The microbiology programme for UK Nirex

Summary The Nirex Safety Assessment Research Programme (NSARP) considers the effect of microbial action on the repository near field. The upper limit of growth for natural soil organisms appears to be pH 12.25. Environmental conditions in the repository will probably allow slow growth particularly on damp wastes. Experiments using packed columns of waste and concrete have shown that an extremely high pH is not conducive to rapid microbial growth. However, viable populations can exist within regions adjacent to the concrete, e.g. where a surface film coats the concrete. Carbon dioxide and methane will be produced by microbial action within the repository but actual rates of production will be lower than that in a domestic landfill. The cellulosic fraction of waste is the main determinant of cell growth. It appears to be the sugar acids arising from alkaline degradation of cellulose which cause enhanced plutonium solubility. The potentially beneficial reduction of chemically derived polyhydroxy acids by the microorganisms is possible. A mathematical model has been constructed to describe the main features of biological action in the repository.