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A study has been undertaken on the interactions of leachates from actinide-doped Synroc and three Australian granites. Leach testing was carried out on Synroc containing 237 Np, 239 Pu and 244 Cm under MCC-1 conditions in close proximity to granite specimens at 70 °C for 28 days.The presence of the granites increased the pH and reduced the total activity released from the Synroc. Preferential uptake of actinides occurred onto specific secondary minerals, especially hematite, other Fe-oxides/hydroxides, leucoxene, sericite, chlorite and clays. The controlling mechanisms for fixation of actinides on the granties appeared to be adsorption, ionic exchange and reduction by Fe(II). The high sorptive capacity of the alteration minerals significantly reduced the concentration of actinides in solution indicating that hydrothermally-altered granites, with a significant proportion of secondary minerals present, are capable of removing actinides from groundwaters.

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In some radwaste applications, such as immobilization of U.S. Tank wastes using vitrification, Tc and Cs/Sr are likely to be separated out from HLW supernates. Simplified Synroc preparations can be devised for the immobilization of separated Tc and Cs/Sr, either together or individually. Under suitably reducing processing conditions, Tc can be immobilized, as metal or Tc4+. The volatility of Tc and Cs/Sr in Synroc processing is very restricted.In Synroc-C, designed for Purex high-level reprocessing wastes, Tc exists as a metal alloy. In air-saturated water, normalized MCC-1 type total differential leach rates at 90°C decreased with leaching time and after 90 days were <10-4 g/m2/d, decreasing by a factor of ∼100 in anoxic conditions. The corresponding results in a pH=6.1 buffer solution were fairly similar to those in deionised water, but in anoxic conditions, the leach rates were higher in the buffer solution than in the deionised water.A single perovskite phase was loaded with ∼40 wt% of TcO2 to form CaTc0.5Ti0.5O3 by a combination of graphite-die hot-pressing and heating in argon. Rutile, intended to be doped with ∼35wt% TcO2, was fabricated by bellows hot-pressing at 1200°C; aproximately 75% of the Tc formed a solid solution with rutile, but some metallic Tc was also present due to imperfect redox control. Results of both MCC-1 and PCT-type leach tests are presented on a preparation containing 70 wt% hollandite + 20 wt% perovskite + 10 wt% rutile containing about 5 wt% of Cs and Sr respectively. Phase distributions resulting from incorporating Cs and Sr in Synroc-B precursor were studied.