Alternative Technology for the Treatment of Waste LiCl Salt by Using Gelation with a Si-P-Al Material System and a Subsequent Thermal Conditioning Method

Abstract

Chloride salt wastes, which are supposed to be generated from a pyrochemical processing of spent nuclear fuels, are one of the wastes that are problematic to treat because of their high solubility in water and the relatively high volatility of some of their nuclides during a high-temperature thermal treatment. In this paper, we propose a new conditioning method, named the gel-route stabilization/solidification (GRSS) method, and present a practical example of its application to fabricate a monolithic waste form for LiCl waste. The GRSS process is carried out in four steps: gelation, drying, mixing with binder glass, and heat treatment (thermal conditioning). The gel-forming material system consists of sodium silicate as a gelling agent, phosphoric acid as a catalyst/stabilizer, and aluminium nitrate as a promoter. Through the drying step, LiCl, CsCl, and SrCl2 are chemically converted into phosphate or aluminosilicate forms, depending on the Si/P/Al molar ratio. The gel products are thermally stable, and there is little possibility of a Cs vaporization up to 1200°C. The final waste form, fabricated by thermally treating a mixture of the gel products and borosilicate glass frit, shows low leach rates (by a product consistency test method for 7 days), 10-2 to 10-3 g/m2.day for Cs and 10-3 to 10-4 g/m2.day for Sr, which are comparable or superior to that of a glass-bonded sodalite ceramic waste form. Also, the amount of waste loading is ~16%, which is double that of the zeolite process, to generate a lesser final waste volume for disposal. From these results, it could be concluded that the GRSS method can be considered as an alternative technology for a sound immobilization of chloride salt wastes.