JP2981580B2 - Manufacturing method of nuclear fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention relates to a method of manufacturing a gadolinium oxide nuclear fuel assembly comprising (Gd) (nuclear fuel pellets) as burnable poison, details, and free UO ₂ affect the combustion characteristics free The present invention relates to a method for producing an oxide-based nuclear fuel body made of a uniform solid solution having extremely little gadolinia (Gd ₂ O ₃ ).

[0002]

The when first operated BACKGROUND ART plant, fuel loaded in the core, all using the new fuel, but a large excess reactivity of the core ^{for 2 3 5} U is not at all the combustion in the case, Since the control rods cannot be used alone, special absorbers must be prepared in advance. To do so, a plate made of stainless steel containing boron is placed between the assemblies only during the initial operation, or a nuclear fuel rod in which gadolinia (Gd ₂ O ₃ ) is mixed with uranium dioxide is attached to each assembly. Four to five wires are being assembled. Gadolinium order to reduce faster than ^{2 3 5} U as the absorption cross section of neutron proceeds combustion so large, no effect of gadolinium in an advanced stage to some extent fuel, can adversely affect against burnup It is convenient.

As a method of producing an oxide nuclear fuel body containing gadolinium, usually, about 2 to 6% of a Gd ₂ O ₃ powder is mixed with an oxide powder containing UO ₂ as a main component and molded. A mixed powder is obtained directly from the solution by sintering or by coprecipitation to obtain a mixed powder by a method such as roasting or reducing a solution of uranyl nitrate containing Gd ₂ O ₃ , etc. There is a method of molding and sintering.

[0004]

According to the latter, Gd ₂
Because a raw material powder in which O ₃ is very uniformly dispersed is obtained,
The shaped and sintered nuclear fuel body becomes a uniform solid solution. However, on the other hand, this method adjusts the Gd ₂ O ₃ content,
There is a drawback that it is difficult to change and leads to an increase in product cost.

On the other hand, in the former method, the Gd ₂ O ₃ content can be adjusted only by mixing a predetermined amount of powders with each other, so that it can be said that this method is suitable for production on an industrial scale. But,
In this method, free UO ₂ and free Gd ₂ are used unless mixing is performed for a long time by a method having a high mixing effect such as a ball mill.
It is difficult to obtain a uniform solid solution with almost no O ₃ .

[0006] The present invention addresses the above circumstances, and in particular finds a novel method in powder mixing, which allows free UO ₂ and free Gd ₂ O ₃ to be obtained even under poor mixing conditions.
It is an object of the present invention to obtain a nuclear fuel body composed of a uniform solid solution having a very low density.

[0007]

That is, the feature of the method for producing a nuclear fuel body of the present invention that achieves the above object is that, when producing an oxide-based nuclear fuel body containing uranium dioxide as a main component and containing a required amount of gadolinium, Using an oxide mainly composed of uranium dioxide as a powder having a particle size of 60 μm or less,
The object is to mix gadolinia powder with the oxide powder, and then form and sinter the mixed powder.

[0008]

According to the production method of the present invention, since the particle size of the uranium dioxide oxide powder is regulated to 60 μm or less, gadolinia powder can be evenly mixed into the oxide powder by simple mixing. It becomes possible, and the nuclear fuel body molded and sintered using the mixed powder obtained as described above becomes a uniform solid solution with very little free UO ₂ and free Gd ₂ O ₃ .

[0009]

EXAMPLES Specific examples of the present invention will be described below together with details of the present invention. First, the present invention is characterized in that the diameter of agglomerated particles of an oxide powder containing UO ₂ as a main component as a main component is regulated. That is, the diameter of the oxide powder before adding Gd ₂ O ₃ is adjusted to 60 by a mesh (sieve) or the like.
μm or less, and then, if necessary, a part of this powder and a Gd ₂ O ₃ powder are master-blended.

Next, the master-blended powder or the Gd ₂ O _{3 is} mixed so as to have a predetermined Gd ₂ O ₃ content.
The O ₃ powder alone is mixed with the oxide powder having a diameter of not more than 60 μm. In this case, it is not necessary to use a device having a high mixing effect such as a ball mill, and a mesh,
Mixing with a V blender or the like is sufficient.

