Patent Number: 042645409
Section: description

DESCRIPTION OF PREFERRED EXAMPLES In typical experiments 0.5% by weight of ball-milled niobium pentoxide was blended with uranium dioxide powder and granulated with 0.2% by weight of zinc stearate as a lubricant before pressing into compacts of diameter 1.1 cm and sintering in moist hydrogen for 4 hours at 1700.degree. C. The following results were obtained by addition of water vapour to provide the oxygen potential. 1. At low moisture contents of less than 1000 parts per million by volume (vpm) density increased with increasing moisture content from 95% theoretical density (TD) to 98.5% while grain size increased from 14-35 microns. 2. At moisture contents between 1000 and 20,000 vpm density remained fairly constant at approximately 98.5% TD while grain size increased to at least 60 microns. For example, with a moisture content of 15,000 vpm a grain size of 50 microns was obtained. 3. Moisture contents over about 20,000 vpm gave a progressive reduction in density (and a decrease in the reproducibility of the density results). A typical result at 25,000 vpm moisture was about 95% TD with a grain size of 80 microns. A similar effect on grain size can be obtained by adding carbon dioxide instead of water to the hydrogen sintering atmosphere as the following results show ______________________________________ Grain size vpm CO.sub.2 % TD (microns) ______________________________________ 1000 94.0 12 7500 98.5 42 15000 99.0 58 ______________________________________ It was also found that the upper limit of moisture content varied with various parameters. High densities and large grain sizes could be obtained at higher moisture contents by (a) Reducing the density of the green (unsintered) compact say from 5.75 g/cm.sup.3 to 5.2 g/cm.sup.3 PA0 (b) Reducing the heating rate during sintering say from 300.degree. C. per hour to 50.degree. C. per hour. From all the above results it follows that, if the presently desired grain size of about 40 microns is to be achieved by adding 0.5% by weight of niobium pentoxide to the nuclear fuel and using the practicable and economic sintering time and temperature of 4 hours at 1700.degree. C., the moisture level in a hydrogen sintering atmosphere should be maintained between 1000 and 9000 vpm. For a most satisfactory microstructure, however, it is considered that the optimum value within this range is 5000-7000 vpm for a water in hydrogen atmosphere and that for a carbon dioxide in hydrogen atmosphere the optimum carbon dioxide content is 7000-8000 vpm. Varying the quantity of niobium pentoxide added affects the results too. Fuel pellets have been made with additions of niobium pentoxide as low as 0.25% by weight. A grain size of 25 microns was easily produced on sintering for 4 hours at 1700.degree. C. in a hydrogen atmosphere containing 6000 vpm water vapour. With an increase in the addition of niobium pentoxide to 0.35% by weight grain sizes in excess of 30 microns were produced under the same conditions. The optimum results were obtained at 1700.degree. C. with additions of 0.5% by weight of niobium pentoxide. With an increase in sintering time or sintering temperature it is to be expected however that less niobium pentoxide will be required to produce a given grain size. Controlled porosity may be introduced into the sintered pellets by the inclusion of a fugitive pore former in the compact, as described, for example in U.K. Pat. No. 1,461,263.