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

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 degrees C, based on the result of the thermo gravimetric analysis. The Cs and Sr leach rates by the PCT-A method were about 1 x 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.

Scientific and Engineering Literature Mini Review of Molten Salt Oxidation for Radioactive Waste Treatment and Organic Compound Gasification as well as Spent Salt Treatment

“…A variety of methods have also been used to make the residual waste containing hazardous or radioactive compounds pass certain disposal requirements [16,19,46,97]. Methods to encapsulate the final residual waste from MSO processing include ceramic materials [16], polyethylene [19], and a silicon-alumina-phosphorous based material [97]. The use of oxides of silicon, alumina, and phosphorus gave higher disposal efficiencies and lower waste volumes compared with other immobilization methods [97].…”

“…Methods to encapsulate the final residual waste from MSO processing include ceramic materials [16], polyethylene [19], and a silicon-alumina-phosphorous based material [97]. The use of oxides of silicon, alumina, and phosphorus gave higher disposal efficiencies and lower waste volumes compared with other immobilization methods [97]. A glass-zeolite ceramic waste form was developed at Argonne National Laboratory for the disposal of radioactive waste salts from electrometallurgical treatment of spent nuclear fuel; the results of characterization studies on the waste form are described in papers [46,88,97].…”

“…A gel-route encapsulation method was developed using borosilicate glass as a chemical binder where silicate produces an amorphous phase and phosphates are encapsulated by the vitrified phase [88,97]. Cesium in the gel product is preferentially situated in the silicate phase, and it is vitrified into a glassy phase after heat treatment.…”

I-Neri-2007-004-K, Development and Characterization of New High-Level Waste Forms for Achieving Waste Minimization From Pyroprocessing

“…The SAP waste form is based on a formulation developed at KAERI involving immobilization of the metal fission products in a silicaͲaluminaͲphosphorus matrix [2]. Producing the waste form involves mixing waste salt with the SAP reagent material.…”

“…This IͲNERI project expands from the previous project with the development of suitable waste forms to immobilize the separated fission products. Researchers at KAERI have developed two novel waste forms using surrogate waste materials for the immobilization of fission products in ER salts [2,3]. Fabrication and characterization of these waste forms and a highͲloaded ceramic waste form using actual ER salt was demonstrated in the hotͲcell facilities at the INL as part of the 3 rd year research activities.…”