γ-Radiolysis of NaCl Brine in the Presence of UO2(s): Effects of Hydrogen and Bromide

A concentrated NaCl solution was γ-irradiated in autoclaves under a pressure of 25 MPa. A set of experiments were conducted in 6 mol (kg H2O)-1 NaCl solution in the presence of UO2(s) pellets; in a second set of experiments, γ-radiolysis of the NaCl brine was studied without UO2(s). Hydrogen, oxygen and chlorate were formed as long-lived radiolysis products. Due to the high external pressure, all radiolysis products remained dissolved. H2 and O2 reached steady state concentrations in the range of 5.10-3 to 6.10-2 mol (kg H2O)-1 corresponding to a partial gas pressure of ∼2 to ∼20 MPa. Radiolytic formation of hydrogen and oxygen increased with the concentration of bromide added to solution. Both, in the presence of bromide, resulting in a relatively high radiolytic yield, and in the absence of bromide surfaces of the UO2(s) samples were oxidized, and concentration of dissolved uranium reached the solubility limit of the schoepite / NaUO2O(OH)(cr) transition. At the end of the experiments, the pellets were covered by a surface layer of a secondary solid phase having a composition close to Na2U2O7. The experimental results demonstrate that bromide counteracts an H2 inhibition effect on radiolysis gas production, even at a concentration ratio of [H2] / [Br-] > 100. The present observations are related to the competitive reactions of OH radicals with H2, Br- and Cl-. A similar competition of hydrogen and bromide, controlling the yield of γ-radiolysis products, is expected for solutions of lower Cl- concentration. (authors)