ATF submittal reviews.
  • Significantly increased levels of communication with all stakeholders. The NRC staff engages early and often with vendors, licensees, industry groups, general public, and other stakeholder groups to closely follow progress on the technologies and obtain any feedback. This close communication allows for the NRC staff to react quickly to any emergent issues.
  • Regulatory changes and guidance development occur prior to the NRC receiving an application. The most recent example of this preemptive approach is the issuance of interim staff guidance (ISG) on chromium-coated cladding. This ISG was issued in January 2020 prior to any topical reports.  Normally, regulatory changes and guidance development occur after an application or license amendment request is received. However, due to the activities outlined above, the NRC staff is able to take action prior to receipt.

With the above four activities implemented for ATF, the shortened timelines for development and approval are possible while still maintaining the health and safety of the public.

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1Typically, a batch load is 1/3 of the fuel assemblies in a reactor core that are replaced in each refueling outage. After 3 or 4 cycles the core would generally consist solely of the new fuel assembly design.

2Testing performed to investigate a specific phenomenon or property with all other test variables held constant.

3Testing performed to investigate how multiple phenomena interact under defined conditions.

4Those conditions of normal operation which are expected to occur one or more times during the life of the nuclear power unit and include but are not limited to loss of power to all recirculation pumps, tripping of the turbine generator set, isolation of the main condenser, and loss of all offsite power.

5Severe accident mitigation, spent fuel pool criticality, transportation, and long-term storage are addressed separately.

Page Last Reviewed/Updated Thursday, July 23, 2020