New insights in long-term alkaline degradation mechanism of cellulosic materials in radioactive waste

Cellulosic materials make up a significant fraction of the current radioactive waste. During storage and disposal, both radiolytic and hydrolytic degradation of such materials can be expected, the latter triggered by the highly alkaline cementitious environment of the waste matrix and disposal facility. The combination of both degradation processes will cause a significant production of radionuclide-complexing agents, which can enhance the migration of certain radionuclides towards the biosphere. Knowledge regarding the degradation mechanisms is therefore required to predict the long-term production rate of these organics. In this study, the physicochemical properties of cellulosic tissues during radiolytic and/or alkaline degradation under disposal conditions were monitored. Our results indicate that the long-term alkaline degradation of cellulose is controlled by two underlying mechanisms, taking place in both the amorphous and the crystalline regions. The first one is a combination of peeling and stopping reactions taking place at the easily available reducing end groups in the amorphous regions of cellulose. This process controls the overall degradation rate until the easily accessible reducing end groups become depleted. Afterwards, the degradation slows down significantly and is rather controlled by a continuous stepwise dissolution and amorphization of the outer layer of crystalline cellulose, resulting in the liberation of reducing end groups, where secondary peeling reactions can take place. These new insights lead to a better-founded choice of the conceptual model for predicting the long-term cellulose degradation in radioactive waste.