CHANGES IN THE DUPLEX GRAIN-STRUCTURE OF UO2 WITH A BIMODAL GRAIN-SIZE DISTRIBUTION AND IRREGULAR GRAINS

Grain growth and the transition from irregular to regular grains have been studied on the duplex grain structure of UO2. The powder compacts were sintered in CO2 at 1300 and 1500-degrees-C for 0.1 to 40 h. The duplex grain structure which consists of fine regular grains and coars e irregular grains develops at 1300-degrees-C after 0.1 h. Grain growt h occurs preferentially in the cluster of fine regular grains. so that the medium-sized grains of about 7 mum develop. Continued grain growt h yields coarse grains, and simultaneously the irregular grain changes into the regular one. The initial bimodal grain size distribution of the duplex grain structure changes eventually into a monomodal distrib ution via transient bimodal distributions. Factors influencing the cha nges in the duplex grain structure are discussed.