Lightbridge fuel rods could outperform MOX in plutonium disposition

March 2, 2023, 9:30AMNuclear News
Mock-up of four-lobed helical fuel rods. (Photo: Lightbridge)

Lightbridge Corporation, which is continuing to work closely with national laboratories on the manufacture and testing of its metallic fuel rod designs for light water reactors, just announced the results of an investigation on the casting process for molten uranium and zirconium with Pacific Northwest National Laboratory under the Department of Energy’s Gateway for Accelerated Innovation in Nuclear (GAIN) program.

At the same time, the company is investigating other uses for its helical, metallic fuel design. According to a peer-reviewed paper recently published in the American Nuclear Society journal Nuclear Technology, a plutonium-zirconium fuel rod could significantly outperform traditional mixed-oxide (MOX) fuel as a vehicle for weapons-grade plutonium consumption in light water reactors. In the computer simulation detailed in the paper, a “plutonium disposition fuel variant” consumed about 5.5 times more plutonium per fuel rod than MOX fuel, according to Lightbridge.

“We are pleased with the acceptance and publication of this paper in this prestigious journal. The findings reported in this study were subject to rigorous review by top scholars, validating the plutonium disposition and proliferation resistance of this variant of Lightbridge fuel rods,” said Seth Grae, Lightbridge president and chief executive officer.

Burnup is key: Previous research has confirmed the proliferation resistance of uranium-zirconium Lightbridge fuel rods, according to the company, and the new research confirms that proliferation resistance persists when uranium is swapped for plutonium, with the high burnup of metallic fuel making reducing the usefulness of any residual plutonium for weapons purposes.

Improved Disposition of Surplus Weapons-Grade Plutonium Using a Metallic Pu-Zr Fuel Design” was coauthored by Braden Goddard, assistant professor in the Department of Mechanical and Nuclear Engineering at Virginia Commonwealth University, and Aaron Totemeier, senior nuclear fuel consultant to Lightbridge.

“Higher burnup not only fissions more atoms, but preferentially fissions plutonium atoms with odd atomic mass numbers,” the authors explain. “This results in the plutonium remaining in the used fuel to primarily consist of Pu-240 and Pu-242, both of which have lower attractiveness for use in nuclear weapons compared to Pu-239 and Pu-241.”

The researchers based their simulated plutonium disposition rod (PDR) on Lightbridge’s solid multilobe helically twisted metallic U-Zr fuel rod design developed for 17×17 pressurized water reactors, substituting the U-Zr alloy with a Pu-Zr alloy.

Monte Carlo N-Particle (MCNP) computer simulations were performed to quantify the mass of plutonium consumed in a Lightbridge-designed fuel rod, compared with traditional MOX fuel, as well as the attractiveness of the plutonium in the used fuel for weapons purposes. The researchers concluded that “although the plutonium mass in the fresh PDR (16 percent Pu) fuel is more than twice that of the MOX (5 percent Pu), the final plutonium mass in the PDR is approximately half that of the MOX rod. This means that the PDR fuel not only consumes plutonium faster than MOX fuel, but that its used fuel rods also contain less residual plutonium.”

Irradiation testing: Lightbridge is backing up simulations of its U-Zr rods with irradiation testing of fuel samples under a seven-year partnership with Idaho National Laboratory governed by two contracts signed with Battelle Energy Alliance, DOE’s operating contractor for INL, in December 2022.

The initial phase of work includes gathering irradiation performance data on thermophysical properties of Lightbridge’s delta-phase U-Zr alloy in fuel samples using enriched uranium supplied by the DOE. That irradiation would be performed in INL’s Advanced Test Reactor (ATR) to support fuel performance modeling and regulatory licensing efforts for the commercial deployment of Lightbridge fuel.

Lightbridge anticipates that later work will include post-irradiation examination of the irradiated fuel samples, loop radiation testing in the ATR, and post-irradiation examination of one or more uranium-zirconium fuel rodlets, as well as transient experiments in the Transient Reactor Test Facility at INL.

The company’s GAIN-enabled work with PNNL, announced February 27, made use of depleted uranium for a demonstration of the casting process. According to Lightbridge, “The results of this work will help Lightbridge determine a final process suitable to produce fuel material coupons for our upcoming irradiation tests in the Advanced Test Reactor.” Lightbridge has received two GAIN awards for collaboration with national laboratories.

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