2, Rensselaer Polytechnic Institute, Troy, New York, United States
To retain iodine-129 with waste forms in geological settings is challenging due to its extremely long half-life and high volatility in natural environments. To evaluate the long-term performance of nuclear waste forms, it is imperative to characterize the release mechanism of radionuclides in the host materials. This study investigated the release mechanism of iodine retained in apatite structure waste form Pb9.85(VO4)6I1.75 to determine whether and how diffusion and dissolution control the chemical durability of apatite waste form in aqueous solutions. A series of standard semi-dynamic leaching tests were conducted in different solutions: deionized water, organic pH buffers, and 1 mol/L of NaCl, Na2CO3, Na3PO4, and Na2SO4 solutions. During the leaching experiment, a sample pellet is periodically exposed to fresh leachant solution in a cap-sealed Teflon vessel under constant temperature 90±0.5°C. The leachant solution is replaced every 24 hours. The leachate solutions were analyzed and the leached surfaces were characterized. The results show that the release of iodine is subjected to both diffusion and dissolution processes and is highly sensitive to the solution conditions: 1) in pH neutral solutions, iodine release is controlled by short-term diffusion through ion-exchange and by long-term dissolution of structure matrix; 2) in acidic and basic solutions, dissolution is enhanced and dominates the iodine release from apatite. In addition, new phases are observed on leached surfaces, such as chervetite at pH 4 and hydroxylvanadinite at pH 10. The finding of this study provides two implications for long-term disposal safety of iodine-apatite waste form: i) avoid ion-rich storage environment, especially chloride; ii) create and maintain neutral pH conditions surrounding waste forms.