Fission product distribution in oxide fuels (LWBR Development Program)

Radial electron probe traverses were made on samples of ThO2 and ThO2-20 w/o UO2 after approximately 15 and 21 x 1020 fissions/cm3, respectively. Concentration profiles of 233U and fission products in the thoria sample, which had not developed columnar grains, varied approximately linearly with the pellet radius. The slope of the profile varied with the volatility of the nuclide being measured; for xenon, the average concentration at the outside diameter was 6.08 percent above that at the center, and for molybdenum, it was 10.74 percent below. The slope observed for uranium did not differ significantly from zero at the 95 percent probability level. When columnar grains were developed, as in the second sample, much larger effects were superimposed on those described above. The region of the columnar grains was depleted of uranium and fission products, which tended to develop a parabolic composition profile with a minimum, at the center, one-third to one-half the average content. The material removed from the columnar grains was relocated in a band at the interface between the columnar and equiaxed grains. Technetium and the noble metals formed alloy ingots in this region with molybdenum. The composition of these ingots was not uniform; some ingots lacked one or more of the noble metals