Stabilization/Solidification of Radioactive Salt Waste by Using xSiO2−yAl2O3−zP2O5 (SAP) Material at Molten Salt State

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Abstract
The molten salt waste from the pyroprocess is one of the problematic wastes to directly apply a conventional process such as vitrification or ceramization. This study suggested a novel method using a reactive material for metal chlorides at a molten temperature of salt waste, and then converting them into manageable product at a high temperature. The inorganic composite, SAP (SiO2−Al2O3−P2O5), synthesized by a conventional sol−gel process has three or four distinctive domains that are bonded sequentially, Si−O−Si−O−A−O−P−O−P. The P-rich phase in the SAP composite is unstable for producing a series of reactive sites when in contact with a molten LiCl salt. After the reaction, metal aluminosilicate, metal aluminophosphate, metal phosphates and gaseous chlorines are generated. From this process, the volatile salt waste is stabilized and it is possible to apply a high temperature process. The reaction products were fabricated successfully by using a borosilicate glass with an arbitrary composition as a chemical binder. There was a low possibility for the valorization of radionuclides up to 1200 °C, based on the result of the thermo gravimetric analysis. The Cs and Sr leach rates by the PCT-A method were about 1 × 10−3 g/(m2 day). For the final disposal of the problematic salt waste, this approach suggested the design concept of an effective stabilizer for metal chlorides and revealed the chemical route to the fabrication of monolithic wasteform by using a composite as an example. Using this method, we could obtain a higher disposal efficiency and lower waste volume, compared with the present immobilization methods.

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