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Technologies: Coated Cladding | Doped Pellets | FeCrAl Cladding | Increased Enrichment | Higher Burnup | Longer Term Technologies

Longer Term Accident Tolerant Fuel Technologies

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Industry has proposed several different concepts that they consider to be part of the accident tolerant fuel (ATF) program. The ATF Project 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, depending on the level of licensing credit requested by the vendors and/or licensees. The concepts that the NRC considers to be near-term are coated claddings, doped pellets, and iron-chromium-aluminum (FeCrAl) cladding. Also, increased enrichment and higher burnup are being explored to help further lower the costs of electricity production over the licensed lifetime of an existing commercial nuclear reactor.

Longer term ATF concepts are those for which substantial new data, models, and methods need to be acquired or developed to support the NRC's safety evaluations. These technologies often need significant research and development and the implementation dates are many years into the future. The following concepts proposed by industry are considered to be longer term: uranium nitride (UN) pellets, silicon carbide (SiC) cladding, and extruded metallic fuel.

Uranium Nitride Pellet

Westinghouse is working with Idaho National Laboratory to develop uranium nitride (UN) to replace uranium dioxide in fuel pellets. Originally, uranium silicide (U3Si2) was being explored for use with lead test assemblies. However, after further research, Westinghouse determined that U3Si2 was not viable for future development and changed their efforts towards the advancement of UN. The potential benefits of UN fuel pellets are:

  • Increase in density of uranium which promotes higher power or longer fuel cycles
  • High melting point
  • Low parasitic neutron absorption
  • Increase in fuel thermal conductivity promotes lower operating temperatures

The potential challenges of UN fuel pellets that need to be overcome are:

  • Require a rare isotope of nitrogen (15N). The process to separate the rare isotope for use adds a significant additional cost.
  • High chemical reaction rate with light water reactor coolants at nominal operating temperatures

UN pellets are currently undergoing research and development. The NRC staff continues to follow developments in the technology.

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Silicon Carbide Cladding

Several silicon carbide (SiC) composite cladding materials are being developed by General Atomics and Framatome. For this technology, SiC fibers are woven, then impregnated with additional silicon carbide to form a rigid tube. SiC has been used in many industrial applications, but not as in-reactor components.

The potential benefits of SiC cladding are:

  • Maintains structural integrity at very high temperatures, even beyond the temperature of uranium dioxide melting
  • Improved high-temperature steam oxidation, which translates into longer coping times and less hydrogen generation under design basis accident and severe accident conditions

The potential challenges of SiC cladding that need to be overcome are:

  • Increased permeability to fission gases
  • Corrosion during normal reactor operation could lead to mass loss of the SiC, resulting in a weakening of the cladding
  • Lack of ductility may be a problem for plant power changes, expected operational occurrences, and postulated accidents
  • Ability to be manufactured. For example, fastening end plugs on the SiC tubing has proven to be a challenge

SiC cladding is currently undergoing research and development. The NRC staff continues to follow developments in the technology.

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Extruded Metallic Fuel

The company Lightbridge is developing a new fuel design that incorporates an extruded metallic bar composed of a zirconium-uranium matrix within a zirconium alloy cladding.

The potential benefits of extruded metallic fuel are:

  • Significant increase in fuel thermal conductivity (compared to ceramics) promotes lower operating temperatures
  • Complete retention of fission products means no burst release of those products upon cladding failure
  • Supports higher power and longer fuel cycles

The potential challenges of extruded metallic fuel that need to be overcome are:

  • Due to the relatively low density of uranium in the zirconium-uranium matrix compared to the uranium density in uranium dioxide fuel, a higher U-235 enrichment is necessary (up to 19.8%, much higher than the currently desired up to 10% for other ATF technologies). This enrichment has a greater criticality risk and significantly higher costs.
  • Little data and experience exists for light water reactors

Extruded metallic fuel is undergoing research and development. The NRC staff continues to follow developments in the technology.

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Page Last Reviewed/Updated Thursday, July 23, 2020