The heart of any nuclear reactor is the core, which contains the fuel, a moderator, and control rods. The fuel used in some reactors consists of uranium oxide, enriched with about 3-4 % of uranium-235. In other reactors, the fuel consists of an alloy made of uranium and plutonium-239. In either case, the amount of fissionable material is actually only a small part of the entire fuel assembly.
The fuel elements in a reactor core consist of cylindrical pellets about 0.6 in (1.5 cm) thick and 0.4 in (1.0 cm) in diameter. These pellets are stacked one on top of another in a hollow cylindrical tube known as the fuel rod and then inserted into the reactor core. Fuel rods tend to be about 12 ft (3.7 m) long and about 0.5 in (1.3 cm) in diameter. They are arranged in a grid pattern containing more than 200 rods each at the center of the reactor. The materials that fuel these pellets are made of must be replaced on a regular basis as the proportion of fissionable nuclei within them decreases.
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