Fast neutron reactors are a type of nuclear reactor that use fast neutrons to sustain the fission process. Unlike traditional thermal reactors, which slow down neutrons using moderators like water or graphite, fast reactors operate without these materials. This allows them to achieve higher energy efficiency and can utilize a wider range of nuclear fuels, including plutonium and uranium. These reactors have the potential to reduce nuclear waste and make better use of existing resources. By converting fertile materials into fissile fuel, fast neutron reactors can help create a more sustainable nuclear energy cycle, contributing to long-term energy solutions.