Mechanisms of photochemical reactions of neptunium ions in nitric acid solutions

Photochemical reactions of Np(V) and Np(VI) in urea-containing HNO3 + KNO3 solutions are studied. UV radiation induces oxidation of Np(V) and reduction of Np(VI). Variation of the H+ concentration from 0.05 to 1.0 M (with constant [NO3] = 1.0 M) accelerates the former and inhibits the latter reaction. Reaction mechanisms are proposed, involving light absorption with nitrate ion, formation of excimers (dimer;) from excited and unexcited neptunium ions, and their decomposition with formation of Np(VI) and H-2 in oxidation of Np(IV) or Np(V) and of H2O2 in reduction of Np(VI).