Effects of γ radiation on the leaching kinetics of various nuclear waste-form materials

Abstract

When a solid used to immobilize high-level nuclear wastes is evaluated with respect to its long-term durability and effectiveness as a barrier against radio-isotope release, the massive doses of ionizing radiation to which the solid will be exposed during storage have to be taken into account. Unlike particle radiation, which can give rise to considerable damage even in dry conditions¹, γ irradiation seems to enhance leachability significantly only when the solid is in contact with an aqueous medium while being irradiated². Even in these conditions studies of the effect of γ irradiation at dose rates similar to those expected in repository conditions show only a modest radiation-induced enhancement in leach rates. This enhancement, expressed in terms of the ratio R_irr = L_i/L₀ between the leach rates measured in irradiated and non-irradiated conditions, respectively, is typically of the order of 1–4 in the case of borosilicate glasses containing typical commericial as well as defence waste stream compositions^2–4. We show here that the radiation-induced enhancement of leach rates can be ascribed primarily to radiolytic acidification of the aqueous medium and secondarily to radiolytic formation of complexing species. As a result, waste-forms based on major components such as alumina, which are more susceptible to acid dissolution and to attack by complexing agents than silica, show much larger radiation-induced enhancements of their leach rates than silicate glasses.