Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten...
Patent
1997-06-24
1999-06-29
Fiorilla, Christopher A.
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
264628, 264642, G21C 2100
Patent
active
059164972
DESCRIPTION:
BRIEF 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 a
REFERENCES:
patent: 3823067 (1974-07-01), Stern et al.
patent: 4199404 (1980-04-01), Mordarski et al.
patent: 4382048 (1983-05-01), Harlow
British Nuclear Fuels PLC
Fiorilla Christopher A.
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