Compacted clay-based buffer surrounds corrosion-resistant waste containersin the Canadian concept for nuclear fuel waste disposal. Clays naturallycontain small quantities of organic matter that may be resistant tobacterial degradation. The containers with highly radioactive material wouldsubject the surrounding buffer to both heat (max. 95°C) and radiation (max.52 Gy/h). Both could potentially break down complex organic material tosmaller, more bioavailable compounds. This could stimulate microbial growthand possibly affect gas production, microbially-influenced corrosion orradionuclide migration. Experiments were carried out in which buffer (50wt.% 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. Unheatedbuffer was also irradiated to 25 and 50 kGy at different moisture contents.The treated materials were subsequently suspended in distilled water, shakenfor 24 h and centrifuged to remove the solids. The 0.22 μm filter-sterilizedleachates were inoculated with equal volumes of fresh groundwater andincubated at room temperature for 10 d to determine the increase in totaland viable bacteria compared to a groundwater control. Results indicatedthat leachates from buffer subjected to heat, radiation or combinations ofthese, had a stimulating effect on both total and viable cell counts ingroundwater, compared to unamended groundwater controls. This stimulatingeffect was generally most pronounced for viable counts and could be largerthan two orders of magnitude. Leachates from untreated buffer material alsostimulated the growth of groundwater bacteria, but to a lesser extent thanleachates from heat-and radiation-treated buffer material. The effects ofheat and radiation on nutrient availability in clay-based sealing materials(at relevant clay/moisture ratios) should, therefore, be taken into accountwhen attempting to quantify the effects of microbial activity on vaultperformance.