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The sorption of thorium and americium has been measured onto crushed samples of freshordinary Portland cement (OPC), degraded OPC (DOPC) and green tuff under a range of aqueous conditions as part of research into the disposal of TRU waste in Japan. Sorption onto OPC was measured from deionised water,3 mol dm−3 sodium nitrate solution and simulated seawater, and onto DOPC from demineralised water; all solutions were pre-equilibrated with the relevant cement. Sorption onto tuff was measured from pre-equilibrated deionised water, 3 mol dm−3 sodium nitrate solution, simulated seawater, OPC leachate and 1 mol dm−3 ammonium hydroxide solution. RD values were determined from the amount of thorium or americium remaining in solution after centrifugation, 0.45 μm filtration and 10,000 nominal molecular weight cut-off (NMWCO) filtration. Centrifugation gave lower RD values than filtration. MeanRD values for sorption onto OPC and DOPC were in the range 4 × 104 to ≥1 × 106 cm3g−1 for thorium, and 3 ×103 to ≥1 × 105 cm3g−1 for americium, after filtration through 10,000 NMWCO filters. Mean RD values for thorium sorption onto tuff increased from2 × 103 cm3g−1 in tuff-equilibrated deionised water, to ≥ 4 × 106 cm3g−1 from ammonium hydroxide solution (10,000 NMWCO-filtration). A similar trend was seen for10,000 NMWCO-iltered americium samples where mean RD values increased from 4 × 103 cm3g−1 in deionised water to 1 × 105 cm3g−1 in ammonium hydroxide solution.

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