JPH0777591A - Production of nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To obtain a nuclear fuel pellet in which the dispersion in the density and outer diametral dimension is suppressed. CONSTITUTION:In the production process of nuclear fuel pellet, a density lowering agent is admixed prior to the granulation step. Consequently, the density lowering agent is diffused uniformly over the entire powder while being contained in and between the granules thus realizing uniform density distribution over the entire mixed powder. It is then admixed with a lubricant and passed through a forming step, a sintering step, etc., thus completing a nuclear fuel pellet. By this method, nuclear fuel pellet suppressed in dispersion in the density and dimensions is produced with significantly high yield.

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 in a nuclear reactor.

[0002]

2. Description of the Related Art Nuclear fuel pellets are usually used in nuclear power generation. This nuclear fuel pellet has a diameter of several millimeters and a height of about the same as the diameter.
Fuel rods are formed by stacking a large number of these inside a cladding tube such as stainless steel.

By the way, generally, when nuclear materials in nuclear fuel are split in a nuclear reactor, gaseous fission products (FP gas) are produced. This FP gas expands the nuclear fuel pellets, which becomes a factor that limits the life of the fuel in the reactor. Therefore, in order to suppress the influence of the FP gas, it is necessary to previously provide a large number of minute gas reservoir cavities (hereinafter referred to as pores) in the pellet. The pores have a size of, for example, several tens to several hundreds of μm, and by forming such pores in the pellet, it is possible to suppress the expansion of the nuclear fuel pellet by the FP gas described above. Pellets having such pores have a low density and are generally called low density pellets.

As described above, when manufacturing nuclear fuel pellets, it is essential to control the density, but there are several methods for controlling the density. The most common of these methods, which have already been put into practical use, is to add a spherically prepared organic substance to a raw material powder to form a green compact, and degrease the green compact in a reducing atmosphere at about 800 ° C. Then, pores are formed in the green compact, and the green compact is baked and solidified by sintering. The density reducing agent used in this method is a substance that does not react with the raw material powder, that can be sufficiently removed in the degreasing step, that it can be adjusted to a predetermined shape and size, water and gas Conditions such as the difficulty of causing adsorption and the difficulty of causing aggregation are required. Examples of those satisfying such five conditions include polyethylene, aromatic carboxylic acids, higher fatty acids such as glycerin-trihydroxy-stearate, and diamine oxalate.
Used practically or experimentally. Further, as described in the previous application by the present applicant (JP-A-5-1812611), it is effective to use crystalline cellulose having heat resistance as a density reducing agent.

In the case of producing a nuclear fuel pellet using such a density reducing agent, conventionally, as shown in FIG. 5, a binder for strengthening the powder bond is added to a raw material powder which is uniformly pulverized and mixed to give a predetermined grain. After granulating to a diameter, a density reducing agent was added to control the density, and then steps such as molding and sintering were performed.

[0006]

However, according to the conventional process, the density-lowering agent has an extremely small specific gravity as compared with the granulated granular powder (hereinafter referred to as granulated powder). Therefore, as shown in FIG. In addition, the granulated powder 11 and the density reducing agent 12 are separated during the powder handling in the step after the addition of the density reducing agent. When molding is carried out in this state, the amount of the density reducing agent contained in each molded green compact varies, and as a result, as shown in FIGS. 7 (a) and 7 (b), pellets after sintering are formed. There is a large variation in density and dimensions. In this figure, N is a parameter and n is the number of samples.

As described above, according to the conventional method, it is difficult to keep the density and size of the sintered pellet within the predetermined allowable range, and it becomes necessary to select the density and size of all the sintered pellets. This was a cause of a large decrease in product yield.

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a method for producing nuclear fuel pellets capable of reducing variations in density and size between pellets.

[0009]

According to a first aspect of the present invention, there is provided a method for producing a nuclear fuel pellet, wherein (i) the nuclear fuel raw material powder is provided with a large number of pores, which are cavities for storing small gas, in the nuclear fuel pellet. A step of adding a density reducing agent for mixing, (ii) granulating the mixture obtained by the adding and mixing step to a predetermined particle size, and (iii) granulating obtained by this granulating step. And a forming and sintering step of forming the particles into a predetermined shape and sintering the formed particles. .

