JPS6042435B2 - Nuclear fuel production method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates in principle to a method for producing nuclear fuel pellets using scrap pellets as part of the raw powder.

In the process of forming, sintering, etc. to produce nuclear reactor fuel pellets made of uranium dioxide, plutonium dioxide, etc.
It is difficult to avoid the generation of scrap pellets due to the occurrence of cracks, etc.

Methods for recovering this scrap are broadly divided into wet recovery methods and dry recovery methods. In the wet recovery method, for example, scrap is dissolved in nitric acid, aqueous ammonia is added to precipitate ammonium deuterate, and this is reduced to produce dioxide such as uranium dioxide, but recovery costs are high. There is a drawback. On the other hand, in the dry recovery method, scrap consisting of dioxide is oxidized to produce octaoxide powder, which is recovered as it is or after being reduced to dioxide. The recovered recycled powder is mixed with the raw material powder, then molded and sintered in a reducing atmosphere to form a dioxide pellet fuel. Such a dry method has the advantage that it requires fewer steps and is more economical than the wet method. Among these, the method of mixing octoxide with raw material powder without reducing it requires fewer steps than the method of reducing it to dioxide and then mixing it, so it can be expected to have an economical effect, and it also reduces fuel sintering. Since the body has a lower density, it has the advantage of being able to absorb the volumetric expansion caused by the accumulation of fission products, but on the other hand, it has the disadvantage that fine bits, surface unevenness, and cracks may occur in the sintered body. Therefore,
This increases the amount of sintered bodies that are used as scrap, and it is difficult to say that this is a practical method for collecting scrap. The present invention solves the problems in the above-mentioned dry recovery method in which octoxide is directly mixed with dioxide (hereinafter referred to as the dry direct recovery method), and makes this method an economic method for scrapping nuclear fuel. The aim is to complete this method as a recovery method. As a result of our research on the dry direct recovery method for this purpose, the present inventors found that the above-mentioned drawbacks of this method are due to the oxidation temperature of octa3 oxide, the particle size of the produced octa3 oxide, and the corresponding formulation. It was found that the direct dry recovery method can be carried out efficiently if these factors are properly controlled. In other words, Figure 1 is about 10T! A cylindrical pellet-shaped uranium dioxide sintered body of $tφ×1 was placed in a batch-type oxidation furnace and oxidized in air at an ambient temperature of 400150016001700°C for 2 and a half hours to obtain a recycled powder mainly composed of uranium octooxide. Sift through a 10 mesh sieve,
The amount of powder under the sieve (corresponding to the amount of octoxide) was measured and its O/U atomic ratio was measured and plotted against the oxidation temperature.

At this time, a theoretical amount of 10 to 2 C@ of oxygen, which was necessary to convert uranium dioxide to uranium octoxide, was supplied to the furnace between 2:30 and 2:30. As is clear from Figure 1, there is a certain optimum value for the oxidation of sintered uranium dioxide, and the maximum rate is reached at about 500°C, and even if the temperature is increased beyond this, the oxidation rate of uranium dioxide The production speed will be slower. Also, the 0/U ratio of the powder is 50.
The value oxidized at 0-600℃ is almost close to the theoretical value of 2.67, while the value oxidized at 700℃ is 2.635.
On the contrary, it is a low value. Figures 2 1 to 4 show the particle size distribution of the powder at each oxidation temperature, and it can be seen that the higher the oxidation temperature, the larger the particle size. Further, even if the oxidation time was changed at 500° C., no substantial change was observed in the particle size distribution or O/U ratio. On the other hand, as will be clear from the examples described later, when the recycled powder thus obtained is mixed with raw material powder, molded and sintered, the larger the particle size of the recycled powder, the greater the effect of reducing the density of the sintered body. However, there is a certain limit on the particle size and the amount added, and it has been found that adding more than this amount causes surface unevenness and cracks in the sintered body. Ta.

