Nuclear Fuel Recycling : Are There NO Issues?

Chimes, Michael and Boxall, Colin (2022) Nuclear Fuel Recycling : Are There NO Issues? PhD thesis, Lancaster University.

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Abstract

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. 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). However, previous work attempting to fit Np(V) oxidation data to the current accepted kinetic expressions has shown inconsistencies, most especially with respect to i. The key oxidant in the forward going conversion of Np(V) to Np(VI); and ii. The nature of the so-generated reductant for the reverse reduction of Np(VI) to Np(V). 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 the reaction outlined in point i. above, 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. In order to support method development prior to experiments on real neptunium samples, work has been performed to look at the electrochemical behaviour of both nitrous acid and nitricoxide, and reduction reactions have been performed using vanadium as a non-radioactive analogue for neptunium. 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 in the vanadium system has been seen to make it kinetically slower. 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.