The chemical and microbial degradation of cellulose in the near field of arepository 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 t he apparent contradiction regarding the representation of microbial activit y in performance assessments of the release of gases and of dissolved radio nuclides, A discussion is presented of the current understanding of the mic robial and chemical degradation of cellulose. The assumptions made in recen t performance assessment calculations of the Nirex disposal concept are the n stated. For the release of gases, it was assumed that the complete conver sion of cellulosic wastes to gases by the action of microbes, was, in princ iple. permitted. However, concerning migration of radionuclides by the grou ndwater pathway, all the cellulose was assumed to be converted to complexan ts that could increase the solubility and decrease the sorption of radionuc lides in the near field. This contradiction in the approach of the groundwa ter 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 ta ken for possible beneficial effects of the microbial destruction of complex ants, whereas the complete conversion of cellulose to gaseous products is a ssumed. (C) 2000 Elsevier Science Ltd. All rights reserved.