Microstructural aspects of grain growth induced by dopants in U O2 sintered pellets

U O2 powder (ADU, route) was mixed with Ti O2, Cr2 O3 or Nb2 O5 in small percents (0-1% wt M/U), pressed at 167 NM/m2, and sintered at 1700 o C for three hours in a reducing atmosphere. For very small concentrations of Nb2 O5 and Ti2, the pellets' density reaches a minimum value, than increases to a value greater than for an undoped pellets. For greater concentrations of dopant, the density slowly decreases. If the pellets are doped with Cr2 O3, the behaviour is different, the minimum for small concentrations does not appear. The average grain size, plotted against doped concentration, shows a growth in direct proportion with the dopant amount at low concentrations and stops at greater amounts of dopant. Grain growth depends on the dopant nature, but the same mechanism occurs. The dopant is mainly distributed at grain boundaries and, at large concentrations, a second phase can be seen, probably an eutectic formed with the matrix and the dopant. (Author)