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How does fuel in a sodium-cooled fast reactor differ from fuel in a light-water reactor?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

Fuel in a light-water reactor (LWR) and a sodium-cooled fast reactor differs primarily in composition and design to accommodate the specific neutron energy requirements of each system.

Light-Water Reactors (LWRs): These reactors use fuel consisting of sintered uranium dioxide pellets enriched to 3 to 5 percent. These pellets are loaded into zirconium alloy cladding tubes, which are bundled into a square lattice.1
Sodium-Cooled Fast Reactors: These systems, also known as liquid-metal reactors, utilize different fuel configurations depending on the design. Some designs use uranium dioxide or uranium-plutonium dioxide pellets, while others—notably in the United States—use uranium-plutonium-zirconium metal alloy pins. Unlike the zirconium cladding common in LWRs, these fuel pins are typically housed in steel cladding.12

Because fast reactors operate without a moderator to slow down neutrons, they require higher concentrations of fissile material than thermal reactors like the LWR to reach criticality.3