Kinetic study on dissolution of UO 2 powders in nitric acid

Abstract

UO 2 powders of 90–150, 300–355, and 850–1000 μm were dissolved in HNO 3 solutions stirred at 300 rpm. Concentrations of HNO 2 in dissolvent solutions were held constant by supplying NaNO 2 solutions of appropriate concentrations at 0.2 ml/min. The dissolution reactions were analyzed on the assumptions that the UO 2 powders are spherical particles and homogeneously dissolved from their external surface. Dissolution rate constants (φ) in mol cm −2 min −1 were measured at various concentrations of HNO 3 and HNO 2, and temperatures. Furthermore, the effect of UO 2 2+ and NO 3 − ions on the dissolution rate was also examined. It was found that the dissolution rate of UO 2 powders is not accelerated by UO 2 2+ ions and expressed as φ = ( k a + k b[HNO 2])[NO 3 −] T 2.3([NO 3 −] T = total nitrate concentration). Activation energies ( ΔE ≠) for k a and k b were evaluated to be 79.5 ± 6.7 and 36.8 ± 2.9 kJ/mol, respectively. The φ value at 80°C, [HNO 3] = 8M, and [HNO 2] = 0.024M (M = mol/dm 3) is 8.5 × 10 −6 mol cm −2 min −1.