JPH09281265A - Production of nuclear fuel pellet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently producing a nuclear fuel pellet having appropriate pore diameter and prosity without using a foreign matter other than the nuclear fuel constituting elements and the oxides obtained by high-temp. treatment. SOLUTION: The UO3 powder obtained by heating ammonium diuranate(ADU) powder to about 450 deg.C and having a low density of 8.25g/cm<3> is added by <=20wt.% to the UO2 powder obtained by a conventional method, further adding a lubricant, as required, mixing 1 the powders and then compacting 2 the mixture. The obtained compact is then degreased 3, if necessary, heated at 1500-1800 deg.C in a hydrogen atmosphere and sintered 4 to produce the pellet.

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, and more particularly to a method for producing oxide fuel pellets containing at least one of uranium (U) oxide and plutonium (Ρu) oxide. Regarding

[0002]

2. Description of the Related Art Nuclear fuel pellets loaded in a light water reactor or a fast breeder reactor are obtained by compression-molding an oxide powder of a nuclear fuel material, followed by heating to a high temperature and sintering.

It is known that the characteristics of the sintered body thus obtained are strongly influenced by the characteristics of the raw material powder, and in particular, the density of the sintered body is strongly influenced by the activity of the raw material powder and becomes too high. There is. Generally, the density of nuclear fuel pellets decreases and the volume thereof increases with combustion, so that the interaction between the pellets and the cladding tube becomes strong, which may cause fuel damage. Here, if the pores appropriately exist in the pellets, the atoms constituting the pellets move to the pores even if a force is applied from the cladding tube, so that the creep rate is high,
The stress is quickly relieved. It is considered that this stress relaxation reduces the interaction between the pellet and the cladding.

As a method for controlling the density of the sintered pellet to a predetermined value, a method of controlling the sintering temperature and the sintering time can be considered. In this method, fine pores and open pores with a diameter of 2 μm or less are increased. Is not preferable. That is, diameter 2μ
It is considered that the micro bubbles of m or less move to the grain boundaries and disappear while repeating re-dissolution into the matrix by the fission fragments and precipitation during combustion of the fuel pellets, which causes compaction. As a result, the gap between the fuel pellets and the cladding tube becomes wider and the heat conduction becomes worse, so the combustion temperature rises, the emission rate of volatile fission products increases, and the internal pressure of the fuel rod rises. It is expected that. Further, since the open pores serve as a release path for the fission products generated during the combustion of fuel, the release rate of the fission products increases, which may eventually cause an increase in the fuel rod internal pressure.

Therefore, it has been attempted to adjust the density of the pellet by forming an appropriate amount of closed pores in the fuel pellet. That is, the powder such as uranium dioxide (UO _2), a method of adding an organic material and triuranium uranium (U ₃ O _8), as volatilize at low temperature have been carried out conventionally.

[0006]

However, in the method using an organic substance, it is considered that the volatilized substance adheres to the exhaust port of the sintering furnace to block the flow path, and further sintering in a strong reducing atmosphere is considered. In, there was a possibility that carbon might remain in the pellet.

Further, U described in Japanese Patent Laid-Open No. 3-9296
In the method of adding ₃ O _8, in order to obtain a U ₃ O _8, U
The O ₂ powder is supposed to be heated and oxidized at a temperature of 450 ° C. to 700 ° C., but in reality, it was necessary to treat it at a high temperature of 700 ° C. or higher. Further, as another method of obtaining U ₃ O ₈ , there is a method of reducing uranium trioxide (UO ₃ ) powder, but this method also requires treatment at a high temperature (a temperature of about 700 ° C.). Then, a method of adjusting the density of the obtained by adding U ₃ O ₈ sintered body by these methods, not be said to be efficient manner.

That is, as a method for preparing a raw material powder of normal uranium, ammonium diuranate (A) obtained by precipitating uranium ions dissolved in nitric acid with an ammonia solution is used.
DU) powder is roasted / reduced at a high temperature to obtain UO ₂ powder, and direct denitration of uranyl nitrate is widely used. In the method of adding U ₃ O ₈ mentioned above,
Since the U ₃ O ₈ powder obtained by further treating the UO ₂ powder at a higher temperature is added to the UO ₂ powder obtained by a method such as roasting and reducing ADU at a high temperature, the production efficiency is improved. Was low.

The present invention has been made in order to solve these problems, and does not use a foreign substance composed of elements other than the constituent elements of the nuclear fuel or an oxide which needs to be treated at a high temperature, and has an appropriate pore diameter. And a method for efficiently producing a nuclear fuel pellet having a porosity.

