A detailed analysis of hyperstoichiometric carbide fuel, which operated under sodium-bonding conditions up to 12.5 at. % burnup in the Rapsodie reactor, yields the description of the four contributions to geometric fuel swelling as functions of temperature and burnup: (a) solid fission products and cesium, (b) fission gas swelling, (c) coarse porosity, and (d) the sum of all direct and indirect statistical swelling effects arising from the fracturing of the pellets. Fission gas swelling has to be separated into the contributions of three bubble populations and gas in solution. Between 7 and 11 at.% burnup, the relative amounts of the four swelling contributions are about the same and do not vary with burnup. The total amount of the cross-sectional swelling ΓA of a pellet can be approximately represented as a function of burnup F and linear heat rating x byΓA = b×Fn,where b and n are empirical constants and b decreases as a function of fuel composition in the order MC > MC  M2C3 > M(C,N) > MN. The carbide pins investigated in this work, having a smear density of 72% and maximum linear heat rating of 88 kW/m at a cladding temperature of 820 K, reach a maximum burnup of 12.5 at.% with very little fuel-cladding mechanical interaction. The most promising development potential for carbide fuel lies in improving its mechanical properties, i.e., in reducing the propensity of the pellets to fracture.