Gamma irradiation accelerates alkaline degradation of cellulosic materials in radioactive waste

Abstract

Cellulosic materials make up a significant fraction of the current low- and intermediate-level radioactive waste. During storage and disposal, radiolytic degradation of such materials is inevitable and can occur under both oxic and anoxic conditions. In addition, the highly alkaline cementitious environment of a disposal system promotes the alkaline degradation of cellulosic materials, producing radionuclide-complexing agents, such as isosaccharinic acid (ISA). As radiolytic degradation changes the physicochemical properties of cellulose, it could also affect its alkaline degradation and thus the production of ISA during disposal. Hence, in the present work, we investigated the alkaline degradation of pre-irradiated cellulosic tissues, which are representative of real radioactive waste. Pre-irradiation occurred by exposing tissues to γ-irradiation under oxic or anoxic conditions at absorbed doses up to 1.4 MGy and at two different dose rates. These irradiated tissues were then submerged in artificially prepared cement water (initial pH of 13.3) and monitored over 2.5 years. The results show a significantly faster production and release of dissolved organic carbon and ISA with an increasing absorbed dose during pre-irradiation, and even more so when oxygen is present during irradiation. The irradiation dose rate did not affect the subsequent alkaline degradation rate. Taken together, this work demonstrates that irradiation of cellulosic materials in radioactive waste during storage and disposal will accelerate their alkaline degradation under disposal conditions. Consequently, radionuclide-complexing agents such as ISA will form at rates far exceeding those anticipated from alkaline degradation alone. These findings are therefore pivotal for improving long-term predictions of the ISA production in radioactive waste.