[0010]

A method for producing a nuclear fuel pellet according to the present invention comprises a method for producing a nuclear fuel pellet containing at least one of an oxide of uranium (U) and an oxide of plutonium (Ρu). Uranium (U
O ₂ ) or plutonium dioxide (ΡuO ₂ ) without passing through the form and compressed at a low temperature of 600 ° C. or less, and oxide powder containing higher-order oxide having a lower density than UO ₂ or ΡuO ₂ is compressed. It is characterized by being molded and sintered at a temperature of 1500-1800 ° C.

In the present invention, a low-density high-order oxide (hereinafter referred to as a low-density oxide) mixed with an oxide powder such as UO ₂ or ΡuO ₂ is produced by the following method. β-, γ-, δ-, etc. uranium trioxide (UO
₃ ) is used. That is, when ADU powder was gently heated at a low temperature of about 450 ° C., β-density of 8.25 g / cm ³
UO ₃ was obtained, and uranyl nitrate was heated at about 200 ° C to homogenize it, and then heated to about 500 ° C to obtain a density of 8.01 g /
cm ³ of γ-UO ₃ is obtained. These UO ₃ can be used.

In the production method of the present invention, the above-mentioned low-density oxide contained in the compression-molded body has a higher density of UO ₂ (density 10.96 g / cm ³ ) or PuO during the reduction process during sintering.
₂ (density 11.45 g / cm ³ ) and UO ₂ and P occupying most of the molded body by changing to mixed oxide of both
Compared to uO ₂ and mixed oxides of both, it shrinks greatly,
Due to the difference in shrinkage ratio, relatively large pores are formed in the portion where the low density oxide was present. And
The porosity in the sintered pellet can be adjusted by adjusting the content ratio of the low-density oxide with respect to the entire oxide. Further, the pore size can be controlled by adjusting the aggregate particle size (secondary particle size) of the low-density oxide.

In the present invention, the content ratio of the low-density oxide to the whole oxide is more than 20% by weight, which is not preferable because the porosity in the sintered pellet becomes too high. desirable. The secondary particle size is 20
It is desirable to use low-density oxide powder with a size of μm or more.
When a low-density oxide powder having a secondary particle size of less than 20 μm is used, the diameter of the generated pores becomes too small, which is not preferable.

Further, in the present invention, if the reduction in the process before the start of sintering between the powders (the initial stage of sintering) is insufficient, the shrinkage is uneven during the sintering after the bonding between the powders. It is conceivable that cracks occur in the sintered pellets.
Therefore, it is desirable to adjust the composition and flow rate of the atmospheric gas so that the molded body is sufficiently reduced in the temperature range of 400 to 500 ° C. or less. The heating and sintering temperature is 1500.
It is desirable to set the temperature to ~ 1800 ° C. That is, the temperature for sufficiently promoting the sintering depends on the type of the oxide raw material powder, but a temperature of 1500 ° C. or higher is necessary considering the diffusion rate of cations, and a temperature of 1800 ° C. or lower is sufficient. Sintering is possible.

[0015]

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

FIG. 1 is a process chart for explaining an embodiment of a method for producing nuclear fuel pellets according to the present invention.

In this example, β-UO ₃ (density 8.25 g / cm ³ ) was selected as the low-density oxide, and this β-U was selected.
As shown in Fig. 1, O ₃ powder and ADU powder were heated to about 450 ° C.
Obtained by heating to the temperature of. Then, mixing step 1
Then, a predetermined amount (0 wt%, 10 wt% and 20 wt%) of the β-UO ₃ powder obtained by the above-mentioned method was added to the UO ₂ powder obtained by the usual method and mixed, and further if necessary. If necessary, a lubricant such as polyvinyl alcohol or paraffin is added and mixed, and then compression molding is performed in the compression molding step 2.
Molded bodies each having a theoretical density of about 49% (relative density of about 49% TD) were obtained. Next, these molded bodies were degreased in degreasing step 3 (only when lubricant was added) and then sintered in a hydrogen atmosphere in a sintering step 4 at a temperature of about 1730 ° C. for 3 hours to sinter. A healthy sintered body having no cracks was obtained.
The porosity of the obtained sintered body is shown in FIG. 2, and the open porosity is shown in FIG.
Are shown respectively. The crystal grain size of the sintered body is shown in FIG. From these figures, the low-density oxide β-UO ₃
The porosity of the sintered body increases with the increase of the powder addition rate, but the open porosity is 0.2 vol% or less, and most of the porosity (about 99% of the total porosity) is closed porosity. all right. In addition, many large pores with diameters exceeding 20 μm were observed. Furthermore, the crystal grain size of the sintered body is β-UO.
_{When 3} was added at a ratio of 10% by weight, it became the maximum.

