Investigation of the influence of alpha-irradiation on the valence states of actinoids. XI. Oxidation of uranium(IV) in perchloric and nitric acid solutions

A spectrophotometric method is used to study the kinetics of the radiochemical oxidation of four-valence uranium in solutions of perchloric (0.5-12 mole/liter) and nitric (0.3-12.5 mole/liter) acids under the action of intense internal alpha-radiation provided by curium-244. For solutions of perchloric acid it is shown that the preparation technique and the method of introduction of the alpha-emitter into the solution have great significance. The dependence of the rate of radiochemical oxidation of uranium(IV) on its concentration (2-42 mmole/liter) and the dose strength (0.5-8 Gy/sec) is found for introduction of curium in the form of a dry salt and in the form of a solution. In the latter case the oxidation of uranium(IV) proceeds primarily via the products of radiolysis of perchloric acid, the initial stage of the formation of which depends on the means of introduction of the alpha-emitter. In nitric acid solutions, in contrast with perchloric acid solutions, the oxidation rate decreases with increase in concentration of acid. The rate varies symbatically with the variations of the coefficient of molar cancellation of the characteristic absorption band of uranium(IV) at 647 nm. This is obviously related to the particular features of the radiolysis of nitric acid solutions and complex formationmore » of uranium(IV).« less