Plastic and nonmetallic article shaping or treating: processes – Shaping or treating radioactive material
Reexamination Certificate
1998-11-30
2001-06-26
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Shaping or treating radioactive material
C264S037100
Reexamination Certificate
active
06251310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of manufacturing UO
2
-based fuel pellets by recycling irradiated UO
2
-based fuel pellets. More particularly this invention relates to a method of making U
3
O
8
-based powder from irradiated UO
2
-based fuel pellets, mixing the U
3
O
8
-based powder with an additive, pressing and sintering the product to produce the UO
2
-based fuel pellets.
2. Definitions of Terminology
A fresh fuel pellet consists of uranium dioxide (UO
2
) or a mixed oxide of uranium dioxide (UO
2
) and plutonium dioxide (PuO
2
), which mixed oxide is hereinafter referred to as “(U, Pu)O
2
”. In this application, the term “UO
2
-based” refers to both UO
2
- or (U, Pu)O
2
-containing products, and the term “U
3
O
8
-based” refers to both U
3
O
8
- or (U, Pu)
3
O
8
-containing products.
3. Description of Related Art
As a UO
2
-based fuel pellet is irradiated in a nuclear reactor, the fissile material in the fuel pellet is depleted and fission products are produced. An irradiated UO
2
-based fuel pellet therefore comprises fissile materials and fission products, of which concentrations are mainly dependent on design burnup specifications and the initial amount of fissile materials. An irradiated fuel pellet discharged from a light water reactor normally has fissile materials of higher than 1% by weight of the irradiated fuel pellet, so it is worthwhile to recycled the fuel pellet for reuse.
According to the literature (G. E. Brand and E. W. Murbach, NAA-SR-11389 (1965)), a UO
2
-based fuel pellet which has been irradiated in a light water reactor can be treated in a so-called AIROX cycle, to refabricate the UO
2
-based fuel pellet. The AIROX cycle comprises the steps of oxidizing irradiated UO
2
-based fuel pellets to U
3
O
8
-based powder, making sinterable UO
2
-based powder from the U
3
O
8
-based powder, mixing the sinterable UO
2
-based powder with enriched fresh UO
2
powder, making granules of the mixed powder, pressing the granules into green pellets, and sintering the green pellets to fabricate UO
2
-based fuel pellets for reuse in a light water reactor.
The art discloses several methods of making sinterable UO
2
-based powder from irradiated UO
2
-based fuel pellets for use in the AIROX cycle. U.S. Pat. No. 3,140,151, discloses a method of making a sinterable UO
2
-based powder comprising oxidizing the irradiated UO
2
-based fuel pellet to U
3
O
8
-based powder in air at a temperature in the range of 300° C. to 500° C., reducing the U
3
O
8
-based powder to UO
2
-based powder at a temperature in the range of 500° C. to 800° C., and repeating the oxidation and reduction steps 3 to 5 times to produce the sinterable UO
2
-based powder. This UO
2
-based powder was able to be sintered to produce UO
2
-based fuel pellets.
A green pellet consisting of U
3
O
8
-based powder which is produced from irradiated UO
2
-based pellets through one round of oxidation can be sintered only up to about 80% theoretical density (TD), and a fuel pellet having such a low density cannot be used in a nuclear reactor, since normal fuel design specifications require the pellet density to be at least about 94% TD. Therefore, oxidation and reduction of the U
3
O
8
-based powder needed to be conducted for many more rounds to enhance its sinterability. In addition, during the oxidation of UO
2
-based pellets or powder to U
3
O
8
-based powder, the irradiated UO
2
-based pellets or powder must be readily pulverized or comminuted to finer powder, since the phase transition of cubic UO
2
to orthorhombic U
3
O
8
causes a volume expansion of about 30%, and thus large stress is generated.
A disadvantage of the prior art is that the oxidation and reduction of UO
2
-based powder needs much time and is hard to control. For example, the oxidation rate of UO
2
-based powder is vety fast, and thus the heat produced from the resultant reaction can increase the temperature of the powder to high temperatures. The UO
2
-based powder so produced is poorly sinterable.
SUMMARY OF THE INVENTION
An object of this invention is to provide a method of manufacturing UO
2
-based fuel pellets by recycling irradiated UO
2
-based fuel pellets.
With the foregoing object and other objects in view, there is provided in accordance with this invention a method of preparing UO
2
-based fuel pellets, comprising oxidizing irradiated UO
2
-based pellets to make U
3
O
8
-based powder; mixing the U
3
O
8
-based powder with an additive comprising at least one oxide of an element selected from the group consisting of Nb, Ti, V, Al, Mg, Cr, Si and Li; making granules of the mixture; pressing the granules into green pellets; and sintering the green pellets in a reducing gas atmosphere to produce new UO
2
-based fuel pellets.
The method according to the invention is preferably conducted such that the additive is in the range of about 0.02% to about 5% by weight of the mixture. It is also preferable that the green pellets are sintered at a temperature above 1500° C.
In another method according to the invention, fresh UO
2
-based powder is added to the U
3
O
8
-based powder to bring the amount of fissile materials within design specifications.
An advantage of the present invention is that the U
3
O
8
-based powder produced through a single oxidation treatment of irradiated UO
2
-based pellets may be pressed without any further oxidation treatments being necessary, and sintered to make new UO
2
-based fuel pellets. Accordingly, the 3-5 oxidation and reduction cycles which were necessarily required in the prior art are not needed in the present invention. Therefore, fuel manufacturing steps and related costs are much reduced by the present invention.
REFERENCES:
patent: 3641227 (1972-02-01), Horsley et al.
patent: 4331618 (1982-05-01), Hoyt
patent: 5882552 (1999-03-01), Song et al.
Jung Youn-Ho
Kim Keon-Sik
Song Kun-Woo
Yang Myung-Seung
Arent Fox Kintner & Plotkin & Kahn, PLLC
Fiorilla Christopher A.
Korea Atomic Energy Research Institute
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