SITE RELATED NUCLEATION AND GROWTH OF HYDRIDES ON URANIUM SURFACES

The characteristics of hydride nucleation and growth on certain surfac es of pure uranium and of U-0.1 wt.% Cr samples were studied (under 1 atm H-2 at temperatures of 50-75 degrees C) using the hot-stage micros cope, microprobe analyzer and atomic force microscope techniques. Diff erent preparation procedures of the samples were applied, in order to check the effects of surface oxidation layer variations on the nucleat ion and growth characteristics. Four families of hydride nuclei, diffe ring in density, size and growth rates, were observed and classified. The smallest and most dense is in the form of submicron blisters forme d instantaneously along mechanical polishing scratches. A larger (1-10 mu m) blister-like family is formed beneath the oxide at point defect sites (but not discontinuities of the oxide), growing very slowly pro bably due to the compression of the coating oxide layer. The third fam ily is characterized by preferential nucleation and rapid growth aroun d carbide inclusions due to the discontinuity in the oxide at the carb ide/oxide interface. The fourth family is found only on the samples ha ving a thick oxide layer, and is characterized by a rapid growth rate, but is not located around inclusions. In this case, the nuclei probab ly originate at some other oxide discontinuities, such as twins or gra in boundaries.