Reprocessing spent nuclear fuel allows the recovery of valuable metals (e.g. U, Pu, Np, and Am) for either fabrication into new fuel elements or promotion of more effective nuclear waste management. Current reprocessing methodologies utilize solvent extraction techniques to separate desired metal ions based on their oxidation state; typically, the tetravalent and hexavalent states are targeted. Separation is achieved through the use of specialized extractant ligands. One of the key challenges in reprocessing is the separation of trivalent Am (Am(III)) from trivalent lanthanide fission products, due to their similar chemical and physical properties. One potential solution to this problem is to selectively oxidize Am(III) to the corresponding hexavalent americyl (Am(VI)) state using bismuthate. Am(VI) is then simply extracted using conventional reprocessing approaches. However, under anticipated large-scale conditions, the reprocessing medium would be continuously subjected to an intense, multi-component, radiation field. Radiolysis induces the formation of a variety of degradation products, many of which are strongly redox active and thus capable of altering the oxidation state distribution of Am (Am(III):Am(V):Am(VI)) and other metal ions. As this ultimately affects separation efficiency it is crucial that we understand the interactions between radiolytic degradation products and actinide metal ions. A combination of experiments and multi-scale modelling have been deployed to quantitatively investigate the gamma-induced radiation chemistry of Am in a range of industrially-relevant aerated nitric acid solutions (1.0, 3.0, and 6.5 mol dm−3), as a function of absorbed gamma dose and Am concentration. Am(VI) is found to be progressively reduced with increasing absorbed gamma dose by the radiolysis products from both water and nitric acid (H2O2 and HNO2), yielding Am(V) and to a lesser extent Am(III). The reduction of irradiated Am(VI) proceeds quantitatively to Am(V) typically within 2.5 kGy total dose.

Gamma radiolysis of americium-nitric acid solutions

253rd National Meeting (2017)

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