This phenomenon is seen not only in uranium, but also in plutonium and gadolinium. However, nium oxide is used only as a mixture with other elements. The method for producing nuclear fuel of the present invention is based on such knowledge,
More specifically, raw material powder consisting of the dioxide of at least one element selected from uranium and plutonium and a sintered mass obtained from this raw material powder are oxidized at elevated temperature in oxygen or an oxygen-containing atmosphere. In a method for producing nuclear fuel, the oxidation is carried out at 400 to 600°C, and the mixed powder is formed and sintered with a recycled powder mainly consisting of 83 oxides of the above elements. The recycled powder made of oxide meets the following requirements (a) to (a) in the mixed powder.
It is characterized by mixing so as to satisfy the condition (e).

(a) 0.5 to 20% in total (b) Substantially does not contain the particle size part above 48 mesh, (
c) 48 to 100 mesh particle size portion of 3% or less, (d)
100-200 mesh particle size portion 10% or less, (e)
The particle size portion of 200 to 325 mesh is 20% or less.

The present invention will be explained in more detail below. The raw material powder used in the present invention is a powder of a dioxide of at least one element selected from uranium, plutonium, and nium oxide (excluding oxide of nium oxide alone).

These elements behave similarly as oxides. These raw material powders are compacted and sintered by a conventional method to obtain a sintered fuel having an arbitrary shape. However, if the surface condition is poor or cracks occur, it cannot be used as a fuel as it is, and is instead used as a sintered lump for producing recycled powder of the present invention. According to the present invention, the raw powder powder sintered mass is sintered in an oxygen or oxygen-containing atmosphere.
It is oxidized at 00 to 600°C for usually 2 to 5 hours.

As a result of this oxidation, octoxide is formed and is pulverized into recycled powder. However, if the temperature is below 400°C, the pulverization speed is slow and uneconomical, and if it exceeds 600°C, as mentioned above. As a result, the particle size of the powder increases, the yield of the powder decreases, the 0/U ratio decreases, and cracks in the sintered body cause surface unevenness. The regenerated powder thus obtained is mixed with the raw powder so that it accounts for 0.5-20%, preferably 1-10% of the total amount.

If it is less than 0.5%, it is difficult to recover substantial sintered body scrap, and if it is added in excess of 20%, it may cause cracks.

The recycled powder must not contain substantially all of the particle size fraction above 48 mesh, and should not contain a particle size fraction of 48 to 100 mesh and 1
The grain size portion of 00 to 200 mesh must be 3% or less and 10% or less, respectively. If this condition is not met, surface irregularities such as bits may occur on the surface of the final sintered body. Also, recycled powder. The particle size fraction of 200-325 mesh should be less than 20%. this is,
This is to prevent cracks from forming. The raw material powder to be mixed with the recycled powder usually has a particle size of 10 mesh or less. Unlike powder, this is not a problem. This is thought to be because the raw material powder, ie, dioxide, is relatively soft and the secondary particles are easily destroyed by mixing or molding. On the other hand, recycled powder is hard and oxidizes especially at high temperatures, resulting in the theoretical value of the 0/U ratio (
If the particle size is lower than 267), this tendency becomes greater, and unless the particle size and the amount used are extremely limited, it is thought that it will appear as a structural defect in the final sintered body. The obtained mixed powder is compacted in a mold of a molding machine having a desired shape at a pressure of about 0.5 to 4.0 ton/CIL, and then the compact is heated with hydrogen, a hydrogen-nitrogen mixed gas, and argon. Terama sintering is performed at a temperature of about 1 to 10 I Terama in a reducing or inert atmosphere such as, for example, about 1600 to 1800°C.

The obtained nuclear fuel sintered bodies are ground to a desired diameter, stacked and sealed in fuel cladding tubes, and collected in fuel rods to serve as a fuel assembly for operation of a nuclear reactor. As described above, according to the present invention, nuclear fuel scrap is oxidized to form octoxide, and when this is mixed with the dioxide raw material powder, molded, and sintered, the oxidation temperature and the temperature in the octoxide mixture are adjusted. By limiting the state of existence, we can take advantage of the advantage of the above-mentioned dry direct recovery method of nuclear fuel scrap, which is economical with a small number of steps, while also preventing structural defects such as roughness and cracks of bits in the sintered body. By preventing the occurrence of
The dry direct recovery method has been completed as a practical and superior scrap recovery method.

