Simcha Stroes-Gascoyne

Microbial, redox and organic characteristics of compacted clay-based buffer after 6.5 years of burial at AECL's Underground Research Laboratory

Abstract Many countries are considering options for long-term management of nuclear waste. One common aspect among deep geological disposal options in granitic host rock is the use of clay-based buffer materials to limit radionuclide migration in case of container failure. The isothermal test (ITT) involved placing ∼2.4 m 3 of clay-based buffer in a borehole at the 240 m level of AECLs Underground Research Laboratory to study the response of buffer to resaturation by groundwater over a 6.5-year period. Results are reported here on measurements taken at the end of the test for microbial, redox and organic characterization of the buffer. Results from enumerations and biomass determinations suggested that the viable population of cells in the buffer was several orders of magnitude larger than could be cultured. It is postulated that, due to the constrictive and nutrient-poor buffer environment, viable and active cells became stressed during burial and lost activity and culturability but not viability. Culturable microbial populations at interfaces in the ITT were about an order of magnitude larger than in comparable bulk buffer samples, suggesting that interfaces may be preferred sites for microbial activity and transport. The presence of culturable SO 4 -reducing bacteria and an increase in solid sulphide concentrations in the buffer suggested SO 4 reduction, which appeared to be very variable locally. Only about 0.02–0.5% of SO 4 was converted to sulphide, suggesting that SO 4 reduction was not (yet) a dominant process. No methanogens could be enumerated from the ITT, and phospholipid fatty acid (PLFA) profiles did not suggest their presence. Gas analysis of samples recovered from the ITT suggested some reduction in O 2 near the top of the experiment, but deeper samples did not show a significant decrease in O 2 and had only a small increase in CH 4 and H 2 levels. This suggested that microbial processes were depressed in the buffer but may have been more active near the concrete/buffer interface. The suggestion of low microbial activity in the buffer was corroborated by the results from the PLFA analysis, which indicated low biomass turnover rates and starvation biomarkers. The combination of enumerations, PLFA and gas analysis results suggested that no significant evolution towards reducing conditions occurred during the duration of the ITT. Fulvic acids made up the largest fraction of water-leachable humic substances but accounted for only about 2% of the total C inventory of the buffer material. The complexing capacity of these humic substances, based on carboxylic functional groups, ranged from 24 to 32 meq/g dissolved organic C. This may provide buffer porewater with considerable complexing capacity for radionuclides.

Model for the Microbiological Corrosion of Copper Containers in a Deep Geologic Repository

A model has been developed to predict the impact of microbiological processes on the long-term corrosion behaviour of copper containers in a deep geologic repository. The model accounts for a range of aerobic and anaerobic microbial processes. Various factors expected to limit the extent of microbial activity in the repository, such as the lack of water, evolving redox conditions, and the nutrient-poor environment, are taken into account in the model. Amongst other effects, the model predicts that microbial activity will not occur close to the container in the presence of highly compacted bentonite buffer material.

The Inpluence of Borehole Flushing on the Concentration of Microbes in Granitic Groundwaters.

A number of groundvater parameters have been studied at AECL’s Underground Research Laboratory (URL) in support of the Canadian Nuclear Fuel Waste Management Program. The concentration of microbes in groundvater is of interest as they may modify the transport of dissolved radionuclides. Preliminary results from an earlier study suggested that the microbe concentrations may be affected by the extent of borehole flushing prior to sampling. A study was therefore carried out in which packer-isolated intervals of two boreholes intersecting a fracture zone at 250-m depth in the URL were flushed and sampled on two occasions at various flow rates. High initial microbial concentrations (most likely due to leaching of nutrients from sample tubes) decreased rapidly as flushing progressed, suggesting enhanced microbial growth near the top of the borehole zone. Also, a tenfold increase in flow rate during flushing caused an increase in microbial concentrations in the groundwater of one of the boreholes, concurrent with an increase in total particle count. This suggests that particulate and biofilm material may be flushed out of the fracture zone at this particular location.

