Interaction of molten chloride salts and zeolite 4A - nature, thermodynamics, and application to nuclear waste treatment and disposal

A glass-zeolite ceramic waste form is being developed at Argonne National Laboratory for the disposal of radioactive waste salts from electro-metallurgical treatment of spent nuclear fuel. Approximately 70 wt% of the process salt is the LiCl-KCl eutectic salt. In addition, the salt contains NaCl, fission product chlorides, and ∝2 mol% actinide chlorides, such as UCl3, NpCl3, and PuCl3. The process salt is treated and/or disposed of by contacting it with zeolite 4A at 823 K. The occlusion of the LiCl-KCl eutectic salt by zeolite has been studied by differential scanning calorimetry. The partial molar enthalpy of occlusion was determined to be -(24±1) kJ.mol-1. The occlusion of the UCl3-LiCl-KCl eutectic salt by zeolite has been studied by temperature-resolved synchrotron powder X-ray diffraction, evolved gas analysis, and differential scanning calorimetry. UCl3 reacts with the zeolitic H2O to form UO2 that is located external to the contracting zeolite lattice. Ion-exchange equilibrium constants at infinite dilution of NaCl, RbCl, and CsCl have been determined by chemical analysis of both the zeolite and the molten salt phases. They are 0.84, 0.87, 2.31, respectively. (orig.)