JP2003107181A - Manufacturing method of nuclear fuel pellet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the inventory of U3 O8 and the cost necessary in a wet recovery process by increasing the addition ratio when adding U3 O8 powder to raw material powder in a manufacturing process of UO2 pellets. SOLUTION: The pellets are formed by adding and mixing a U3 O8 powder obtained by heating and oxidizing the UO2 pellet and UO2 powder in air, to a UO2 powder which can be sintered, and carrying out preliminary pressing, disintegration, pressing, sintering and grinding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing nuclear fuel pellets used for nuclear reactor fuel.

[0002]

_2. Description of the Related Art UO ₂ pellets used for nuclear reactor fuel are usually used as a raw material U
O ₂ powder was pressure-molded into a cylindrical molded body, which was mixed with hydrogen gas or hydrogen mixed with nitrogen by adding 0 to 2% by volume of water vapor in a furnace to obtain a mixed gas atmosphere. In a furnace, after sintering at a temperature of 1650 ° C. to 1800 ° C. for about 2 hours to 6 hours, in order to obtain a cylindrical body having a predetermined diameter,
It is manufactured by grinding a cylindrical surface.

Pellets that have cracks and chips due to impact during handling during the manufacturing process of the pellets should be added to the raw material powder as a minute powder of U ₃ O ₈ obtained by a thermal oxidation treatment in air. Although it is reused due to, etc., if the addition ratio is large, abnormal voids occur in the metallurgical microstructure of the pellet after pressure molding and sintering,
In some cases, the specifications of resintering stability (burning stability), which is one of the characteristics required for pellets, cannot be satisfied. In the conventional method of adding U ₃ O ₈ powder to the raw material powder and press-molding and sintering, the upper limit of the addition ratio is about 5 to 7% for the above reason, and therefore it is reused, for example. Since there is a large amount of U ₃ O ₈ powder, it is not possible to add more than that. As a result, the stock amount of U ₃ O ₈ powder increases and wet recovery treatment with higher cost is required. ..

In view of such a point, the present invention provides the above UO.
_TwoU in the pellet manufacturing process _ThreeO₈Raw powder
By increasing the addition ratio when adding at the end,
U_ThreeO₈The amount of powder in stock and the cost required for wet recovery processing
The goal is to reduce

[0005]

The invention according to claim 1 is
The sinterable UO ₂ powder, UO ₂ pellets or UO ₂ powder using a powder added to and mixed with U ₃ O ₈ powder which is obtained by thermal oxidation treatment in air, the powder pressing, baked In a method for producing a nuclear fuel pellet by binding and grinding to UO ₂ pellets, U ₃ O ₈ powder is used as a raw material powder UO ₂
It is characterized in that it is added to the powder, mixed, and then preformed and crushed before being pressure-formed, sintered and ground.

According to a second aspect of the invention, in the invention according to the first aspect, the preforming pressure performed after the U ₃ O ₈ powder is added and mixed is 0.
It is characterized by being 8 times or less.

The invention according to claim 3 is the invention according to claim 1 or 2.
U added in the invention according to_ThreeO ₈The powder addition rate is
It is not more than 20% by weight based on the weight of powder after addition.
To collect.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, U ₃ O ₈ powder obtained by heating and oxidizing a sintered pellet in air is added to UO ₂ powder which is a raw material, pre-molding and crushing are performed, After that, UO ₂ pellets are manufactured by performing pressure molding, sintering, and grinding in the same manner as conventional ones.

That is, unlike the conventional method, U ₃ O ₈ powder is added to UO ₂ powder which is a raw material, and thereafter, UO ₂ pellets are not manufactured by directly press-molding, sintering and grinding. Even if the weight ratio of the U ₃ O ₈ powder to the raw material powder is increased to 20%, abnormal voids can be eliminated.

FIG. 1 is a diagram showing a specific example of the case where UO ₂ powder is used as a raw material powder and U ₃ O ₈ powder is added to this to produce pellets, and three types of production methods are shown. .

That is, the manufacturing method 1 is a method in which the raw material powder is pre-molded and crushed, and then pressure molding, sintering and grinding are performed without adding U ₃ O ₈ powder, and the manufacturing method 2 is After the raw material powder is preformed and crushed, U ₃ O ₈ powder is added, followed by pressure molding, sintering, and grinding. These two methods are conventional pellet manufacturing methods.

On the other hand, the manufacturing method of the present invention is manufacturing method 3, in which U ₃ O ₈ powder is added to the raw material powder, followed by preforming and crushing, followed by pressure forming, sintering and grinding. Is.

Therefore, pellets were manufactured by the above-mentioned three kinds of manufacturing methods, and the microstructure was observed. In that case, the method of adding the U ₃ O ₈ powder in the second and third manufacturing methods was the same, and the measured U ₃ O ₈ powder was added to the raw material powder and then thoroughly mixed in the V-type mixer. Other manufacturing conditions are shown below, but the same method was used in all manufacturing methods.

