Patent Number: 039490270
Section: summary

This invention relates to nuclear fuel and more particularly to ceramic fuel tablets or pellets used in nuclear reactors. These ceramic pellets generally consist of uranium, plutonium, thorium, etc.. in oxide, nitride, carbide or other form. The manufacturing process for these pellets comprises the compression of appropriate powder in a cylindrical mould, followed by sintering to confer to the pellets the necessary characteristics, such as mechanical resistance and density. The sintering cycle however gives rise to shrinkage of the raw material, which may attain about 50 % of the green volume. Thus the sintering of the pellet influences its final form; more particularly, the pellet will not shrink uniformly on sintering and will shrink more in the less dense areas if the density of the raw pellet is not homogeneous after compression. The peculiar friction conditions on compression do not allow a homogeneously dense pellet to be obtained. Therefore the pellets are at present rectified after sintering, in order to correct their form, so that the final product obtained is properly cylindrical. The diametrical tolerances set by the users of nuclear fuel pellets are generally very strict, so that rectification, mostly effected by grinding, can hardly be avoided. It has been noted that density differences in the raw pellets on compression are a function of the relative movement of the punches in the compaction chamber. Indeed, when using a double-acting press, i.e. compressing by simultaneous shifting of the upper and lower punches, the lateral surface of the obtained cylindrical pellet presents, after sintering, an inwardly incurved configuration or "diabolo" configuration. If, on the other hand, one uses a single-acting pelletizing machine, i.e. with only one punch moving on compression, the obtained pellet will, after sintering, present a configuration resembling a cone trunk, the broad base of which corresponds to the part of the pellet in contact with the mobile punch. In order to make these pellets comply with the specifications, their "diabolo" or conical form must be rectified by grinding their lateral side so as to obtain a cylindrical surface. The invention proposes an improved compression phase, allowing avoidance of rectification of the pellets after sintering. According to the invention, the pellet is compressed solely by shifting the lower punch of a pelletizing machine in a compaction chamber of which at least the upper part of the walls is inclined in the direction of a diameter increase towards the die-bearing table, on a height corresponding at least to the height of the compressed pellet. The inclination angle, i.e. the angle formed by the vertical axis of the die and the intersection line between an axial plan and the inclined wall, will generally lie between 1.degree.30' and 0.degree.6' as a function of the compaction conditions, the diameter of the pellets and the nature of the powder. The walls are preferably inclined on their whole height, but it is evident that, if the pellet to be compressed is rather high and the compression ratio rather elevated, the walls may have a lower cylindrical part and an upper inclined part. Generally it is enough to have walls inclined on a height corresponding to the tablet to be compressed to obtain the advantages according to the invention. The inclined part should then be situated on the upper part of the wall. A pellet, compressed according to the invention, will thus have the configuration of a cone trunk, the large base of which will be the less dense part. After sintering, this pellet will generally have a substantially cylindrical form and rectification will be superfluous.