Microbiology and radioactive waste disposal

The present Nirex Safety Assessment Research Programme on microbiology is based on experimental as well as theoretical work. It has concentrated on the study of how mixed, natural populations of microbes might survive and grow on the organic component of Low Level Radioactive Wastes (LLW) and PCM (Plutonium Contaminated Waste) in a cementitious waste repository. The present studies indicate that both carbon dioxide and methane will be produced by microbial action within the repository. Carbon dioxide will dissolve and react with the concrete to a limited extent so methane will be the principal component of the produced gas. The concentration of hydrogen, derived from corrosion, will be depressed by microbial action and that this will further elevate methane levels. Actual rates of production will be lower than that in a domestic landfill due to the more extreme pH. Microbial action will clearly affect the aqueous phase chemistry where organic material is present in the waste. The cellulosic fraction is the main determinant of cell growth and the appearance of soluble organics. The structure of the mathematical model which has been developed, predicts the general features which are intuitively expected in a developing microbial population. It illustrates that intermediate compounds will build up in the waste until growth of the next organism needed for sequential degradation is initiated. The soluble compounds in the pore water and the mixture of microbes present in the waste will vary with time and sustain biological activity over a prolonged period. Present estimates suggest that most microbial action in the repository will be complete after 400 years. There is scope for the model to deal with environmental factors such as temperature and pH and to introduce other energy sources such as hydrogen. (author)