Reduction of U₃O₈ and (U,Pu)₃O_8-x powders in flowing (N₂+7% H₂) gas mixture by microwave heating

The reduction of U₃O₈ and (U,Pu)₃O_8-x is reported at temperatures in the range 300-900 °C using several reductants like H₂, NH₃, CO, C, CH₄, etc. The reduction of U₃O₈ to UO₂ by H₂ is a surface controlled gas-solid reaction which proceeds according to contracting-sphere model involving desorption of water as the rate-limiting step. The stoichiometric reduction of U₃O₈ and (U,Pu)₃O_8-x is time consuming process which initially is fast and becomes slow at the end. The conversion of U₃O₈ to UO₂ proceeds via formation of reaction intermediates (U₃O₇/U₄O₉) which are unstable at temperature > 300 °C. The efficient reduction of U₃O₈ and (U,Pu)₃O_8-x to MO_2-x (M: U or U+Pu) using microwave heating is being reported using flowing (N₂+7% H₂) gas mixture in the present article. The method is fast and advantageous in comparison to the more traditional method of reduction in the electrical resistance furnaces. The reduction of U₃O₈ or the oxidized mixed oxide (U,Pu)₃O_8-x powders is a vital step in the feed preparation for oxide and MOX fuel fabrication