Reduction Reactions of Neptunium & Neptunium Analogues with Nitrogen Oxide Species

In reprocessing flowsheets for spent nuclear fuel, one challenge which needs to be addressed is the controlled routing of neptunium. This is of importance as neptunium, along with the other minor actinides, contributes significantly to the long-term radiotoxicity of radioactive waste and can be highly mobile in the environment. Its presence across various reprocessing streams also contributes to the radiolysis of the nitric acid medium as well as other species present. Radiolysis of nitric acid, largely due to Pu/minor actinide alpha emission and fission product gamma emission, gives rise to significant in-process concentrations of redox-active nitrous acid; the following chemical reactions of this radiolytically generated HNO₂ then giving rise to a range of similarly active nitrogen-oxygen redox species such as NO₂, N₂O₄ and NO.<br/>The effect of the concentrations of nitrous and nitric acid on the extent of oxidation of neptunium is dependent on the HNO₃:HNO₂ ratio - control of the Np(V)/nitrous ratio has been found to be key in achieving near-complete Np extraction as Np(VI).<br/>NpO₂⁺ + 0.5NO₃^- + 1.5H⁺ ↔ NpO₂²⁺ + 0.5HNO₂ + 0.5H₂O (1)<br/>However, work in these laboratories attempting to fit Np(V) oxidation data to the current accepted kinetic expression has shown inconsistencies, most especially with respect to;<br/>(i) The key oxidant in the forward going conversion of Np(V) to Np(VI); and<br/>(ii) The nature of the so-generated reductant for the reverse reduction of Np(VI) to Np(V).<br/>Thermodynamic analysis based on the redox potentials of possible nitrogen-oxygen species present within a spent fuel reprocessing scheme suggests that the key oxidant in reaction (i) is N₂O₄. This oxidation of NpO₂⁺ by N₂O₄ would thereby generate NO, a hitherto unconsidered species with the capacity to act as the reductant in the reverse reduction of Np(VI) back to Np(V) – thus addressing point (ii) above. Therefore, examination of the kinetics with respect to the net production of the known reducing agent NO is needed to determine its role in the oxidation/reduction reactions of the actinides.<br/>In order to support method development prior to experiments on real neptunium samples, vanadium was used as an analogue. Whilst vanadium shows similar electrochemical potentials for the VO₂⁺ reduction to VO²⁺ to the reduction of Np(VI) to Np(V), making it a good thermodynamic analogue, the removal of the bonded oxygen has been seen to make it kinetically slower. Additionally, this analogue is seen to form the pervanadyl ion, making them similar in bonding to the neptunyl form seen for Np(VI). With previous experiments demonstrating the analogous behaviour between the VO₂⁺ and Np(VI) reduction with nitrous acid, experiments have been performed to investigate the reduction of both the VO₂⁺ and NpO₂²⁺ species by NO. These experiments have allowed for the deduction of a mechanism of reaction to be proposed which addresses the inconsistences in the accepted kinetic expression previously found and detailed above. This, along with the limitations of the vanadium analogue, will be discussed and experiments with real neptunium samples presented.