Which type or types of nuclear reactors have these characteristics? \(\begin{array}{l}{\text { (a) Can use natural uranium as a fuel }} \\ {\text { (b) Does not use a moderator }} \\ {\text { (c) Can be refueled without shutting down }}\end{array}\)

The only type of nuclear reactor that fulfills all three given characteristics is the Fast Breeder Reactor (FBR). Let's review each characteristic for the FBR: a) Can use natural uranium as a fuel: FBRs can use natural uranium as well as a mixture of uranium and plutonium as a fuel. Fast neutrons are more likely to cause fission in natural uranium than in enriched uranium, making it suitable for FBRs. b) Does not use a moderator: FBRs don't use a moderator because they rely on fast neutrons for fission reactions. Instead, they use a coolant, usually liquid sodium, to transfer heat from the reactor core to the power cycle. The coolant also helps to slow down the neutrons without moderating their energy levels to the thermal range. c) Can be refueled without shutting down: FBRs have a continuous fueling process, known as on-load refueling, which allows fuel to be added and removed without stopping the reactor. This increases the plant's uptime and operational efficiency. In conclusion, the Fast Breeder Reactor (FBR) is the type of nuclear reactor that fits all three given characteristics.