S Stroes-Gascoyne | PubMed Health

PubMed1997

Microbial studies in the Canadian nuclear fuel waste management program.

Atomic Energy of Canada Limited (AECL) has developed a concept for permanent geological disposal of used nuclear fuel in Canada. This concept, based on a multibarrier system, would involve disposal of nuclear fuel waste in titanium or copper containers, surrounded by compacted clay-based buffer and backfill materials, in a vault 500-1000 m deep in granitic rock of the Canadian Shield. Subsurface environments will not be sterile and an experimental program was initiated in 1991 by AECL to address and quantify the potential effects of microbial action on the integrity of the disposal vault. This microbial program focuses on answering specific questions in areas such as the survival of bacteria in compacted clay-based buffer materials under relevant radiation, temperature and desiccation conditions; mobility of microbes in compacted buffer materials; the potential for microbially influenced corrosion of containers; microbial gas production in backfill material; introduction of nutrients as a result of vault excavation and operation; the presence and activity of microbes in deep granitic groundwaters; and the effects of biofilms on radionuclide migration in the geosphere. This paper summarizes the results to date from the research activities at AECL.

Bacterial Physiological Phenomena↗

PubMed1997

Occurrence and identification of microorganisms in compacted clay-based buffer material designed for use in a nuclear fuel waste disposal vault.

A full-scale nuclear fuel waste disposal container experiment was carried out 240 m below ground in an underground granitic rock research laboratory in Canada. An electric heater was surrounded by buffer material composed of sand and bentonite clay and provided heat equivalent to what is anticipated in a Canadian nuclear fuel waste repository. During the experiment, the heat caused a mass transport of water and moisture content gradients developed in the buffer ranging from 13% closest to the heater to 23% at the rock wall of the deposition hole. Upon decommissioning after 2.5 years, microorganisms could be cultured from all samples having a moisture content above 15% but not from samples with a moisture content below 15%. Heterotrophic aerobic and anaerobic bacteria were found in numbers ranging from 10(1) to 10(6) cells/g dry weight buffer. Approximately 10(2), or less, sulphate-reducing bacteria and methanogens per gram of dry weight buffer were also found. Identification of buffer population members was performed using Analytical Profile Index (API) strips for isolated bacteria and 16S rRNA gene sequencing for in situ samples. A total of 79 isolates from five buffer layers were identified with API strips as representing the beta, gamma and delta groups of Proteobacteria and Gram-positive bacteria. Sixty-seven 16S rRNA clones that were obtained from three buffer layers were classified into 21 clone groups representing alpha and gamma groups of Proteobacteria, Gram-positive bacteria, and a yeast. Approximately 20% of the population comprised Gram-positive bacteria. Members of the genera Amycolatopsis, Bacillus, and Nocardia predominated. Among Gram-negative bacteria, the genera Acinetobacter and Pseudomonas predominated. Analysis of lipid biomarker signatures and in situ leucine uptake demonstrated that the buffer population was viable. The results suggest that a nuclear fuel waste buffer will be populated by active microorganisms only if the moisture content is above a value where free water is available for active life.

Aluminum Silicates↗