Radioactive wastes containing Iodine-129 are to be disposed of in Japan in an underground facility together with TRU wastes. Iodine-129 has a long half-life (1.6 × 107 y) and it is strongly adsorbed in the thyroid gland when it intrudes into the human body. The main chemical formulae of iodine in an alkaline solution are I− and IO3−, and these anion species are absorbed only to a very small extent on silicate minerals. Iodine-129, therefore, is one of the key nuclides to be studied in the geological disposal of radioactive wastes. Recently, a new inorganic ion-exchanger, BiPbO2NO3, has been developed which reacts with iodide ions in a solution by forming BiPbO2I (BPI). The leach resistance of BPI encapsulated in cement (BPIC) was studied in a solution under geological conditions.
The leaching experiment was carried out in an inert glove box in which the concentrations of oxygen and carbon dioxide were maintained at less than 1 ppm. Pure water was degassed 12 hours prior to use. Two kinds of solution were prepared: one was low salinity solution (RW), and the other was high salinity solution (SW). Leachants were prepared by adding a reductant (N2H4) to each solution and pH was adjusted to a fixed value. BPIC was mixed with the leachant in a plastic container. The container was shaken continuously at ambient temperature for six months. The concentrations of iodide ions, bismuth ions and lead ions in the leachant were analyzed periodically using ICP-AES.
Limited numbers of iodide ions (2%–4%) were released from BPIC in the initial period of leaching, following which no additional release of iodide ion was observed for six months. No significant difference was observed in the X-ray diffraction patterns of BPI in BPIC before and after the experiment. These results indicate that iodine is fixed tightly in BPIC. A mechanism of the leaching resistance is discussed.