Surface Alteration of Sintered Product of Strontium Aluminum Silicate Under Hydrothermal Conditions
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SURFACE ALTERATION OF SINTERED PRODUCT OF STRONTIUM ALUMINUM SILICATE UNDER HYDROTHERMAL CONDITIONS H. MIMURA*, A. KOYAMA* AND K. AKIBA* *Tohoku University, Institute for Advanced Materials Processing, Katahira 2, Aoba-ku, Sendai 980-77, Japan. ABSTRACT The surface alteration of strontium aluminum silicate has been studied under hydrothermal conditions in connection with the leachability of Sr from solidified radioactive wastes. Samples exposed to a 3 M NaCI solution at 300'C produced a precipitated layer of analcime (NaAlSi20d with high crystallinity. The normalized leach rates for Sr in 3 M NaCl and 3 M 2 KC1 solutions were found to be less than 10-3 g/cm "d, yielding high resistance to leaching in these solutions. On exposure to the CaCl 2 solution at 300'C, anorthite (CaAl 2Si 2O8 ) was formed on the surface, indicating the replacement of Sr with Ca through isomorphous substitution. In the case of MgCI 2 solution, a thick precipitated layer of clinochlore (Mg 5 AI(Si 3Al)O 10(OH) 8 ) built up on the outermost surface. This layer serves to depress the migration of Sr, resulting in the deposition of SrCl 2 salts in the altered layer. INTRODUCTION High-level liquid wastes (HLLW) generated from reprocessing of spent nuclear fuels are usually solidified, stored and finally isolated in geological formations [1]. A variety of crystalline solid forms have been developed as host materials consisting of insoluble crystal phases immobilizing radioactive nuclides. Aluminum silicates [2][3] and titanates [4][5] have been proposed for the immobilization of 'Sr, a major heat-generating nuclide in HLLW. A strontium aluminum silicate, SrAl 2 Si208 was found to be a useful crystalline phase for the immobilization of 9°Sr, owing to its high resistance to leaching in water and high thermal stability [6]. Solidified wastes containing heat-generating nuclides may be exposed to hydrothermal conditions, when they contact with ground water in a repository environment. This requires the evaluation of chemical durability of solid forms under high temperatures and pressures. Several tests of hydrothermal alteration have been performed on a number of crystalline solid forms and borosilicate glasses by exposure to solutions containing NaCI [7]-[9]. As for the solid form of SrAl 2 Si208 the leaching behavior of Sr in deionized water has been studied under relatively mild conditions below 90°C [6]. The present study was undertaken to examine the surface alteration of the solid form of SrAI 2Si2O. and to clarify the migration and precipitation behavior of its constituents under hydrothermal conditions. EXPERIMENTAL Materials The procedure for the production of the solid form of SrAl 2Si2 0 is shown in Fig. 1. Zeolite A with binder (TOSOH, Zeoramu A-4) was pulverized to give a particle size less than 100 mesh and contacted with a 1 M SrCl 2 solution. The strontium form of zeolite A was pressed at 78.5 MPa into a cylinder of 15 cmo and 15 cm in height and then converted into solid SrAl 2Si 20 8 by calcination at 1200'C for 1 h [2]. The solid for
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