Resolving the H-2 effect on radiation induced dissolution of UO2-based spent nuclear fuel

摘要

In recent years, the impact of H-2 on alpha-radiation induced dissolution Of UO2-based spent nuclear fuel has been studied and debated extensively. Experimental results on the effect of H-2 on the concentration of H2O2 during alpha-radiolysis have been shown to disagree with numerical simulations. For this reason, the reaction scheme used in simulations of aqueous radiation chemistry has sometimes been questioned. In this work, we have studied the impact of H-2 on the H2O2 concentration in alpha-irradiated aqueous solution using numerical simulations. The effects of H-2 pressure, alpha-dose rate and HCO3- concentration were investigated by performing systematic variations in these parameters. The simulations show that the discrepancy between the previously published experimental result and numerical simulations is due to the use of a homogeneous dose rate (the energy is assumed to be equally distributed in the whole volume). Taking the actual dose rate of the alpha-irradiated volume into account, the simulation is in perfect agreement with the experimental results. This shows that the H-2 effect is strongly alpha-dose rate dependent, and proves the reliability of the reaction scheme used in the simulations. The simulations also show that H-2 influences the H2O2 concentration under alpha-radiolysis. The magnitude of the effect depends on the dose rate and the H-2 pressure as well as on the concentration of HCO3-. The impact of the radiolytic H-2 effect on the rate of alpha-radiation induced dissolution of spent nuclear fuel is discussed along with other (alpha- and gamma-) radiation induced processes capable of reducing the concentration of uranium in solution. The radiolytic H-2 effect is quantitatively compared to the previously presented noble metal catalyzed H-2 effect. This comparison shows that the noble metal catalyzed H-2 effect is far more efficient than the radiolytic H-2 effect. Reduction of U(VI) in solution due to low dose rate gamma-radiolysis in the presence of H-2 is proposed to be the cause of the H-2 effect observed in leaching experiments on alpha-doped UO2.