The method for producing nuclear fuel pellets according to the second aspect of the present invention is characterized in that, in the first aspect, the nuclear fuel raw material powder is uranium oxide.

According to a third aspect of the present invention, there is provided the method for producing nuclear fuel pellets according to the first aspect, wherein the nuclear fuel raw material powder is uranium carbide.

According to a fourth aspect of the present invention, there is provided the method for producing a nuclear fuel pellet according to the first aspect, wherein the nuclear fuel raw material powder is uranium nitride.

According to a fifth aspect of the present invention, in the method for producing nuclear fuel pellets according to the first aspect, the nuclear fuel raw material powder is a mixture of uranium oxide and plutonium oxide.

According to a sixth aspect of the present invention, in the method for producing nuclear fuel pellets according to the first aspect, the nuclear fuel raw material powder is a mixture of uranium carbide and plutonium carbide.

The method of producing a nuclear fuel pellet according to the invention of claim 7 is the method of producing a nuclear fuel pellet according to claim 1, wherein the nuclear fuel raw material powder is a mixture of uranium nitride and plutonium nitride.

[0016]

In the method of producing nuclear fuel pellets according to the present invention,
In the mixed powder after the granulation step, the density reducing agent is uniformly dispersed inside and outside the granulated powder, and the density distribution of the entire mixed powder is homogenized. Therefore, variations in density and dimensions between the sintered pellets produced by molding and sintering this are reduced.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

The characteristic feature of this embodiment is that the density reducing agent is added before the granulation step, and the density reducing agent is uniformly dispersed inside and outside the granulated powder before molding and sintering. In point.

Here, heat-resistant crystalline cellulose is used as the density reducing agent. This crystalline cellulose has a high thermal decomposition temperature of about 350 ° C., is easy to be prepared into a spherical shape having a predetermined size, and is resistant to adsorption and aggregation of water and the like (I to I described in the conventional example). All the conditions of M) are satisfied.

FIG. 1 shows a density of 85% T. It is an example of a manufacturing process of D (theoretical density) nuclear fuel pellets. As shown in this figure, powder of uranium dioxide (UO ₂ ) and mixed oxide (M
(OX) powder is mixed and pulverized, and then a density reducing agent and a binder are added thereto. Here, the above-mentioned crystalline cellulose is used as the density reducing agent, and zinc stearate is used as the binder. Here, the binder plays a role of a glue for improving the binding of the powder, and is necessary for mass production in order to improve the handling strength before the green pellet stage.

In this state, that is, in the state where the density reducing agent is added to and mixed with the raw material powder, the mixed powder is as shown in FIG.
The state becomes as shown in. As shown in this figure, the crystalline cellulose 12 as the density reducing agent is contained not only between the granulated powders 11 but also within the granulated powders 11 and is uniformly dispersed throughout the powders.

Next, after mixing these, the process proceeds to the granulation step. This granulation step consists of three steps of rough molding, crushing (crushing) and classification. Next, zinc stearate is added to this as a lubricant, and then mixed and molded into a columnar shape. Through this molding process, so-called green pellets are formed.

Next, a step of degreasing the green pellets is performed. This degreasing is performed at a temperature of about 800 ° C. or higher in a mixed gas atmosphere of argon and hydrogen. By this degreasing step, the density reducing agent is thermally decomposed and evaporated to be removed. Next, sintering is performed at about 1700 ° C. for about 2 hours in a mixed gas atmosphere of argon and hydrogen, and further about 8
Degas in a high temperature vacuum at 00 ° C. Finally, the outer periphery of the molded pellet is ground to complete the final sintered pellet. If the outer diameter accuracy of the pellet after sintering is extremely good, this grinding step may be omitted.

In this embodiment, crystalline cellulose spheroidized to a predetermined particle size is used. The particle size varies with the desired size of the cavities present in the pellet.

