The Sorption of Thorium and Americium onto Fresh and Degraded Ordinary Portland Cement and onto Green Tuff

摘要

The sorption of thorium and americium has been measured onto crushed samples of fresh ordinary 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{sup}(-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{sup}(-3) sodium nitrate solution, simulated seawater, OPC leachate and 1 mol dm{sup}(-3) ammonium hydroxide solution. R{sub}D values were determined from the amount of thorium or americium remaining in solution after centrifugation, 0.45μm filtration and 10,000 nominal molecular weight cut-off (NMWCO) filtration. Centrifugation gave lower R{sub}D values than filtration. Mean R{sub}D values for sorption onto OPC and DOPC were in the range 4 × 10{sup}4 to ≥1 × 10{sup}6 cm{sup}3g{sup}(-1) for thorium, and 3 × 10{sup}3 to ≥1 × 10{sup}5 cm{sup}3g{sup}(-1) for americium, after filtration through 10,000 NMWCO filters. Mean R{sub}D values for thorium sorption onto tuff increased from 2 × 10{sup}3 cm{sup}3g{sup}(-1) in tuff-equilibrated deionised water, to ≥9 × 10{sup}6 cm{sup}3g{sup}(-1) from ammonium hydroxide solution (10,000 NMWCO-filtration). A similar trend was seen for 10,000 NMWCO-filtered americium samples where mean R{sub}D values increased from 4 × 10{sup}3 cm{sup}3g{sup}(-1) in deionised water to 1 × 10{sup}5 cm{sup}3g{sup}(-1) in ammonium hydroxide solution.