Designs SMRs are envisioned in multiple designs. Some are simplified versions of current reactors, others involve entirely new technologies. All proposed SMRs use nuclear fission with designs including thermal-neutron reactors and fast-neutron reactors. Thermal-neutron reactors Thermal-neutron reactors rely on a moderator to slow neutrons and generally use as fissile material. Most conventional operating reactors are of this type. Fast reactors Fast reactors don't use moderators. Instead they rely on the fuel to absorb higher speed neutrons. This usually means changing the fuel arrangement within the core, or using different fuels. E.g., is more likely to absorb a high-speed neutron than . Fast reactors can be breeder reactors. These reactors release enough neutrons to transmute non-fissionable elements into fissionable ones. A common use for a breeder reactor is to surround the core in a "blanket" of , the most easily found isotope. Once the undergoes a neutron absorption reaction, it becomes , which can be removed from the reactor during refueling, and subsequently used as fuel. Technologies Cooling Conventional reactors typically use water as a coolant. SMRs may use water, liquid metal, gas and molten salt as coolants. Coolant type is determined based on the reactor type, reactor design, and the chosen application. Large-rated reactors primarily use light water as coolant, allowing for this cooling method to be easily applied to SMRs. Helium is often elected as a gas coolant for SMRs because it yields a high plant thermal efficiency and supplies a sufficient amount of reactor heat. Sodium, lead, and lead-bismuth are common liquid metal coolants of choice for SMRs. There was a large focus on sodium during early work on large-rated reactors which has since carried over to SMRs to be a prominent choice as a liquid metal coolant. SMRs have lower cooling water requirements, which expands the number of places a SMR could be built, including remote areas typically incorporating mining and desalination. Thermal/electrical generation Some gas-cooled reactor designs could drive a gas-powered turbine, rather than boiling water, such that thermal energy can be used directly. Heat could also be used in hydrogen production and other commercial operations, such as desalination and the production of petroleum products (extracting oil from oil sands, creating synthetic oil from coal, etc.). Load following SMR designs are generally expected to provide base load power; some proposed designs can adjust their output based on demand. Another approach, especially for SMRs that can provide