THE INTRODUCTION OF MICROBIAL NUTRIENTS INTO A NUCLEAR WASTE-DISPOSALVAULT DURING EXCAVATION AND OPERATION

AECL has developed a concept for permanent geological disposal of used nuclear fuel waste in Canada. This concept would involve disposal of the waste in corrosion-resistant metal containers, surrounded by compa cted clay-based buffer and backfill materials, in a vault 500-1000 m d eep in granitic rock of the Canadian Shield. Such a vault would not be a sterile environment. Microbial activity would be expected in those areas of a vault where effects of heat, moisture content, and radiatio n would not exclude microbial life and where sufficient nutrients woul d be present. Although the granitic rock environment is naturally nutr ient-poor, a substantial amount of nutrients could be introduced from residues of explosives used in the excavation of a vault. Using standa rd rock leaching techniques, measurements of the concentrations of suc h residues were made in excavated rock, tunnel walls and mine service- water supplies at AECL's Underground Research Laboratory. The effects of these residues on the bacterial population size in groundwater were also determined. Results showed that the largest potential nutrient a ddition (both N and C) to a vault would result from using untreated br oken rock as part of the backfill. Nitrate in the residues could incre ase groundwater bacterial populations by several orders of magnitude.