Wet chemical synthesis of apatite-based waste forms – A novel room temperature method for the immobilization of radioactive iodine

Abstract

Iodine isotopes make up ∼0.69% of the fission products of ²³⁵U slow neutron fission, i.e. ∼360 g iodine per metric ton of ²³⁵U. The currently proposed technology for the removal of iodine from reprocessing plant off-gas is to pass it through a caustic scrubber (using ∼1–2 M NaOH) to form NaI, NaIO₃ and possibly NaOI, or to pass it through Ag-containing solid sorbents to form chemisorbed AgI. Since iodine is not amenable to conventional vitrification routes because of its poor incorporation into borosilicate glass chemistry and high volatilization at temperatures >500 °C, considerable effort is being made to develop alternative waste forms for the immobilization of iodine at low temperatures (<100 °C). In this paper, a novel wet chemical synthesis method has been described for immobilizing radioactive iodine obtained from caustic scrubber solutions and silver solid sorbents into the apatite, Pb₁₀(VO₄)₆I₂, at room temperature. The uptake of iodine for the synthesized waste form has been shown to be 94.6%. In addition, investigation into the formation of solid solutions in the systems, Pb_(10−x)Ca_x(VO₄)₆I₂ and Pb₁₀(VO₄)_(6−y)(PO₄)_yI₂, by the proposed method has been discussed. The study provides an easy pathway to design novel waste forms for the immobilization of large volumes of radioactive iodine at room temperature and a methodology to potentially synthesize new compositions of iodide-containing apatites.