The influence of the sintering atmosphere on properties of refractories for UO2 sintering furnace and microstructure of UO2 pellets

The impact of various sintering atmosphere on thermochemical behavior of main refractory materials of the UO2 sintering furnace and on microstructure of sintered UO2 pellets has been investigated. It is found that Mullite (3Al2·2SiO2) and Andalusite/Sillimanite (Al2O3·SiO2) which are main constituents in the transition sintering zone have low possibility of pure H2-induced reduction, whereas Fire Clay and Chamotte in the Presintering Zone which are composed of SiO2 bonded with Al2O3 or free SiO2 have high possibility of pure H2 gas-induced reduction. This is because Fire Clay and Chamotte have negative free energy change over the test temperature, while Mullite and Andalusite/sillimanite positive free energy change. In refractory materials, two kinds of mixed gas such 70%H2-30%N2 and 50%N2 are used. The H2-N2 mixed gas experiment show that the larger amount of N2 will retard the H2 gas-induced reduction rate of the refractory materials with the increasing sintering temperature. The 50%H2-50%N2 mixed gas test indicates the H2 gas-induced corrosion resistance improvement of about 50% at 1400 deg C, but negligible improvement at 1200 deg C. On the other hand, the H2-N2 mixed gas has negligible impact on microstructure and properties of UO2 pellet such as density, grain size, pore size distribution and open porosity. (author). 5 refs., 11 tabs., 4 figs