The development of an iodine immobilization technique that can fix radioactive iodine in waste form for a long period and constrain its leaching into pore water is necessary in order to secure the long-term safety of geological disposal of transuranic (TRU) waste. Lead borate glass vitrified at a low temperature is regarded as a promising material for immobilizing the Iodine-129 that is recovered from spent AgI filters generated by reprocessing plants in Japan and which may have a significant effect on the long-term safety of geological disposal.
Batch leaching tests were conducted to understand glass dissolution behavior in various solutions that account for geological disposal conditions. Boron dissolved at the highest rate in all types of solutions to be used as an index element for measuring the glass dissolution rate. On the other hand, lead dissolved in these solutions at a much lower rate. These results are consistent with an electron micro-probe analysis (EPMA) of the altered glass surfaces that indicated the depletion of boron and enrichment of lead near the surfaces.
The altered glass surfaces were further examined by scanning and transmission electron microscopy (SEM/TEM) and X-ray diffraction (XRD). SEM/TEM observation showed formation of a porous altered layer consisting of fine crystallites on the pristine glass and euhedral crystals on the altered layer. XRD analysis indicated that the fine crystallites and euhedral crystals are hydrocerussite, Pb3 (CO3)2(OH) 2, which was predicted by geochemical calculation as the precipitate for the experimental system.