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Instant-release fractions were measured for release of14C,99Tc,90Sr and137Cs 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% for14C, 0.001 to 0.23% for99Tc 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 for14C were also lower than values reported in the literature for PWR fuels.

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Five thermal-convection loops, constructed of granite, were operated for 350 d with a hot-side temperature of 60°C and a cold-side temperature of 20°C, to examine the effects of a thermal gradient on the convective transport of seven radionuclides. Radionuclide concentrations on the surface of the loop flow channels indicated that 125Sb, 60Co and 54Mn and precipitated iron oxyhydroxides were concentrated in the hot side of the loops. Concentrations of 144Ce and 99Tc were higher in the cold side of the loops. Both of these patterns were exhibited by 75Se. The concentration of 137Cs was not significantly affected by the thermal gradient. Radionuclide solubility and sorption were affected by variations in pH, Eh and ionic strength. Incorporation of the radionuclide into mineral lattices was found to play a key role in retarding the transport of all radionuclides tested.