Oxidation Behavior of Am-containing MOX Fuel Pellets in Air

Americium-containing MOX (Am-MOX) fuels were subjected to heating tests using thermogravimetric and differential thermal analysis measurements in a flowing gas atmosphere of dry air to investigate the effect of Am addition on oxidation behavior of MOX fuel. Three kinds of Am-MOX fuel pellets containing 3, 5 and 10 wt.% Am were prepared for the examination together with MOX fuel and UO2 fuel pellets as references. Sintered fuel pellets were heat-treated to adjust the oxygen-to-metal ratio to 2.00 and were crushed into small pieces. The weight gain due to oxidation was monitored and the pulverization behavior of the fuel pellets was observed. The specimens were analyzed by X-ray diffraction in order to investigate the change of the phase relation. The specimens were subjected to programmed rate heating test (6 K/min) from room temperature to 1073 K. The UO2 pellet specimen was oxidized rapidly to an O/M ratio of 2.67 (i.e. U3O8 was formed) and it was pulverized easily. The specimens of MOX fuel and Am-MOX fuel pellets, however, were oxidized gradually to O/M ratio around 2.3 (i.e. MO2+x and/or M4O9 were formed) and there was no pellet crumbling. Although the oxidation rate slightly decreased with increasing the Am content in the Am-MOX fuels, the oxidation curve shapes of Am-MOX fuels were similar to the curve of MOX fuel. The isothermal heating test was carried out for the specimens of MOX, 3 wt.% Am-MOX and 5 wt.% Am-MOX fuels. The kinetic analysis of the oxidation in the isothermal heating test was evaluated by the Johnson-Mehl equation.