Thorium-Based Seed & Blanket Technology

Lightbridge's patented thorium-based seed-and-blanket fuel assembly consists of two sections: (1) metallic fuel rods in the central region called the seed that is surrounded by (2) oxide fuel rods in the outer region called the blanket.

The seed incorporates proprietary all-metal fuel rods which differ from conventional fuel rods and offer certain reactor safety and cost benefits. The seed rods are comprised of a zirconium-based alloy produced using a co-extrusion fabrication process.

The blanket is comprised of thorium-uranium oxide pelletized fuel rods similar to that of conventional fuel rods presently used in commercial light water reactors. The main difference between the blanket fuel rods and conventional fuel rods is the material contained in the pellets: thorium-uranium oxide (Th,U)O2 mixture in the blanket rods vs. uranium oxide (UO2) in conventional uranium fuel rods.

The benefits of this design include:

  • Once-through fuel cycle based on patented seed and blanket fuel assembly design that efficiently utilizes thorium – no reprocessing of used fuel needed to take advantage of thorium (U-233 bred from thorium is fissioned in situ providing up to one-third of total reactor power output);
  • Full compatibility with existing light water reactor designs (no modifications to reactor internals are required);
  • Enhanced proliferation resistance of the used fuel (reduced quantity of plutonium in used fuel and increased fraction of even Pu isotopes);
  • Reduced natural uranium requirements (up to 10% natural uranium savings);
  • Reduced decay heat of spent fuel inventory (up to 22% reduction at discharge, up to 35% reduction at 3 years after discharge, and up to 50% reduction at 100 years after discharge) ;
  • Reduced volume (up to 40% reduction) and long-term radio-toxicity (up to 90% reduction) of used fuel; and
  • Improved fuel cycle economics (cost competitive on the front-end with significant cost savings expected on the back-end).


We currently have variants of the thorium-based fuel designs under development. The first is designed to provide reactor owners/operators with a proliferation resistant fuel technology that generates significantly less plutonium in the spent fuel. In some earlier technical publications, this fuel design has also been referred to as Radkowsky Thorium Fuel or RTF. The second is designed to dispose of reactor-grade plutonium that has been extracted from spent fuel from commercial reactors. The third is designed to dispose of weapons-grade plutonium that is stockpiled primarily in Russia and the United States. These two variants utilize plutonium-zirconium metal fuel rods in the seed region and thorium-uranium oxide fuel rods in the blanket region. In some earlier technical publications, this fuel design was referred to as Radkowsky Thorium Plutonium Incinerator (RTPI). This fuel design has similar proliferation resistance and waste reduction benefits as the uranium-zirconium metal seed and thorium-uranium oxide blanket fuel design and offers the most efficient way to burn surplus plutonium stockpiles in existing light water reactors. In 2005, Westinghouse Electric Company conducted an independent technical review of this design, paid for by the US Department of Energy, and issued a favorable assessment.