A nuclear reactor is used to control nuclear fission. In most reactors, uranium is encased in metal tubes that are inserted into the reactor. The tubes are surrounded by a moderator, such as graphite, that slows sown the reaction.
In another form of reactor the fuel rods are surrounded by low-grade uranium, and the neutrons escaping from the reactor strike this material. The radiation converts the low-grade uranium into plutonium, which can be used as a nuclear fuel. This type of reactor, called a fast-breeder reactor, produces more plutonium that it can use as fuel. Although this sounds very useful, plutonium is much more dangerous to handle than uranium, and there are serious safety problems in its use.
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