Reprocessing Silicon Carbide Inert Matrix Fuel by Molten Salt Corrosion
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Reprocessing Silicon Carbide Inert Matrix Fuel by Molten Salt Corrosion Ting Cheng, Ronald H. Baney1, James Tulenko2 1 Material Science and Engineering Department, University of Florida, 32611, Gainesville, FL, U.S.A. 2 Nuclear Engineering Department, University of Florida, 32611, Gainesville, FL, U.S.A. ABSTRACT Silicon carbide is one of the prime matrix material candidates for inert matrix fuels (IMF) which are being designed to reduce plutonium and long half-life actinide inventories through transmutation. Since complete transmutation is impractical in a single in-core run, reprocessing the inert matrix fuels becomes necessary. The current reprocessing techniques of many inert matrix materials involve dissolution of spent fuels in acidic aqueous solutions. However, SiC cannot be dissolved by that process. Thus, new reprocessing techniques are required. This paper discusses a possible way for separating transuranic (actinide) species from a bulk silicon carbide (SiC) matrix utilizing molten carbonates. Bulk reaction-bonded SiC and SiC powder (1 µm) were corroded at high temperatures (above 850 ºC) in molten carbonates (K2CO3 and Na2CO3) in an air atmosphere to form water soluble silicates. Separation of Ceria (used as a surrogate for the plutonium fissile fuel) was achieved by dissolving the silicates in boiling water and leaving behind the solid ceria (CeO2). INTRODUCTION During the past decade, considerable efforts have been made to search for a material to be utilized as the matrix for inert matrix fuels (IMF) to transmute plutonium and minor actinides (Np, Am, Cm) in a nuclear reactor. The properties of silicon carbide attracted extensive attention to function as an IMF’s due to the small thermal neutron absorption cross section of both silicon and carbon, its chemical inertness, and high thermal conductivity [1-4]. Issues related to the application of SiC for IMF’s including fabrication, in-reactor behavior, reprocessing and waste disposal remained. Recently, S. Bourg [4] developed a separation method for SiC matrix fuel using Cl2 to oxidize SiC powder (45 µm), in which 75 percent of SiC was volatilized in the form of SiCl4 at 900 ºC. The remaining solid carbon was removed by oxidation under oxygen at 400 ºC. This paper did not consider the hazardous and corrosive nature of this process, if the system were applied on a large scale. In our work, separation can be achieved by filtering ceria, a widely used surrogate of plutonium [5-7], from a solution of water-soluble SiC corrosion products (e.g. silicate) formed by SiC corrosion in molten carbonates. Experimental In our experiment, a tube furnace with caps on both ends was preheated to 900 ºC for SiC powder and 1050 ºC for SiC monoliths contained in carbonate baths. In all experiments, a compressed air cylinder provided constant airflow (658 ml/min) through the furnace. SiC powder (purchased from Alfa Aesar) with 1 µm diameter was studied. After ball milling for 9 hours, the
mixture of SiC powder (1 g) and Na2CO3 at approximately a 1:2 molar rat
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