Compacted clay-based buffer surrounds corrosion-resistant waste containers
in the Canadian concept for nuclear fuel waste disposal. Clays naturally
contain small quantities of organic matter that may be resistant to
bacterial degradation. The containers with highly radioactive material would
subject the surrounding buffer to both heat (max. 95°C) and radiation (max.
52 Gy/h). Both could potentially break down complex organic material to
smaller, more bioavailable compounds. This could stimulate microbial growth
and possibly affect gas production, microbially-influenced corrosion or
radionuclide migration. Experiments were carried out in which buffer (50
wt.% Na-bentonite, 50 wt.% sand) was heated at 60 and 90°C for periods of 2,
4 and 6 weeks, in some cases followed by irradiation to 25 kGy. Unheated
buffer was also irradiated to 25 and 50 kGy at different moisture contents.
The treated materials were subsequently suspended in distilled water, shaken
for 24 h and centrifuged to remove the solids. The 0.22 μm filter-sterilized
leachates were inoculated with equal volumes of fresh groundwater and
incubated at room temperature for 10 d to determine the increase in total
and viable bacteria compared to a groundwater control. Results indicated
that leachates from buffer subjected to heat, radiation or combinations of
these, had a stimulating effect on both total and viable cell counts in
groundwater, compared to unamended groundwater controls. This stimulating
effect was generally most pronounced for viable counts and could be larger
than two orders of magnitude. Leachates from untreated buffer material also
stimulated the growth of groundwater bacteria, but to a lesser extent than
leachates from heat-and radiation-treated buffer material. The effects of
heat and radiation on nutrient availability in clay-based sealing materials
(at relevant clay/moisture ratios) should, therefore, be taken into account
when attempting to quantify the effects of microbial activity on vault
performance.