Study on iodine-leaching mechanism for HIP rock solidified waste form

In Japan, a preliminary safety assessment for geological isolation of TRU waste has reported that the rate of radioactive exposure can be controlled by radioactive iodine (I-129). In order to decrease the rate of radioactive exposure, the HIP (Hot Isostatic Pressing) solidified method has been developed for an iodine adsorbent waste (silver loaded silica gel) to produce a stable 'HIP rock solidified waste form'. Static leaching tests for the HIP rock solidified waste form, which were performed in a reducing condition with 3x10-3 M Na2S solution for 300 days, showed that the normalized leaching rates for I and Si were extremely low (10-7 - 10-8 g/dm2/day). For initial 60 days of the tests the cumulative amount of leached iodine was proportional to square root of time, which indicates that the leaching can be controlled by diffusion. Therefore, this phenomenon will be controlled by diffusion. By TEM analysis for the reacted waste form surface, formation of sulfides was observed at quartz grain boundary, which indicates that the diffusion path of iodine can be the grain boundary. Consequently, it is possible to adapt 'grain boundary diffusion model' as iodine-leaching mechanism of the HIP 'rock' solidified waste. (author)