Patent Number: 059164972
Section: summary

FIELD OF THE INVENTION The present invention relates to the manufacture of ceramic articles and, particularly but not exclusively, to the manufacture of ceramic pellets such as nuclear fuel pellets. BACKGROUND OF THE INVENTION In general, when a particulate material is compacted in a die, particle-particle friction and die-wall-particle friction result in variations in the applied pressure in the particulate body and, in consequence, the compacted body has a non-uniform pressed density. The non-uniform pressed density of the compact gives rise to a differential shrinkage during subsequent sintering, resulting in distortion of the ceramic article. Even when sintering is not accompanied by shrinkage, the non-uniformity of density remains in the sintered component and is a source of weakness. Nuclear fuel pellets are ceramic substantially cylindrical solid or hollow bodies which are composed mainly or wholly of an oxide of uranium, especially UO.sub.2. Collections of such pellets are used together in a sheath providing a fuel rod or pin. Assemblies of such rods or pins are employed as the active elements in a nuclear reactor. In a conventional process for the manufacture of nuclear fuel pellets, a uranic oxide powder, eg manufactured in the manner described in EP 0277708, is compacted or compressed in a die or mould and then sintered usually over a period of several hours at least, usually in a reducing atmosphere at elevated temperatures, eg 1500.degree. C. to 1800.degree. C. It is desirable for the pellets to be produced with a body shape which is a right circular cylinder. Product specifications normally require such a shape. However, pellets produced in the conventional manner described above may not, after sintering, be obtained with a right circular cylindrical shape even though the cavity of the die or mould used to form the pellet shape is itself a perfect right circular cylinder. The pellets are often produced with a shape which has a body waist in a wheatsheaf-like profile, ie the cross-sectional area (perpendicular to the pellet axis) taken at different points along the length of the pellet is less near the middle of the pellet than near the ends of the pellet. Grinding of the pellets is normally required after sintering to meet product specifications. Such grinding is both time consuming and costly. SUMMARY OF THE INVENTION The purpose of the present invention is to reduce or eliminate the need for such grinding following sintering. According to the present invention there is provided a method of manufacturing a ceramic article including the steps of forming a body of particulate material, compressing the body at its ends and sintering the body and characterised in that the body is formed, prior to compression, of particulate material layers of different bulk particle density. The ceramic article may comprise a ceramic pellet having a substantially cylindrical shape. The ceramic pellet may comprise a nuclear fuel pellet and the particulate material may comprise nuclear fuel material. We have found by experimental analysis that the wheatsheaf shaped profile obtained in the prior art is caused by sintering a green pellet having a density which is less in its interior region than at its ends. This differential density profile is caused in the following way. In the usual method of pressing a green pellet from a powder, the powder is introduced into a right circular cylindrical die cavity and the powder is compressed at its ends. Owing to friction effects within the powder body, there is a variation in load experienced by different regions within the body and a slight barrelling of the pellet body occurs near the middle of the pellet. The barrelled region has a lower density than the end regions of the body because the same particles occupy a greater volume. By employing regions of particulate material having different densities prior to compaction in accordance with the present invention the reduction in density caused upon compaction by the aforementioned barrelling effect is compensated for and a compact having a more uniform density distribution is obtained. This in turn provides, upon sintering, a sintered pellet having a more uniform cross-sectional body shape. In the method according to the present invention the body of particulate material may be formed by introducing powder or particles of the nuclear fuel material into a mould or die cavity having a substantially right circular cylindrical shape. The said body may prior to compaction comprise discrete layers having different densities. Such layers may have interfaces which are substantially planar, eg in a plane orthogonal to the axis of the body. Alternatively, the layer interfaces may be non-planar eg convex or concave to provide a suitable density profile. Alternatively, the density change in the said body prior to compaction may take place gradually, eg linearly with distance, over a portion of the length of the body. The said body prior to compaction desirably includes regions at the ends of the body which have a density less than that in the interior region ie near the middle of the body. The different densities in the said body may be achieved by the introduction of particulate material into a die or mould cavity from a plurality of sources containing different particulate materials providing different bulk densities, the release of material from the two sources being controlled so as to give the required density profile in the body. The different particulate materials may comprise for example materials of the same composition but which have been treated differently or which have different particle morphology. The different particle types may comprise, for example, (a) particles produced from a single powder but pre-compacted using different pressures to form different density granules, or (b) particles of the same powder which have on the one hand been milled and on the other hand have not been milled or (c) particles which on the one hand have a plate-like morphology and on the other hand, have a sphere-like morphology or (d) combinations of these different types. Where granulation is employed to produce different density granules various known granulation methods may be used for the production of granules one or more of the different density types. For example, pre-compaction in a die followed by breaking through a sieve may be employed. Alternatively, roll compaction may be employed. The body of particulate material, eg produced in one of the ways described, may comprise uranium dioxide which may contain optional additives, eg niobia or gadolinia, or plutonium dioxide (eg up to 6 percent by weight) to provide a mixed oxide (MOX) fuel pellet. The particles may also be coated with a small quantity, eg less than 1 percent by weight, of a solid lubricant such as zinc stearate which has been employed in a pre-treatment process or is used in the compaction step, to treat the powders from which the body is formed (directly or indirectly as pre-compacted particles). The pre-compaction and sintering steps in the method according to the present invention may be carried out in a manner similar to that employed in the prior art although the pressure in the compaction step may be increased with time gradually to a maximum, eg 6-7 tonnes per cm.sup.2. The maximum compaction pressure may be from 1 to 10 Te/cm.sup.2. The sintering step may be carried out as in the prior art in an inert gas atmosphere or in a reducing atmosphere, eg hydrogen or a mixture of hydrogen and an inert gas containing up to 99% percent by weight of hydrogen. The sintering step may be carried out for at least one preferably several hours, eg &gt;5 hours, at a temperature in the range 1500.degree. C. to 1800.degree. C. A pressure of greater than one atmosphere is desirably applied during the sintering step. The present invention beneficially and unexpectedly allows ceramic, eg nuclear fuel, pellets to be produced which in profile have sides which are more straight than those obtained immediately after sintering in the prior art. The need for costly post-sinter grinding is thereby reduced or eliminated. Although the present invention is primarily intended for the production of ceramic, eg nuclear fuel, pellets which in axial cross-section are straight sided pellets having grooves in their curved surfaces or barrel-shaped pellets required for particular applications may also be produced using the invention. The pellets produced by the present invention may be substantially all solid. Alternatively the pellets may include an axial hole therethrough. The ends of the pellet may be flat or curved, eg convex. The exact shape of the pellet will be as usually specified for the particular type of nuclear reactor (eg AGR or LWR) in which such pellets are to be used in assemblies of fuel rods or pins.