A model has been developed to describe the transient pressure field within the interconnected porosity of solid mixed-oxide fast reactor fuel during a reactor transient. The pore gas may be composed of up to two distinct chemical species, so that gas released from fuel grains may differ chemically from the fill gas originally present within the porosity of the fuel. The volume expansion of fuel upon melting is accounted for, but mechanical deformation of the solid fuel is not modeled. Results are presented for a hypothetical unprotected transient over-power accident in a gas-cooled fast reactor with ramp rates of 0.10, 1.0, and 10.0 dollar/s. In these calculations, fuel cladding failure is computed from a linear accumulative damage law and a Larson-Miller parameter correlation.