The purpose of the fuel rod is to keep the fuel in a well-defined geometry and to provide the first barrier separating the fission products from the environment. Preserving its integrity is therefore the primary goal of fuel design and rules for reactor operation. As illustrated in Fig. 9.8, a fuel rod consists of a cladding tube

Lower end plug j — Fuel pellet j- Cladding

into which fuel pellets are inserted, a compression spring keeping the pellets axially in place during handling and transport, some extra space (plenum, about 10% of the fuel volume) to accommodate gaseous fission products and thermal expansion of the pellet stack, and end plugs welded onto the tube closing it and making it leakproof. The rod is filled with helium at a pressure depending on reactor type. Typical dimensions of cladding and fuel pellets are listed in Table 9.2.