First, the uniaxial pressure type press is used for preforming and crushing.
After forming a columnar shape using, a mesh with an opening of 1.5 mm
Crushed on. Pellet pressure molding is preforming and crushing
Cylindrical shape by pressure molding using a uniaxial press similar to
It was a molded body. Sintering is a mixed gas of 75% hydrogen + 25% nitrogen.
In a sintering furnace with a heating atmosphere at a heating rate of 600 ° C / hour
Raise the temperature to 1750 ° C, hold for 3 hours, and then lower to room temperature
Then, the cylindrical surface was ground by a coreless grinding machine. Also, the same
As shown in FIG. 1, U in the manufacturing method 3_ThreeO ₈Powder
Addition rate of a (%), preforming in all manufacturing methods
Pressure is b (ton / cm^Two), U in manufacturing method 2_ThreeO₈
The powder addition rate is c (%), and in all manufacturing methods
Pressing pressure is d (ton / cm^Two) And these a, b,
C and d are parameters shown in FIG.
18 pellets of 18 kinds were produced.

FIG. 2 shows the results of observation of the metallic microstructure of the pellets produced by each method.

First, in the pellet numbers 1 to 3 produced by the conventional production method 1, pellet numbers 1 and 2 in which the preforming pressure b is 0.24 times and 0.76 times the press forming pressure d, respectively. In the case of No. 2, the fine structure was good, but 0.
Unacceptable voids were observed on the microstructure in pellet No. 3, which was 91 times larger. A photograph of the microstructure of pellet No. 1 is shown in FIG.

Next, among the pellet numbers 4 to 9 produced by the conventional production method 2, pellets 4 to 6 in which the addition rate c of the U ₃ O ₈ powder is 4% are also pellets. Similar to Nos. 1 to 3, the pellets Nos. 4 and 5 in which the preforming pressure b was 0.24 times and 0.76 times the press forming pressure d had a good microstructure,
Unacceptable voids were observed on the microstructure in pellet No. 6 which was 91 times larger. A photograph of the fine structure of pellet No. 5 is shown in FIG. In addition, the addition rate c of U ₃ O ₈ powder is 8%
In all of the pellet numbers 7 to 9, unacceptable voids were observed on the microstructure. A photograph of the fine structure of pellet No. 7 is shown in FIG.

Then, in the pellet numbers 10 to 18 produced by the production method 3 which is the method of the present invention, the pellet numbers 10 to 12 and a having the addition ratio a of the U ₃ O ₈ powder of 4% are 8
%, The preforming pressure b is 0.24 times the pressure forming pressure d and 0.76 times in any of the pellet numbers 16 to 18 in which a is 20%.
The doubled pellet had a good microstructure. However, in pellet numbers 12, 15, and 18 in which the preforming pressure b was 0.91 times the pressure forming pressure d, unacceptable voids were observed on the microstructure. A photograph of the fine structure of pellet No. 16 is shown in FIG.

From the above results, in the conventional manufacturing method, U ₃
When the addition rate of the O ₈ powder is set to 8% or more, unacceptable voids appear on the metallurgical microstructure of the pellet, whereas in the production method of the present invention, the preforming pressure is set to the pressure during pressure forming. By making it smaller than 0.8 times, U
_It can be seen that even if the addition rate of ₃ O ₈ powder is increased to 20%, pellets having a fine microstructure can be obtained. That is, the method of the present invention shows that even if the addition rate of the U ₃ O ₈ powder is made larger than the permissible limit of the conventional method, the metallurgical microstructure of the pellet is equivalent to the conventional one.

As described above, when U ₃ O ₈ powder was added to the raw material powder in the production of UO ₂ pellets, the addition rate was up to 7% in the conventional production method. You can add at least 20 percent,
There is an advantage that the stock amount of U ₃ O ₈ powder for reuse can be reduced, and in some cases, the costly wet recovery treatment amount can be reduced.

[0021]

As described above, according to the present invention, in the case of reusing pellets which are cracked due to impact in the pellet manufacturing process, U ₃ O ₈ powder is used as raw material powder UO ₂ powder. After adding and mixing, preforming and crushing were performed, and then pressure forming, sintering, and grinding were performed.
₃ O ₈ it is possible to increase the amount of powder, amount of stock U ₃ O ₈ powder for recycling can be reduced, the effect of such can reduce the high wet recovery throughput cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a flow of a pellet manufacturing method.

FIG. 2 is a diagram showing observation results of pellet manufacturing parameters and a microstructure.

FIG. 3 is a micrograph of a pellet metal phase.

FIG. 4 is a micrograph of a pellet metal phase.

FIG. 5 is a micrograph of a pellet metal phase.

FIG. 6 is a micrograph of a pellet metal phase.

Claims (3)

[Claims] 1. A uranium dioxide (UO ₂ ) powder that is sinterable, to which uranium trioxide (U ₃ O ₈ ) powder obtained by heating and oxidizing UO ₂ pellets or UO ₂ powder in air is added. Using mixed powder, press molding, sintering,
In a method for producing a nuclear fuel pellet that is ground into UO ₂ pellets, U ₃ O ₈ powder is added to UO ₂ powder that is a raw material powder, and after mixing, preforming and crushing are performed, followed by pressure forming and sintering. A method for producing a nuclear fuel pellet, which comprises grinding. 2. The pre-forming pressure after adding and mixing U ₃ O ₈ powder is 0.
It is 8 times or less, The manufacturing method of the nuclear fuel pellet of Claim 1 characterized by the above-mentioned. 3. The method for producing nuclear fuel pellets according to claim 1, wherein the addition rate of the U ₃ O ₈ powder to be added is 20% by weight or less based on the weight of the powder after the addition.