The effect of aqueous-gas partitioning on water radiolysis under γ-irradiation

Water radiolysis produces H₂, and when a headspace is present, the aqueous-gas interfacial transfer of volatile species can alter the net radiolytic production of H₂. Accumulation of H₂ (and O₂) in a cover-gas volume can lead to an unacceptable combustion hazard. In addition, the H₂ level in the cover gas is often used as an indication of system health in power plants. The effect of gas-aqueous partitioning on steady-state water radiolysis chemistry was studied by performing experiments using different gas-to-liquid volume ratios at various solution pH and cover-gas compositions. The concentrations of molecular water radiolysis products (H₂(g) and H₂O₂(aq)) were monitored as a function of irradiation time, and the experimental results were compared with computer simulations. At all pHs, the rate of increase in [H₂(g)] in the headspace of a small volume is proportional to the aqueous concentration of H₂, and the concentration of H₂ in the headspace can be used to infer the aqueous concentration of H₂. However, the impact of the interfacial mass transfer on the concentrations of the other aqueous species generated by radiolysis strongly depends on pH. At pH ≤ pK_a of ·H (9.6) the effect is negligible. However, at higher pHs, not only the radiolytic production of H₂(aq) and H₂O₂(aq), but the production of secondary radiolysis species (such as O₂(aq)) are considerable. The interfacial transfer of both H₂ and O₂ becomes significant, influencing and the aqueous concentrations of H₂ and O₂ and thereby further affecting the concentrations of other radiolysis products. Thus, when the solution pH can change considerably, the measurement of H₂(g) in the gas phase alone is not sufficient, and the measurements of both H₂O₂(aq) in the aqueous phase and H₂(g) in the cover gas are required to determine water redox chemistry under continuous irradiation conditions. (author)