Hereinafter, the present invention will be explained in more detail with reference to Examples. In the examples, 1% is based on weight. EXAMPLE As mentioned above in connection with FIG.
A cylindrical pellet-shaped uranium dioxide sintered body of 10 mmL was placed in a batch-type oxidation furnace at 400150016001700'C.
The bottom part of the 10 mesh sieve of the regenerated powder mainly composed of uranium octooxide (U3O8) obtained by oxidation in a stream of air for 2.5 hours at a temperature of
%, 5%, 10% of raw material uranium dioxide powder (
(average particle size approximately 1P1 subsieve sizer particle size),
The U3O8 recycled powder was molded into a pellet of approximately 10TrUnφ×1 at 2t0n/d, and then sintered at 1750°C for 3 hours in a hydrogen atmosphere. Figures 3 to 5 are plots of the average particle size of the particles.

Looking at these figures, the oxidation temperature ranges from 400℃ to 700℃.
As the value increases, the particle size of the recycled powder increases, and accordingly,
The amount of decrease in sintered density becomes large, which is preferable from this point of view. Furthermore, as the amount of recycled powder added increases, the sintered density also decreases. Example 2 Example 1 with oxidation temperature of 700°C
The lower part of the regenerated powder obtained by 10 mesh sieve was further classified into 48-100 mesh part and 100-20 mesh part.
0 mesh part, 200-325 mesh part, 325
The mesh part and the 325 mesh lower part were mixed with the raw material uranium dioxide powder so that their proportions in the mixture were 3%, 5%, 10%, and 20%, respectively, and the molding pressure was 1.5.
．． 25 and 3.8t. 0n/C! The material was molded in the same manner as in Example 1 except that 1 was changed, and further sintered to obtain a sintered body, and the outer surface was ground. The visual judgment results of the ground sintered pellets were as shown in Table 1 below, and the large particle diameter portion of 48 to 100 mesh was 5.
The pellet containing 10120% had surface unevenness that appeared to be fine bits. Products containing recycled powder of other particle sizes had no problem in terms of surface unevenness, but
In the pellets to which 0% was added, large racks were observed on the end faces of the pellets, except for the pellets in which the portion below 325 mesh was added. Also for reference, below 325 mesh and 48
Surface micrographs (4 times magnification) of sintered bodies obtained by adding 10% of recycled powder of ~100 mesh are shown in FIGS. 6 and 7, respectively.

[Brief explanation of the drawing]

Figure 1 shows the oxidation temperature, oxidation amount, and 0/U of U3O8 powder.
A graph showing the relationship between oxidation temperature and U3O8.
Graphs 3, 4, and 5 showing the relationship with the powder content were obtained by adding 3%, 5%, and 10% of U3O8 powder obtained by changing the oxidation temperature, depending on the particle size, respectively. Graphs 6 and 7 showing the relationship between the density reduction of the sintered body and the U3O8 average grain size are 325 mesh and 4 mesh, respectively.
Micrograph of the outer surface of ground pellets obtained by adding 10% U3O8 with a particle size of 8 to 100 mesh (4
times).

Claims (1)

[Claims] 1. A raw material powder consisting of a dioxide of at least one element selected from uranium, plutonium, and gadolinium (excluding the case of gadolinium oxide alone), and sintered material obtained from this raw material powder. A method for producing nuclear fuel comprising molding and sintering a mixed powder consisting of a powder of a high-temperature oxide of the above-mentioned elements obtained by oxidizing a lump at an elevated temperature in oxygen or an oxygen-containing atmosphere. ~60
A process for producing nuclear fuel, which is carried out at 0°C, and the obtained high-temperature oxidized powder mainly consisting of octaoxide is mixed in a mixed powder so that the following requirements (a) to (e) are satisfied. Manufacturing method. (B) 0.5 to 20% of the total amount (based on mixed powder, weight %. The same applies hereinafter) (B) Substantially does not contain particles with a particle size above 48 mesh (JIS standard sieve. The same applies below); (C) 48
~100 mesh particle size portion 3% or less, (d) 100~
200 mesh particle size portion of 10% or less, and (e) 2
00-325 mesh particle size portion is 20% or less.