THERMOCHEMISTRY OF TITANOSILICATE MINERALS RELEVANT TO NUCLEAR ENERGY WASTE FORMS

To continue utilizing nuclear energy, nuclear waste management is a prerequisite. Titanosilicates are possible oxide forms for immobilization of short half-life nuclides, such as Cs-137 and Sr-90, in radioactive waste. Cesium decays to barium, while strontium decays to yttrium and then zirconium through beta decay. Therefore, not only the stability of Cs-/Sr-loaded waste forms, but also that of a potential decay product series with charge-balance in multicomponent oxide phases is of fundamental importance. In nature, the mineral analogues of many ceramic forms provide evidence of long-term stability. Here, we report thermochemistry of titanosilicate minerals and their decay products with pollucite, titanite, and fresnoite structures. Ba-substituted Cs-titanosilicates (Cs_1-xBa_0.5xTiSi₂O_6.5), Y-substituted Sr-titanosilicates (Sr_1-xY_0.67xTiSiO₅, Sr_2-xY_0.67xTiSi₂O₈) and Ba-titanosilicates (Ba₂TiSi₂O₈) were prepared using nonradioactive analogues. The enthalpies of drop solution of the samples were obtained from calorimetry in a molten lead borate (2PbO-B₂O₃) solvent at 702 ^oC. The enthalpies of formation for the samples from constituent oxides were calculated and used to analyze the thermodynamic stability with changing composition due to radioactive decay. The energetics between glass and crystal were studied.