Preliminary Studies of the Disposition of Cesium in a Glass-Bonded Sodalite Waste form
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Preliminary Studies of the Disposition of Cesium in a Glass-Bonded Sodalite Waste Form Marsha J. Lambregts and Steven M. Frank Argonne National Laboratory-West, PO Box 2528, Idaho Falls, ID 83401 ABSTRACT Argonne National Laboratory has developed an electrometallurgical treatment for DOE spent metallic nuclear fuel. Fission products are immobilized in a durable glass bonded sodalite ceramic waste form (CWF) suitable for long term storage in a geological repository. Cesium is estimated to be in the waste form at approximately 0.1 wt.%. The exact disposition of cesium was uncertain and it was believed to be uniformly distributed throughout the waste form. A correlation of X-ray diffractometry (XRD), electron microscopy (EM), and nuclear magnetic resonance spectroscopy (NMR) performed on surrogate ceramic waste forms with high cesium loadings found a high cesium content in the glass phase and in several non-sodalite aluminosilicate phases. Cesium was not detected in the sodalite phase. INTRODUCTION Argonne National Laboratory has recently completed the Spent Fuel Demonstration Project for the DOE involving the use of electrometallurgical conditioning of spent metallic nuclear fuel. This process uses a molten lithium chloride/potassium chloride eutectic salt to anodically dissolve the fuel. Three materials are produced during this process [1]: uranium, cladding hulls and the fission product bearing salt. Various isotopes of cesium reside in the salt. From a repository point of view, cesium-135 is particularly problematic because of its high radioactivity and high solubility in water. The CWF developed to immobilize the salt is composed primarily of two phases— sodalite and glass [2]. Minor phases present in the CWF include: nepheline, halite crystallites, rare earth silicates and actinide oxides. The sodalite is produced from a solid state reaction of salt with zeolite 4A under heat or heat and pressure. Production of the CWF involves heated mixing of dry zeolite A powder with ground salt from the electrorefiner at 500oC. A glass binder is mixed in at a 1 to 3 ratio with the salt-occluded zeolite. The mixture is heated to 915oC. It is during this step that the zeolite transforms into sodalite and the glass melts, binding the sodalite particles together into a consolidated waste form. This is accomplished in an inert atmosphere hot cell environment. Sodalite contains chlorine as part of the crystal structure. The empirical formula for sodalite is M8Al6Si6O24X2, where M is a metal and X represents halides. The overall structure is cubic and consists of alternating SiO4 and AlO4 tetrahedra joined at the corners to form four unit and six unit rings. These rings make up beta-cages similar to those found in zeolite 4A. For the sodium chloride form of sodalite, a chlorine atom lies in the center of each cage tetrahedrally coordinated to four sodium atoms. The sodium atoms are further coordinated to the oxygen atoms of the six unit rings. The cesium content of the CWF is approximately 0.1% by weight. The fission product cesium
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