SOME PREPARATION METHODS AND PHYSICAL CHARACTERISTICS OF U02 POWDERS

摘要

The results of the Anderson, et al. , studies concerning the dependence of UO2 properties upon their mode of preparation are confirmed and their conclusions extended to UO2 powders made by a large variety of oxidation and reduction methods. Although physical properties such as particle size and distribution, microstructure, porosity, crystallite size, surface area, and density can vary over a wide range, these properties can be controlled through the judicious choice of vari¬ables in the preparation procedure.nIn a first approximation, the crystallite size of UO2 powders depends more on the temperature than on the method of preparation. Low temperature preparations (350 °C) produced the smallest crystallite sizes (200 A);the crystallite size of UO2 made at 800°C was considerably larger (1000 A).nThe real density of a UO2 preparation depends upon its O/U ratio and on the density of its parent higher oxide. The density was hot dependent on the reduction temperature up to 1200°C. However, heating low density UO2above its temperature of preparation can cause an increase in its density. In many cases the liquid displacement densities were considerably below the helium displacement values, indicating measurable amounts of open porosity. Grinding the UO2 powders did not increase their densities (as determined by liquid or helium displacement) significantly, but did decrease their bulk densities. Milling the less abrasive UO3 produced some increase in its density which was retained upon reduction to UO2.nThere is no correlation between density and total BET surface area;a high surface area can be associated with either a high or a low density;a low surface area powder can have either a low or a high density. There appears to be no relationship between the density and total surface area of the parent ammonium diuranate precipitates and the derivative UO2 powders.nGrinding UO3and UO2 powders increased the surface area and decreased the particle size. The increase in surface area for ball-milled UO3 was retained upon reduction to UO2. The influence of oxide source, wet-and dry-milling, amount and type of grinding media, milling time, and comminution processes en the particle size distribution of UO2 was studied and little difference in the particle size distribution of the milled powders was observed.nThe particle size distribution and total BET surface area of a UO2 preparation depend upon the particle size distribution and surface area of the higher oxide from which it has been prepared by reduction and on the reduction temperature itself. In the reduction of UO3, the uranium dioxide reduction product was most similar in surface area and particle size distribution to the parent oxide at low temperatures (480 °C). Particle growth occurred at higher temperatures. No change in particle size distribution but an increase in surface area was observed on low temperature reduction (500°C) of U3O8 to UO2,. There was less particle breakdown at higher reduction temperatures. This suggests that the UO2 particles are fractured U3O8 particles whose strains anneal out at higher temperatures.