Instant Release of 14C, 99Tc, 90Sr and 137Cs from Used Candu Fuel at 25°C in Distilled Deionized Water

Instant-release fractions were measured for release of 14 C, 99 Tc, 90 Sr and 137 Cs from segments of clad used CANDU fuels in distilled deionized water at pH 8.5 and 25°C. Measured–instant release values, 0.002 to 0.07% for 14 C, 0.001 to 0.23% for 99 Tc and 0.0005 to 0.04% for 9 0 Sr, were one to two orders of magnitude lower than the estimated values currently used in the safety assessment of the Canadian concept of geological disposal of intact used CANDU fuel. The values for 14 C were also lower than values reported in the literature for PWR fuels.

The Effects of Dry Density and Pore-water Salinity on the Physical and Microbiological Characteristics of Compacted 100% Bentonite

This study examined the conditions required to suppress microbial activity in compacted bentonite, such that microbially influenced corrosion (MIC) of copper waste containers, surrounded by compacted bentonite in a future deep geologic repository, would become insignificant. Experiments were carried out to determine the effects of dry density and pore-water salinity on swelling pressure, water activity (a{sub w}) and the culturable microbial community in compacted bentonite. A dry density {>=} 1.6 g/cm{sup 3} ensures that a{sub w} is 2 MPa. Both conditions suppress microbial culturability below background levels (2.1 x 10{sup 2} Colony-Forming Units/g) in as-purchased bentonite. Under such conditions, cells likely survive as dormant cells or inactive spores, which greatly reduces the possibility of significant MIC. Observations in natural clay-rich environments support these findings. (authors)

Measurements of Grain-Boundary Inventories of 137 Cs, 90 Sr and 99 Tc in Used Candu Fuel

Two methods were used to measure grain-boundary inventories of 137 Cs, 90 Sr and 99 Tc in used CANDU fuel, to corroborate source term estimates based on a fission gas release code. Used fuels were partially oxidized at 200°C in air to overall compositions of UO 2+x (0.15≤ × ≤0.25) to expose UO 2 grain boundaries, followed by leaching in aqueous solution. Only a fraction (2 to 18%) of the calculated gap + grain-boundary inventories for 37 Cs was released. This suggests that the calculations overestimate Cs release or that oxidation does not expose all grain boundaries, or that Cs release from grain boundaries is slow. Release of 90 Sr (0.01 to 0.7%) agreed reasonably well with the source term estimates (0.001 to 0.3%). Release of 99 Tc (0.3 to 1.5%) suggests that the source term estimate for the upper boundary of 99 Tc release (25%) may be too high. A second technique involved leaching of crushed and size-fractionated used fuel in either a static or dynamic system. A direct one-to-one correlation between calculated and measured gap + grain-boundary inventories for 137 Cs was found for low- and medium-power fuels.

Implications of Microbial Redox Catalysis in Analogue Systems for Repository Safety Cases

A detailed assessment of studies of oxidising redox fronts around fractures at depth in otherwise “reducing” environments suggests that the usual explanation, in terms of past disturbances that have resulted in deep penetration of oxidising water, are incompatible with hydrogeological and/or geochemical observations. An alternative hypothesis, microbial catalysis of kinetically slow or hindered reactions involving oxyanions such as sulphate or carbonate, appears potentially more credible. Although still not always taken into account by the geochemical community, the role of microbial metabolism in low temperature geochemistry is supported by the rapidly expanding database on subsurface microbial populations. These populations are demonstrated to be viable and, therefore, could potentially be active at levels close to or below current detection limits in deep geological systems. Indeed, inspection of information available from several analogue studies or repository site characterisation programmes suggests that such activity may explain some of the geochemical anomalies encountered. This paper examines the current (indirect) evidence for microbial redox catalysis in relevant subsurface rock matrix environments and considers the implications that this would have for the development of site understanding — and in particular the identification of factors that may distinguish between different locations during site selection. Further, it examines the wider implications of more extensive roles of microbes in repository systems on the overall post-closure safety case and the need for further focused analogue studies to develop answers to these open questions.Copyright