State of Radiorhodium in High-Level Liquid Waste from Regeneration of Spent Nuclear Fuel

The state of radiorhodium in liquid waste from processing of spent nuclear fuel was analyzed in detail by ESR and NMR spectroscopy on several nuclei. The most probable oxidation state of rhodium in nitric acid solutions is +3. Three procedures for preparing nitric acid solutions of rhodium were studied. The composition of Rh(III) complexes in these solutions is similar but not identical. The method for identifying the ionic composition of similar objects was proposed. The speciation of rhodium in nitric acid solutions with the acid concentration ranging from 2×10-2 to 15 M was studied. Polynuclear oligomers with (μ-ONO2)2 bridges, mainly tetramers, are formed in solutions with CRh > 2 M, [NO3-] ≥ 8 M, and [H+] ≥ 0.7 M. At CRh = (1-2)×10-2, [NO3-] = 1-4 M, and [H+] = 0.4-4 M, the (μ-OH, μ-ONO2) dimer and trimer prevail; at [NO3-] < 1 M and [H+] < 0.2 M, the (μ-OH, μ-ONO2) dimer dominates. No Rh(III) aqua ion and no monomeric nitrate complexes were detected in the quasiequilibrium system at any concentrations of the components. In nitrate-nitrite solutions, both “subnitrated” (μ-OH, μ-ONO2) Rh(III) oligomers and mononuclear Rh(III) nitroaqua complexes exist. The equilibrium between these species is attained very slowly and depends on the equilibrium concentration of nitrite anion. Forecast was given on isolation of rhodium from nitric acid solutions of SNF by extraction, ion-exchange, electrolytic, and precipitation procedures.