ADVANCED FUELS | FUEL & FUEL CYCLE
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Lightbridge fuel will use High Assay Low Enriched Uranium (HALEU) Source: INL
Advantages of novel fuel Because of its design features, the metallic fuel has inherently superior technical performance in several key areas compared to conventional cylindrical fuel rods consisting of UO2
pellets in zirconium-alloy cladding tubes:
● Lower fuel temperatures: Lightbridge metallic fuel has significantly higher thermal conductivity than ceramic UO2
fuel, and since Lightbridge cladding is metallurgically
bonded to the fuel, there is no fuel-cladding gas gap as in conventional fuel rods. This results in Lightbridge Fuel operating at lower fuel temperatures, which translates into increased safety margins and better performance in normal, off-normal, and accident scenarios.
● Enhanced cooling: The multilobed, helically twisted design of the Lightbridge Fuel rods provides a significantly higher heat transfer area and greatly improved coolant mixing than cylindrical fuel rods. Since Lightbridge fuel rods are self-spacing and do not require spacer grids, they can accommodate increased coolant flow rates with lower pressure drop across the core. The use of the low pressure-drop Lightbridge Fuel is notably advantageous in SMRs that use natural circulation and passive safety systems.
● Use of HALEU: Lightbridge Fuel uses HALEU, containing adequate fissionable material to achieve power uprates, cycle-length extensions, and reach high burn-ups in light water reactors, reaching up to 50% or more burn-up beyond the current commercial fuel limits.
● Mechanical integrity of the cladding: Lightbridge Fuel utilises standard zirconium-alloy cladding material. However, unlike conventional pellet-in-tube fuel rods, the cladding is metallurgically bonded to the fuel and serves primarily as a barrier to fission product release instead of being the fuel rod’s primary structural component. As such, Lightbridge cladding is not subjected to the same mechanical stresses as in conventional fuel rods, allowing the fuel rod to maintain its mechanical integrity (both structurally and as a fission product barrier) to high burn-ups. Additionally, since the cladding is metallurgically bonded to the fuel, any breach of the cladding will only result in an isolated and localised amount of fuel being exposed to the coolant, reducing the consequences of a cladding breach.
● Mechanical integrity of the fuel: The fuel exhibits low swelling and excellent fission product retention in water- cooled reactor conditions, ensuring that the uranium- zirconium fuel alloy maintains its mechanical integrity and that low stresses are imparted on the cladding from the fuel. The co-extruded metallic fuel and cladding comprise a robust fuel rod design that withstands other mechanical loads from power transients, flow-induced vibration, seismic events.
● Proliferation resistance: The significantly reduced amount of U-238 in fresh Lightbridge metallic fuel contributes to improved proliferation resistance, since much less plutonium is produced during irradiation and it is in a proliferation-resistant isotopic mix.
● Reduced waste volume: Since Lightbridge Fuel can achieve high end-of-life burn-ups, the throughput of fuel required to operate water-cooled reactors (including for uprated power and extended cycle lengths) is significantly reduced, resulting in lower waste volumes.
These advantages inherent to the fuel design enable it to operate at higher power over longer time at lower fuel operating temperatures, with increased safety margins, while producing less waste that is more proliferation resistant than current fuel. The robust mechanical design makes it inherently better suited to withstand power adjustments, offering enhanced operational flexibility and enabling both large plants and SMRs to load follow.
Fuel development The fuel design is an improved version of a mature fuel technology that incorporated uranium-zirconium fuel alloy composition and was used for decades in water- cooled maritime reactors. Improvements made in the design to optimise it for use in commercial water-cooled reactors include adding the central displacer to reduce fuel temperatures, using a modern nuclear fuel cladding alloy, and implementing the multi-lobed geometry for optimised heat transfer and coolant mixing. The maritime fuel used high-enriched uranium, and exhibited excellent performance to very high burn-ups, up to more than double the current end-of-life burn-up limits for commercial nuclear fuel. Since Lightbridge Fuel uses HALEU instead of high-enriched uranium, the expected burn-up levels are
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