Radiolytic degradation of cellulosic materials in nuclear waste - Effect of oxygen and absorbed dose

Lignocellulosic materials can be found in a significant fraction of the current low- and intermediate-level radioactive waste. During storage and disposal, radiolytic degradation of such materials can be expected, under oxic or anoxic conditions. This degradation may lead to a significant gas production and changes in the physico-chemical properties of the lignocellulosic materials, which can affect the formation of the known radionuclide-complexing agent isosaccharinic acid (ISA) as well as other (possibly complexing) degradation products during disposal. Hence, in the present work the radiolytic degradation of cellulosic tissues – realistically found in radioactive waste – was investigated under various storage and disposal conditions. For this, cellulosic tissues were exposed to γ-irradiation in gas-tight containers under oxic or anoxic conditions, at an absorbed dose ranging up to 1.4 MGy and at two different dose rates. Our results show that mainly H2, CO and CO2 are produced during irradiation of tissues, though also small amounts of CH4 are formed. The presence of oxygen does not affect the generation of H2, but results in a significant increase in the yields of CO, CO2 and CH4. Furthermore, radiation-induced chain scission is observed, causing a decreasing polymerization degree with increasing absorbed dose. Amorphization of the cellulose microstructure occurs significantly at high doses of gamma rays (≥ 0.8 MGy). An increase in the concentration of reducing functional groups is observed with increasing absorbed doses as well. For irradiation under anoxic conditions, this increase is correlated with the observed chain scission. In contrast, additional oxidation processes occur when irradiating cellulosic tissues in the presence of oxygen, resulting in a partially oxidized polymer backbone without causing considerably more chain scission or amorphization. These radiolytic changes to the cellulose structure, both under anoxic and oxic conditions, may enhance its hydrolytic degradation under the hyper-alkaline conditions of long-term final disposal, resulting in a faster production of radionuclide-complexing agents.