Chemical reactions during ThO2 and ThO2-UO2 fuel fabrication

摘要

The chemical reactions that occur during the fabrication of fuel pellets for the Shipping port Light Water Breeder Reactor are discussed. These include (1) precipitation and pyrolysis of thorium oxalate, (2) precipitation, calcination, and hydrogen reduction of ammonium diuranate, (3) comminution, granulation with an organic binder, and cold compaction of ThO(sub 2) and ThO(sub 2)-UO(sub 2) powders, (4) decarburization of the organic binder in CO(sub 2) at temperature up to 925 C, and (5) sintering in moist hydrogen at temperature up to 1790 C. Thorium oxalate precipitation and pyrolysis temperatures were the primary process variables for controlling the resulting thoria powder properties. Coprecipitated metal sulfates were converted to transition metal sulfides during calcining - the conversion of thorium oxalate to thorium oxide. The critical variable for controlling the urania powder properties was the hydrogen reduction temperature. Thermodynamic analyses showed that the efficiency of carbon removal from cold-compacted ThO(sub 2) and ThO(sub 2)-UO(sub 2) pellets by CO(sub 2) oxidation increases with temperature and, at temperatures around 900 C, substantially complete oxidation of carbon to carbon monoxide gas should occur. The carbon content of the ThO(sub 2) and ThO(sub 2)-UO(sub 2) fuels was further reduced during the initial heating in the hydrogen sintering cycle through the formation of methane gas. Additions of water vapor to the hydrogen sintering atmosphere also aided in carbon removal.