Radiolytic oxidation behavior of neptunium in sodium chloride solutions

Abstract

Radiolytic oxidation behavior of neptunium was investigated in order to predict its migration behavior at the disposal site for radioactive waste. Neptunium, in 5 mol/dm3 sodium chloride solutions of several pH values, was irradiated by α-particles of 238Pu which had been placed in the solutions as dioxide powder. Solution neptunium redox behavior was compared with that of an unirradiated sample. Pentavalent neptunium, which was stable in the absence of 238Pu, was found to be oxidized to hexavalent and even to heptavalent neptunium. Oxidizing species would be chloride molecule anion (Cl−2) and/or hypochlorite anion (ClO−) which were generated by the reaction between radiolytically generated hydroxide radical (OH) and chloride ion (Cl−). The oxidation rate of pentavalent neptunium was independent of its concentration, but dependent on solution pH. The measured rate constant was (19±4)[H] mol/dm3/d. © 1996 Taylor and Francis Group, LLC.