AbstractThe dissolution process for sintered uranium dioxide into nitric acid falls into two regimes of behaviour which depend on the overall transfer rate. The critical dissolution rate is approximately 8 mg. cm.−2min−1, its precise value depending upon the solvent composition. Below this critical value there is a large degree of chemical kinetic control, the dissolution rate being proportional to the cube of nitric acid concentration, the experimental activation energy being 15 kcal. mole−1. Above the critical value the process appears to be controlled by the diffusion of a nitric acid species as the transfer rate is directly proportional to acid concentration with an activation energy of about 4 kcal. mole−1.In addition, the reaction is autocatalysed by the nitrite product so that forced convection can reduce rates by a factor of four times, and nitrite neutralisers can reduce them by a factor of one hundred times.To a first approximation the dissolution rate depends simply on the total nitrate concentration although polyvalent nitrates and, in particular, ferric nitrate, tend to accelerate the r