The nuclear fuel body molded and sintered using the mixed powder thus obtained becomes a very uniform solid solution. In addition, when mixing a base material and an additive, in order to perform good mixing, it is often the case that the additive is first mixed with a part of the base material, and then this mixture is mixed with the remaining base material. However, the above-mentioned master blend refers to a mixture of a part of the base material and the additive. A V-blender is a mixing device mainly composed of a V-shaped container, and cannot perform high mixing like a ball mill, but can easily perform mixing.

Next, an example of producing a UO ₂ nuclear fuel body having a Gd ₂ O ₃ addition concentration of 6 wt% by the production method of the present invention will be described below. The UO ₂ powder was reduced to particles of 60 μm or less by a mesh, and this powder and a Gd ₂ O ₃ powder having a particle diameter of 2 μm were master-blended so that the weight ratio of the UO ₂ powder to the Gd ₂ O ₃ powder was 5: 2. . At this time, mixing was performed only by passing through a coarse mesh (opening of about 850 μm) five times.

Next, this master-blended powder is mixed with another UO ₂ having a diameter of 60 μm or less by a mesh.
The powder was mixed at a weight ratio of 21:79 by the same mixing method as in the master blend, and after dry granulation by press molding, press molding was performed and sintering was performed. The sintering temperature is 1750 ° C., the sintering time is 4 hours, and the sintering atmosphere is a mixed gas of hydrogen and nitrogen to which steam has been added. The above manufacturing flowchart is shown in FIG.

Observation of the structure of the nuclear fuel assembly obtained as described above revealed that a very uniform solid solution was obtained even though only mesh mixing was performed. For comparison, the particle size was (1) unregulated,
(2) 100 μm or less, (3) 75 μm or less, (4) 6
Nuclear fuel bodies were manufactured by the above method using UO ₂ powder restricted to 0 μm or less, and (1), (2), (3)
180μm, 30μm, 10μm respectively in case of
Free UO ₂ of observed became homogeneous solid solution with the following 60μm in (4).

FIG. 2A shows an example of a cross-sectional structure observation of a nuclear fuel body manufactured by the method of the present invention, and FIG.
Each of the cross-sectional structure observation examples of a nuclear fuel body manufactured by a conventional method that does not regulate the O ₂ powder particle diameter is shown.

As shown in FIG. 2 (b), when the particle size was not regulated, free UO ₂ (a densely formed lump) was recognized, and the major axis reached a maximum of 180 μm. In FIG. 2 (a) according to the invention manufacturing method, no such free matter is recognized. It should be noted that those having a small diameter which appear dark in FIG. 2 (a) are not free matter but bubbles (pores).

As described above, according to the method of the present invention, UO
₂ By regulating the particle size of the powder to 60 μm or less,
A nuclear fuel body structure consisting of a uniform solid solution was obtained. Also,
In the above-described embodiment, a method of performing master blending has been described. However, even if Gd ₂ O ₃ is directly mixed with UO ₂ having a size of 60 μm or less, a structure in which a solid solution is obtained can be obtained. Note that according to the present invention, not only about 2-6%, which is the current practical Gd ₂ O ₃ addition concentration, for higher concentrations Gd ₂ O ₃ added since uniform structure is obtained in addition a concentration of 10 number% It is also useful for fuel.

[0018]

As described above, the method for producing a nuclear fuel body according to the present invention provides a method for producing an oxide nuclear fuel body containing uranium dioxide as a main component and a required amount of gadolinium. Oxide having a particle size of 6
0 gm or less, gadolinia powder is mixed with the oxide powder, and then the mixed powder is molded and sintered. By regulating the particle diameter of the uranium dioxide oxide powder to 60 m or less, a ball mill is used. Gadolinia powder is evenly mixed with the above oxide powder by simple mixing without strong mixing as described above, and free UO ₂ · free Gd equivalent to a nuclear fuel body manufactured using a coprecipitation method. ₂ O
₃ has a remarkable effect that a very small uniform solid solution can be obtained.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating an example of a manufacturing method according to the present invention.

FIG. 2 (a) is a photomicrograph (× 100) showing a metal structure of a nuclear fuel body manufactured by the manufacturing method of the present invention. (B) A photomicrograph of 100 times showing the metal structure of a nuclear fuel assembly manufactured by a conventional manufacturing method.

Claims (1)

(57) [Claims] When producing an oxide-based nuclear fuel body containing uranium dioxide as a main component and a required amount of gadolinium, the oxide containing uranium dioxide as a main component has a particle diameter of 6%.
A method for producing a nuclear fuel body, characterized in that gadolinia powder is mixed with the oxide powder, and then the mixed powder is molded and sintered.