Photochemical Oxidation of Neptunium(V) to Neptunium (VI) in Nitric Acid Solution Containing Reductants.

The behavior of photochemical reactions of Np(V) in a nitric acid solution was studied under various test conditions as follows. As light sources, a mercury lamp and a semiconductor laser were used, and the light from the sources was irradiated on 2 ml of neptunium nitric acid solutions in a quartz cell. The variables in these photochemical tests were the light irradiation rates (0.2 and 1.5 W/cm2), the wavelength regions of the irradiation light (ultraviolet (UV) (250–400 nm)), visible (400–600 nm) and 980 nm), the acidities of the nitric acid (1 and 3M) and the kinds of addition reagents (hydroxylamine + hydrazine and urea). The photochemical changes in Np valences in the test solutions depending on the irradiation times were analyzed using a spectrophotometer. Based on these tests, the following results were obtained. • Neptunium(V) was photooxidized to Np(VI) by the irradiation of UV light on a nitric acid solution. • The higher the irradiation rate and the concentration of nitric acid, the easier the photo-oxidation reaction of Np(V) progressed. • The addition reagents influenced the oxidation reaction rate and the degrees of the progress in the photooxidation reaction of Np(V)→Np(VI). Based on the results of these tests, the quantitative control of the photooxidation reaction of Np(V) was found to have much potential for a valence adjustment by selecting the appropriate conditions of the UV irradiation rates, the acidity of HNO3 and the kinds of reductants.