The chemical and microbial degradation of cellulose in the near field of a repository for radioactive wastes

This paper focuses on one aspect of the calculations of risk in performance assessments of the deep disposal of radioactive wastes in the UK, namely the apparent contradiction regarding the representation of microbial activity in performance assessments of the release of gases and of dissolved radionuclides. A discussion is presented of the current understanding of the microbial and chemical degradation of cellulose. The assumptions made in recent performance assessment calculations of the Nirex disposal concept are then stated. For the release of gases, it was assumed that the complete conversion of cellulosic wastes to gases by the action of microbes, was, in principle, permitted. However, concerning migration of radionuclides by the groundwater pathway, all the cellulose was assumed to be converted to complexants that could increase the solubility and decrease the sorption of radionuclides in the near field. This contradiction in the approach of the groundwater and gas pathway assessments stems from the consistent need to provide a cautious approach in the face of uncertainty about the actual evolution of microbial activity in the repository. Therefore, no credit is currently taken for possible beneficial effects of the microbial destruction of complexants, whereas the complete conversion of cellulose to gaseous products is assumed.