Unravelling the origin of long-term stability for Cs + and Sr 2+ solidification inside sodalite.

Cesium (Cs + ) and strontium (Sr 2+ ) ions are the main fission byproducts in the reprocessing of spent nuclear fuels for nuclear power plants. Their long half-live period (30.17 years for 137 Cs and 28.80 years for 90 Sr) makes them very dangerous radionuclides. Hence the solidification of Cs + and Sr 2+ is of paramount importance for preventing them from entering the human food chain through water. Despite tremendous efforts for solidification, the long-term stability remains a great challenge due to the experimental limitation and lack of good evaluation indicators for such long half-life radionuclides. Using density functional theory (DFT), we investigate the origin of long-term stability for the solidification of Cs + and Sr 2+ inside sodalite and establish that the exchange energy and the diffusion barrier play an important role in gaining the long-term stability both thermodynamically and kinetically. The acidity/basicity, solvation, temperature, and diffusion effect are comprehensively studied. It is found that solidification of Cs + and Sr 2+ is mainly attributed to the solvation effect, zeolitic adsorption ability, and diffusion barriers. The present study provides theoretical evidence to use geopolymers to adsorb Cs + and Sr 2+ and convert the adsorbed geopolymers to zeolites to achieve solidification of Cs + and Sr 2+ with long-term stability.