Dissolution of a Multiphase Waste Form
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Dissolution of a Multiphase Waste Form Michele A. Lewis, Nancy L. Dietz, and Thomas H. Fanning Argonne National Laboratory 9700 S. Cass Avenue Argonne, IL 60439 ABSTRACT The dissolution behavior of the ceramic waste form (CWF) is being investigated to support its qualification for disposal in the proposed repository at Yucca Mountain. The CWF consists of sodalite and glass phases and has been consolidated either by hot isostatic pressuring (HIP) or by pressureless consolidation (PC). In this paper we compare the dissolution behavior of the two materials using in MCC-1 type tests at 90°C in a simulated silicate groundwater. The test solutions were periodically exchanged limit feedback effects. The solid surface area to volume ratio was 10 m-1. Five types of samples were tested: (1) HIP CWF, (2) binder glass vitrified by HIP, (3) PC CWF, (4) binder glass vitrified by PC, and HIP sodalite. Boron releases were used to monitor glass dissolution; these were similar in tests with HIP CWF, HIP glass, and PC CWF, but about 3X higher in tests with PC glass. At the end of the tests, the surfaces of the reacted materials were examined with scanning electron microscopy for signs of preferential dissolution. Differences in the dissolution behaviors of the materials are described and the implications of the test results regarding the performance of the CWF in the disposal system are discussed. INTRODUCTION A ceramic waste form is being developed for disposition of the radionuclide-bearing salt waste stream generated as part of the spent fuel treatment program at Argonne National Laboratory. Preparation of the CWF has been described in detail previously [1]. Briefly, the salt is mixed with zeolite 4A at 500°C to form salt-loaded zeolite, which is subsequently mixed with glass frit at a 3 to 1 zeolite to glass mass ratio. Two consolidation methods have been employed; these are hot isostatic pressing (HIP) and, more recently, pressureless consolidation (PC) [1 and references therein]. The HIP CWF is obtained by isostatically pressing at 850°C for one hour and 14,500 psi pressure while the PC CWF is obtained by heating at 915°C for 16 h. During the consolidation step, the salt-loaded zeolite is converted to sodalite, and small amounts of oxides, silicates, and free salt (primarily NaCl) are formed. The glass becomes fluid and flows around the other phases encapsulating them, and the mixture is converted to a multiphase material. The dissolution of the ceramic waste form (CWF) is being studied to obtain data to evaluate its performance and to support qualification for disposal in the proposed repository at Yucca Mountain. Previous studies with the HIP CWF have been used to develop a bounding model for radionuclide release [2, 3]. In those studies, short-term MCC-1 tests in pH-buffered solutions were used to identify the pH and temperature dependence of HIP glass and HIP sodalite dissolution. One result was that sodalite dissolved faster than glass in demineralized water. In solutions with pH values greater than 9, glass dissol
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