Patent Number: 059164972
Section: description

DETAILED DESCRIPTION OF THE INVENTION As shown in FIG. 1, a pellet profile having a barrel shape 1 is produced by compressing a homogenous UO.sub.2 or UO.sub.2 -based powder in a die or mould cavity of right circular cylindrical cross-section in the conventional manner described above. Upon sintering of the compressed pellet, a pellet of wheatsheaf-like shape 2 is formed as shown in FIG. 2. The diameter at the waist of the shape 2 may for example by typically 50 .mu.m (ie 25 .mu.m at each side) less than that at the ends of the pellet for a pellet having dimensions of about 6 mm (diameter) by 10 mm (length). FIG. 3 shows a pellet profile as obtained by using the method of the present invention. A right circular cylindrical pellet body shape 3 is formed by compressing a UO.sub.2 or UO.sub.2 -based powder having three regions 5, 6, 7. In regions 5 and 7 at the ends of the pellet body the density of particles is d.sub.1, and in the region 6 at the middle of the pellet body the density is d.sub.2. The density d.sub.2 may be three times greater than d.sub.1 for regions 5, 6 and 7 of equal depth when the compaction pressure is 2 tonnes/cm.sup.2. When the pellet shown in FIG. 3 is sintered in a conventional way, eg as described in the following example, a sintered pellet profile substantially the same as the shape 3 is obtained. The following illustrative example demonstrates the benefit of the invention as compared with the prior art. Ceramic grade uranium dioxide powder manufactured by the Applicants was used in the experiments. Two types of granules, Type A and Type B, were prepared from the same UO.sub.2 powder as follows. Type A: Powder was poured into a steel die of diameter 2.54 cm and pressed at a pressure of 0.25 tonnes cm.sup.-2 to form a disc shaped powder compact of thickness approximately 1 cm. The compact was then broken through a #14 mesh sieve with orifice size 1.2 mm to produce granules. The resultant granules were placed in a glass jar and 0.2 wt % zinc stearate added. The zinc stearate acts as a lubricant during die pressing. The jar was then placed on rollers and tumbled for 10 minutes to improve granule flowability and mix in the stearate. Type B: These granules were prepared in exactly the same manner as Type A except that they were pressed at 0.75 tonnes cm.sup.-2. The granules were then pelleted as follows: Experiment 1 (prior art): 8 g of granules of Type B were poured into a right cylindrical die cavity with diameter 11 mm and then pressed with a pressure of 2 tonnes cm.sup.-2. The resulting compact was then fired under the following conditions: heating rate 5 C/min ramp rate to 300 C and then a 20 C/min ramp rate up to 1750 C, held at 1750 C for 5 hours and then cooled at 20 C/min. The atmosphere used was pure hydrogen with 0.5 volume % carbon dioxide at a flow rate through the furnace of 10 /min. No pressure was applied during firing. The fired pellet side profile was then measured on a stylus profile measuring device. The result of profiling is shown in FIG. 4 which clearly shows the previously described `wheatsheaf` effect. In FIG. 4, the vertical axis represents pellet radius variation from an arbitrary datum value, as profiled on one side of the pellet and the horizontal axis represents pellet length in distance from an arbitrary point, as measured along the same side of the pellet. Experiment 2 (invention): 3 g of granules of Type A were first placed in the die cavity and the die lightly tapped to ensure the granules lay level. 2 g of granules of Type B were placed on top of the Type A granules and again the die tapped. Finally 3 g of granules of Type A were laid on top of the Type B layer. The granules were then pressed, fired and measured as in Experiment 1 above. The resultant fired pellet profile shown in FIG. 5 is much improved in comparison to the profile obtained by the prior art procedures as illustrated in FIG. 4. As in FIG. 5, the vertical axis represents pellet radius variation and the horizontal axis represents distance along the pellet both as profiled along the side of the pellet. While the various embodiments of the present invention have been described in detail, it is apparent that various modifications and adaptations of those embodiments will occur to those skilled in the art. It is expressly understood, however, that such modifications and adaptations are within the scope of the present invention as set forth in the following claims.