Patent Number: 039490270
Section: description

FIG. 1 shows the compaction chamber 1 of an hydraulic press; the punches 2 and 3, which move in the direction shown by the arrows; and the UO.sub.2 pellet 4. FIG. 2 shows the pellet 4 after sintering, and the deformation of its lateral wall 5, which is incurved inwards. This pellet must be brought down to the prescribed dimensions by grinding it along the dotted lines 6 before it can be used. FIG. 3 represents the compaction chamber 11 of a mechanical press; the upper punch 12, mobile during the compression phase, shifts in the direction shown by the arrow; the lower punch 13, fixed during the compression; as well as the UO.sub.2 pellet 14. FIG. 4 shows the pellet 14 after sintering and its cone trunk form. Before using this pellet it will equally be necessary to bring it down to the prescribed dimensions by grinding it along the dotted lines 16. FIG. 5 represents the compression of a UO.sub.2 pellet according to the invention. It shows the compaction chamber 21, the upper punch 22, fixed during the compaction cycle, and the lower mobile punch 23 shifting in the direction of the arrow during the compaction phase. The compressed pellet is represented by reference number 24. As shown on FIG. 5, the upper part 28 of the walls of the compaction chamber 21 is slightly widened, so that the section near the upper punch 22 has a slightly larger surface than the section near the lower punch 23; the lower part 27 of the walls is cylindrical. The angle .alpha., as defined above, is equally shown on the figure. After compaction, the tablet 24 thus has slightly flared lateral walls. Since, owing to the particular friction conditions on compression, the green density of the part 30 of the pellet 24, situated near the upper punch 22 on compression, is smaller than the green density of the part 31 of the pellet, situated near the punch 23 on compression, the part 30 will shrink more than the part 31 of the pellet when sintered. Thus, after sintering, the pellet will have a substantially cylindrical form and does not need rectification. The angle .alpha., formed by the vertical axis of the mould and the intersection line between an axial plan and the inclined wall, will be determined for each fabrication batch separately, as a function of the interfering parameters, i.e. the nature of the powder, the diameter of the pellet, the compression density, the friction between the powder and the die wall, the internal friction between the grains, the state of the equipment surfaces, the kind of apparatus, the lubrification means etc. The invention will be hereinafter more fully described with the help of two different examples of pelletizing. EXAMPLE 1 Ceramic sinterable uranium oxide powder is compressed in an hydraulic press comprising a tungsten carbide-tipped die. The characteristics of the powder are the following: oxygen content : O/U = 2.08 PA1 apparent density : 2.1 PA1 average diameter of the grains : 1.6 .mu.m PA1 specific surface : 4 m.sup.2 /g. PA1 oxygen content : 2.07 PA1 apparent density : 2.1 PA1 average diameter of the grains : 0.6 .mu.m PA1 specific surface : 3 m.sup.2 /g Before the compression cycle, the equipment will be lubrified by means of a preliminary compression cycle with polystyrene balls containing 3 % zinc stearate. The diameter of the die is 12 mm and the conicity of the walls is defined by an angle .alpha. = 26'10", corresponding to a radius increase of 75 microns per cm height. After 10 seconds compression, the compressed UO.sub.2 pellet has a green density of 5.63 and a height of 11.9 mm. The pellet is then sintered for an hour, at 1650.degree.C in an argon atmosphere containing 5 % hydrogen. The density then reaches 10.47 or 95.5 % of the theoretical density. The height of the sintered pellet is 9.62 mm, and its diameter 9.83 mm. The shape of the pellet is then inspected with an apparatus having an accuracy of 2 microns. The difference in diameter found on the pellet is 12 microns. EXAMPLE 2 Uranium oxide powder is compressed in a rotative press with steel equipment, lubrified by means of a preliminary lubrification cycle with polystyrene balls containing 2 % zinc behenate. The characteristics of the powder are the following: The diameter of the mould is 15 mm and the conicity of the walls is defined by an angle .alpha. = 20'43", corresponding to a radius increase of 60 microns per cm height. The pellet, compressed to a density of 5.4, is sintered for 4 hours at 1650.degree.C in an argon atmosphere containing 5 % hydrogen. The obtained pellet then has a density of 10.36 or 94.5 % of the theoretical density. The diameter divergences of this pellet lie below 15 microns. After the same manufacturing cycle with classical double compression (as shown in FIG. 1) the obtained pellets have a "diabolo" form with diameter divergences of 70 microns. These examples clearly show the efficiency of the method according to the invention. The method according to the invention allows manufacture of pellets with diameter divergences lying within the prescribed diametrical tolerances. Thus, rectification of the pellets may be avoided and the fabrication cycle notably shortened. Avoiding rectification is particularly advantageous because no rectification scraps are formed and the recovery of said scraps is thus superfluous. Of course, this absence of rectification wastes is still more advantageous when the fuel pellets contain fissile material which should be very carefully handled. It is evident that the examples described above are not at all limitative and that the man skilled in the art will find numerous modifications or improvements without leaving the field of the invention. Compaction chambers with slightly inclined walls have already been proposed for facilitating the ejection of tablets. The very slight inclination proposed to this end could be defined by an angle of hardly 0.degree.3' and does not allow the advantages of the compression according to the invention to be obtained. This conicity, moreover, reduced the regularity form of the tablets, since the presses used had either two mobile punches or an upper mobile punch; the use of a lower mobile punch was as yet exceptional.