The mixing ratio of the crystalline cellulose and the raw material powder in the above description varies depending on the characteristics of the raw material powder (ease of sintering) and the density of the product pellets.
When producing D pellets, the amount is about 2 wt%.

% TD is the pellet density when the theoretical density (the density when the constituent atoms (U, PU, O) of the nuclear fuel are packed without gaps according to the crystal structure) is 100% TD, For example, in the case of the fast breeder reactor "Monju", 85% TD is specified.

3 (a) and 3 (b) respectively show an example of the density distribution and the size distribution of the nuclear fuel pellets manufactured by the manufacturing method of this embodiment. As shown in this figure, by adding and mixing a density reducing agent (crystalline cellulose) before the granulation step, the range of density distribution and size distribution of the nuclear fuel pellets as a product is extremely narrow,
It can be seen that most of the pellets are within specifications. Therefore, the product yield is extremely high.

In the present embodiment, the case where crystalline cellulose is used as the density reducing agent has been described, but the present invention is not limited to this, and higher fatty acids such as polyethylene, aromatic carboxylic acid, and glycerin-trihydroxy-stearate, Alternatively, diamine oxalate or the like may be used.

[0029]

As described above, according to the present invention,
Since the density reducing agent was added before the granulating step of the raw material powder, in the mixed powder after the granulating step, the density reducing agent is uniformly dispersed inside and outside the granulated powder, and the density of the entire mixed powder is increased. The distribution is homogenized. Therefore, there is an effect that the density and the dimensional variation between the sintered pellets produced by molding and sintering this are reduced, and the product yield is significantly improved.

[Brief description of drawings]

FIG. 1 is a process drawing showing a method for producing nuclear fuel pellets in an example of the present invention.

FIG. 2 is a diagram showing a state of a mixed powder after a granulation step in this manufacturing method.

FIG. 3 is a diagram showing a density distribution and a size distribution of nuclear fuel pellets manufactured by this manufacturing method.

FIG. 4 is a process diagram showing a conventional method for producing nuclear fuel pellets.

FIG. 5 is a diagram showing a state of a mixed powder after a granulation step in a conventional manufacturing method.

FIG. 6 is a diagram showing a density distribution and a size distribution of nuclear fuel pellets manufactured by a conventional manufacturing method.

[Explanation of symbols]

 11 Granulated powder 12 Density reducing agent

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazutaka Suzuki Tokai-mura, Naka-gun, Ibaraki Prefecture 4-3 Muramatsu, Tokai Plant, Reactor and Nuclear Fuel Development Corp. (72) Masayuki Morihira Tokai-mura, Naka-gun, Ibaraki Prefecture Muramatsu 4-33 33 Reactor and Nuclear Fuel Development Corporation Tokai Works

Claims (7)

[Claims] 1. A step of adding to a nuclear fuel raw material powder a density-reducing agent for forming a large number of pores, which are microscopic gas storage cavities in a nuclear fuel pellet, and mixing, and a step of adding and mixing the same. Production of nuclear fuel pellets, characterized by comprising: a step of granulating the mixture into a predetermined particle size; and a molding and sintering step of molding the granule obtained by this granulation step into a predetermined shape and sintering. Method. 2. The method for producing a nuclear fuel pellet according to claim 1, wherein the nuclear fuel raw material powder is uranium oxide. 3. The method for producing a nuclear fuel pellet according to claim 1, wherein the nuclear fuel raw material powder is uranium carbide. 4. The method for producing a nuclear fuel pellet according to claim 1, wherein the nuclear fuel raw material powder is uranium nitride. 5. The method for producing a nuclear fuel pellet according to claim 1, wherein the nuclear fuel raw material powder is a mixture of uranium oxide and plutonium oxide. 6. The method for producing a nuclear fuel pellet according to claim 1, wherein the nuclear fuel raw material powder is a mixture of uranium carbide and plutonium carbide. 7. The method for producing a nuclear fuel pellet according to claim 1, wherein the nuclear fuel raw material powder is a mixture of uranium nitride and plutonium nitride.