Molecular Scale Interactions Between Hydrated Cement Phases and Radionuclides Using Molecular Dynamics Modeling
Cesium (137Cs) and technetium (99Tc) are two radionuclides of significant concern in spent nuclear fuel because of their long half-life, high decay energy, and high solubility. Cementitious materials are considered as potential candidates for nuclear waste immobilization. The study of the interaction of radionuclides with the cement matrix is thus essential to determine the capability of cementitious materials to immobilize radioactive isotopes such as 137Cs and 99Tc. Calcium-silicate-hydrate (C-S-H) and calcium-aluminate-sulfate hydrate (ettringite) are two main products of cement hydration in Portland Cement (PC). The interactions of CsCl solution with different crystalline C-S-H structures (9Å tobermorite, 14Å tobermorite and jennite) and the interactions of KTcO4 solution with crystalline C-S-H structures (14Å tobermorite and jennite) and ettringite were studied using Molecular Dynamics (MD) simulations. MD simulation results indicated that Cs+ ions were mainly adsorbed as inner-sphere complexes at the tetrahedral SiO4 surface ((0 0 1) plane) of 9Å tobermorite, 14Å tobermorite, and jennite. The tobermorites showed higher adsorption capacity of Cs+ ions than jennite. Jennite had weak affinity to Cs+ ions. MD simulation results indicated that TcO4- ions can be adsorbed on the surfaces of 14Å tobermorite and ettringite. On the octahedral CaO6 surface ((0 0 -1) plane) of 14Å tobermorite, inner-sphere Tc complexes coexisted with outer-sphere Tc complexes. Co-ion adsorption affected the adsorption of outer-sphere Tc complexes and pushed outer-sphere complexes further away from the surface. On the octahedral CaO6 surface ((0 0 -1) plane) of ettringite, TcO4- ions were mainly adsorbed as outer-sphere complexes. Two types (i.e. type 1: tetragonal geometry and type 2: tetrahedral geometry) of outer-sphere Tc complexes were predicted on both surfaces of 14Å tobermorite and ettringite. Jennite had no affinity to TcO4- ions. In conclusion, 9Å tobermorite and 14Å tobermorite were better candidates for the immobilization of Cs+ ions than jennite. Ettringite and 14Å tobermorite also showed better affinity to TcO4- ions than jennite. Ca2+ ions on the surface played an important role and formed complexes with TcO4- ions.