Next, another embodiment of the present invention will be described.

In this embodiment, as shown in FIG.
In the step (roasting / reduction step) 5 of preparing UO ₂ powder by roasting / reducing U powder, a temperature distribution is provided, and a part of the temperature is reduced.
By maintaining a low temperature of 450 ° C., UO ₃ in UO ₂
Mixed in. Then, the UO ₂ -UO thus obtained
_{The 3} mixed powders are crushed and mixed in the crushing and mixing step 6, and further, if necessary, a lubricant is added and mixed, and thereafter, the compression molding step 2 and the degreasing step 3 (adding a lubricant is carried out in the same manner as in the above-mentioned embodiment. In the hydrogen atmosphere in sintering step 4.
When heated at a temperature of 1730 ° C for 3 hours and sintered, cracks,
A healthy sintered body without chipping was obtained. Further, when the porosity and open porosity of the obtained sintered body were examined, respectively, the porosity was moderate (3 to 8% by volume), and most of the porosities were closed.

In the above examples, the heat sintering was carried out in a hydrogen atmosphere, but the oxygen potential was -200.
It is also possible to reduce after sintering in a weakly oxidizing atmosphere up to about kJ / mol. Furthermore, in the embodiment, as the low density oxide, but density using beta-UO ₃ of 8.25 g / cm ^3, as a low density oxide is not limited to this, for example, heating the uranyl nitrate at about 200 ° C. And homogenize, then about 500
Γ-UO ₃ (density 8.01 g / cm ³ ) obtained by heating to ℃,
Alternatively, δ-UO ₃ or ε-UO ₃ may be used.

[0021]

EFFECTS OF THE INVENTION According to the present invention, a nuclear fuel pellet having a proper porosity and pore diameter can be efficiently prepared without using a foreign substance composed of elements other than the constituent elements of the nuclear fuel or an oxide obtained by high temperature treatment. Can be obtained.

[Brief description of drawings]

FIG. 1 is a process drawing for explaining an embodiment of a method for producing nuclear fuel pellets according to the present invention.

FIG. 2 is a graph showing the porosity of nuclear fuel pellets manufactured according to an example.

FIG. 3 is a graph showing the open porosity of nuclear fuel pellets manufactured according to the examples.

FIG. 4 is a graph showing the crystal grain size of the nuclear fuel pellets manufactured according to the example.

FIG. 5 is a process drawing for explaining another embodiment of the present invention.

[Explanation of symbols]

 1 ………… Mixing process 2 ………… Compression molding process 3 ………… Degreasing process 4 ………… Sintering process 5 ………… Roasting / reduction process 6 ………… Grinding and mixing process

Claims (5)

[Claims] 1. A method for producing a nuclear fuel pellet containing at least one of an oxide of uranium (U) and an oxide of plutonium (Ρu), comprising: uranium dioxide (UO ₂ ) or plutonium dioxide (Ρ).
uO ₂ ), which is obtained at a low temperature of 600 ° C. or lower without passing through the morphology of uO ₂ ) and is compression-molded with an oxide powder containing a higher-order oxide having a lower density than that of UO ₂ or ΡuO _2.
A method for producing a nuclear fuel pellet, which comprises sintering at a temperature of 00 ° C. 2. The method for producing a nuclear fuel pellet according to claim 1, wherein uranium trioxide (UO ₃ ) is used as the low-density high-order oxide. 3. The content ratio of the low-density high-order oxide is
The method for producing nuclear fuel pellets according to claim 1 or 2, wherein the content of the oxide is 20% by weight or less based on the whole oxide. 4. The product is roasted and reduced by providing a temperature distribution so that a part of the product becomes UO _3, and then the obtained oxide powder is pulverized and mixed, and then compression molded and sintered. The method for producing a nuclear fuel pellet according to any one of claims 1 to 3, characterized in that. 5. The composition of the atmosphere gas so that the reduction of the powder is completed before the sintering between the powders is started in the step of sintering the oxide powder containing the low-density high-order oxide. And adjusting the flow rate.
A method for producing a nuclear fuel pellet according to any one of 1.