Fission products, which include solid, volatile and gaseous species, accumulate in the fuel as irradiation proceeds. Since each heavy metal atom that fissions is generally replaced by two fission product atoms, the accumulation of fission products causes the pellets/bars to swell. The fission products also degrade the thermal conductivity of the fuel. However, the behaviour is complicated by the fission product chemistry, the different ways in which the fuel matrix accommodates the fission product atoms (occupying lattice positions, as interstitials, as separate phases, etc.), and the migration of the volatile and gaseous species. Swelling is a particular problem in metallic fast reactor fuel (IAEA, 2003b), in Magnox fuel (Harris and Duckworth, 1982) and in carbide and nitride fast reactor fuel (Bailly et at., 1999). The overall swelling is often divided into two components: ‘inexorable swelling’ due to solid fission products and volatile fission products that are solid at the fuel temperatures of interest; and ‘gaseous swelling’ due to gaseous fission products and volatile fission products that are gaseous at the fuel temperatures of interest. Gaseous swelling is described in more detail below.