A Fast Breeder Reactor (FBR) is a type of nuclear reactor that generates more fissile material than it consumes. It uses fast neutrons to initiate fission reactions, allowing it to convert non-fissile isotopes, like uranium-238, into fissile isotopes, such as plutonium-239. This process helps to utilize nuclear fuel more efficiently and extends the availability of nuclear resources. FBRs are designed to operate without a moderator, which distinguishes them from traditional reactors that slow down neutrons. By breeding fuel, FBRs can potentially provide a sustainable energy source, reducing the need for mining new uranium and minimizing nuclear waste.