Compositions – Radioactive compositions – Nuclear reactor fuel
Reexamination Certificate
1999-07-19
2001-06-26
Bos, Steven (Department: 1754)
Compositions
Radioactive compositions
Nuclear reactor fuel
C264S000500
Reexamination Certificate
active
06251309
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing uranium dioxide fuel pellets. Particularly, this invention relates to a method of making U
3
O
8
single crystals and manufacturing large-grained uranium dioxide (UO
2
) fuel pellets through the use of a mixture comprising UO
2
powder and U
3
O
8
single crystals.
Uranium dioxide (UO
2
) fuel pellets have been manufactured using UO
2
powder according to the following processes; homogenizing UO
2
powder or mixing UO
2
powder with other additives, pressing UO
2
powder into green pellets, and sintering the green pellets at about 1700° C. in a reducing gas atmosphere to produce UO
2
pellets. The UO
2
pellet has a cylindrical shape of about 8 mm diameter and 10 mm length, and it has a density of about 95% of theoretical density (TD) and a grain size of about 8 &mgr;m. UO
2
pellets are loaded into a zirconium-based tube, which is then seal-welded to fabricate a fuel rod.
The defective UO
2
pellets, which do not meet pellet specifications, are usually made in a small quantity during the process of pellet manufacture. Since defective UO
2
pellets contain expensive enriched uranium, they are commonly recycled in the manufacture of new UO
2
pellets according to the following method. Firstly, U
3
O
8
powder is made by heating defective UO
2
pellets at around 450° C. in air so as to oxidize UO
2
to U
3
O
8
, and the U
3
O
8
powder is then mixed with UO
2
powder. Secondly, the mixture of UO
2
and U
3
O
8
powder is pressed and sintered to produce UO
2
pellets in the same way as the single UO
2
powder. The U
3
O
8
powder is much less sinterable (capable of getting a high pellet density) than the UO
2
powder, so that its content is generally limited within 15% by weight of the mixture of UO
2
and U
3
O
8
powder. The art to improve the sinterability of U
3
O
8
powder has been disclosed in the literatures of U.S. Pat. No. 3,140,151 and U.S. Pat. No. 3,578,419.
While a fuel rod is irradiated (burned) in a nuclear reactor, the fission gas such as xenon and krypton is generated in a fuel pellet and is released to the outside of the fuel pellet. The pressure in a fuel rod builds up increasingly with burnup. The fission gas released should be maintained as low as possible, for an excessive pressure gives rise to the failure of a fuel rod. In high burnup, the fission gas released may restrict the performance of a fuel rod.
It has been known that a fuel pellet having a large grain provides a good performance since the amount of the fission gas released during irradiation decreases with increasing the grain size of a fuel pellet. It is a common sense that increasing sintering temperature and time makes a large-grained fuel pellet, but such a sintering method is not economical. Therefore, the art has been disclosed which provides a method of manufacturing large-grained UO
2
pellets with the aid of sintering additives.
According to the method of U. S. Pat. No. 4,869,867, UO
2
fuel pellets having an average grain size of at least 20 &mgr;m are produced by adding aluminosilicate to UO
2
powder, pressing and sintering. A shortcoming of the prior art is that the sintering additive may have an adverse effect on the other properties of the UO
2
fuel pellet. For example, the sintering additive may degrade thermal properties such as thermal conductivity and melting point.
On the other hand, the art has been known which provides a method of manufacturing a large-grained UO
2
pellet with no sintering additive added. U.S. Pat. No. 4,578,229 disclosed a method of sintering UO
2
green pellets at a temperature range of 1000° C. to 1400° C. in an oxidizing gas such as carbon dioxide and reducing the sintered pellet at that temperature range in a reducing gas. The prior art has a problem in that a special sintering furnace is needed in which an oxidizing gas and a reducing gas can be used separately.
SUMMARY OF THE INVENTION
The above disadvantages of the prior arts are overcome by the present invention. A principal object of this invention is to provide a method of manufacturing large-grained UO
2
fuel pellets in a reducing gas atmosphere with the aid of U
3
O
8
single crystals.
With the foregoing object and other objects in view, there is provided in accordance with the invention a method of manufacturing large-grained UO
2
pellets, comprising annealing U
3
O
8
powder at temperatures of 1000° C. to 1500° C. in order to produce U
3
O
8
polycrystalline aggregates having a large crystal size; dividing the U
3
O
8
polycrystalline aggregate into its constituent U
3
O
8
single crystals; forming a mixture of UO
2
powder and the U
3
O
8
single crystals; making granules of the mixture; pressing the granules into green pellets; and sintering the green pellets at temperatures above 1600° C. in a reducing gas atmosphere to produce large-grained UO
2
pellets.
The method according to the invention is characterized in that the U
3
O
8
powder is produced preferably by heating defective UO
2
pellets at temperatures of 300° C. to 800° C. in an oxidizing gas to oxidize UO
2
to U
3
O
8
.
The method according to the invention is characterized in that the mixture includes 1% to 15% by weight of the U
3
O
8
single crystals.
An advantage of the present invention is that a large-grained UO
2
pellet is produced in a reducing gas atmosphere without adding any sintering additives. Another advantage is that defective UO
2
pellets can be reused in producing large-grained UO
2
pellets, so fuel fabrication cost is saved.
REFERENCES:
patent: 4985183 (1991-01-01), Yato et al.
patent: 5211905 (1993-05-01), Wood
patent: 5514306 (1996-05-01), Larson et al.
patent: 5597538 (1997-01-01), Taylor et al.
Jung Youn-Ho
Kang Ki-Won
Kim Jong-Hun
Kim Keon-Sik
Song Kun-Woo
Arent Fox Kintner & Plotkin & Kahn, PLLC
Bos Steven
Korea Atomic Energy Research Institute
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