JPH02259596A - Production of sintered body of nuclear fuel - Google Patents

Abstract

PURPOSE:To adjust the crystal density and grain size of the sintered body of nuclear fuel to the values within desired values by adjusting any one of the grain size of the triuranium octoxide U3O8 to be added to uranium dioxide UO2, the amt. of the U3O8 to be added, and the roasting temp. at the time of production. CONSTITUTION:Nuclear fuel powder of an oxide mixture system contg. other oxides is usable for the raw material powder of the UO2. The U3O8 is obtd. by roasting the UO2 scrap or the unheated UO2 of moldings, etc. The crystal density and crystal grain size of the sintered body of the nuclear fuel are adjusted to the desired values by changing the roasting temp. thereof. The adjustment of the crystal density and crystal grain size of the sintered body is possible as well even if the mixture composed of the U3O8 and the UO2 obtd. by the roasting and the U3O8 of a prescribed grain size are mixed. Further, the crystal density is adjusted by mixing the U3O8 having about 250 to 500mum average grain size at about 5 to 10wt.% ratio with the mixture formed by once crushing the U3O8 obtd. by the roasting to the UO2 and mixing the same. The mixture adjusted in such a manner is subjected to molding, sintering and reducing treatments, by which the sintered body having the desired density and grain size is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a nuclear fuel sintered body, including a method for adjusting the crystal grain size and crystal density in the nuclear fuel sintered body.

[Prior Art and Problems to be Solved by the Invention] Uranium dioxide-based nuclear fuel sintered bodies, which are nuclear fuel sintered bodies used as nuclear fuel materials in nuclear reactors, have come into widespread use with the spread of light water reactors.

This uranium dioxide-based nuclear fuel sintered body has an average grain size of 5
However, it is believed that nuclear fuel sintered bodies with small grain size crystals release a large amount of fission product gas (FP gas), and in order to suppress the release of FP gas, It is desired that the nuclear fuel sintered body has a large particle size with an average crystal grain size of about 20 to 80 g.m.

Conventionally, as a method for producing large-grain uranium dioxide-based nuclear fuel sintered bodies, there is a low-temperature oxidizing atmosphere sintering method by KWU (West Germany). Uranium (U308) and uranium dioxide (UO
2) It may be mixed with raw material powder and sintered.

In the above manufacturing method, the amount of triuranium octoxide to be mixed is mainly adjusted to adjust the density of the sintered body and the compaction of the sintered body, and the grain size is adjusted by adjusting the sintering temperature, etc. Large grain size nuclear fuel sintered bodies are manufactured by setting specific conditions.

However, in the above manufacturing method, in order to manufacture a nuclear fuel sintered body with a large particle size and a constant density [83 to 88% TD], it is necessary to set the range of the amount of triuranium octoxide to be mixed. This method has the problem that conditions such as sintering temperature are limited to a specific setting range.

This invention was made based on the above circumstances.

That is, an object of the present invention is to provide a nuclear fuel including a method for adjusting the crystal grain size and crystal density of a nuclear fuel body, which solves the above problems and makes it possible to easily produce a nuclear fuel body having a desired crystal grain size and a desired crystal density. An object of the present invention is to provide a method for manufacturing a sintered body.

[Means for Solving the Problems] This invention for solving the problems described above involves molding a mixed powder of 75 to 55% by weight of uranium dioxide raw material powder and 25 to 45% by weight of triuranium octoxide. 1. The obtained molded body is placed in an oxidizing atmosphere. After sintering at a temperature of IOQ ~ 1.400℃, 1.100 ~ 1.400 in a reducing atmosphere
Heating at a temperature of ℃, the oxygen chamber metal ratio (O/U) was 1.
In a method for manufacturing a nuclear fuel sintered body, which manufactures a nuclear fuel sintered body adjusted to 38 to 2.02.

Adjustment of grain size of nuclear fuel sintered bodies.

[1] At least one of the following: (1) adjustment by the roasting temperature when roasting uranium oxide to produce the triuranium octoxide; and (1) adjustment by the amount of the triuranium oxide mixed into the uranium dioxide raw material powder. [1] Adjustment by the roasting temperature when roasting uranium oxide to produce the triuranium octoxide.

■ Adjustment by the amount of the triuranium octoxide mixed into the raw material powder of uranium dioxide, and ■ Divide the mixing period of the triuranium oxide into a period and a later period, and determine the particle size of the triuranium octoxide during the latter period. Adjustment by.

This is a method for producing a nuclear fuel sintered body, characterized in that it is carried out by adjusting at least one of the following.

FIG. 1 is an explanatory diagram showing an example of the method of the present invention.

As shown in FIG. 1, the method of the present invention includes, for example, a mixing process in which uranium dioxide raw material powder and triuranium octoxide obtained by roasting uranium dioxide scrap, etc., and an f8& type process. Adjustment of grain size and crystal density of nuclear fuel sintered bodies when producing uranium dioxide-based nuclear fuel sintered bodies by performing sintering treatment in an oxidizing atmosphere and reduction treatment in a reducing atmosphere in this order. It's a method.

Below, the uranium dioxide raw material powder, triuranium octoxide, mixing treatment, molding treatment, sintering treatment, and reduction treatment will be explained in detail in this order.

Secondary uranium raw material powder The uranium dioxide raw material powder used in the method of the present invention contains uranium dioxide.

Examples of the uranium dioxide include ammonium deuterate (ADU) and 1M uranyl ammonium (AUG).
Examples include uranium dioxide obtained by roasting and reducing uranium dioxide.

The content of the uranium dioxide in the nuclear fuel powder is usually 100% by weight.

However, in the method of the present invention, mixed oxide nuclear fuel powder in which the uranium dioxide raw material powder contains other oxides together with the uranium dioxide can also be suitably used.

As the mixed oxide nuclear fuel powder, for example, UO2
Mixed oxide of and Pu 02 [(■, Pu) 021,
Mixed oxide of UO2 and GdO2 [(U, Gd)Oz
] etc.

Triuranium octoxide This triuranium octoxide includes, for example, uranium dioxide nuclear fuel sintered body scrap (UO2 scrap) such as polishing waste and defective pellets generated in the manufacturing process of uranium dioxide nuclear fuel sintered body (U02 pellet), or molded bodies. Examples include triuranium octoxide obtained by roasting unheated UO2.

Note that, for example, when U 2 O 2 scrap is gradually heated from room temperature, U 3 O 8 is obtained through U 40 q and U 3 O y , and when this U 30 8 is heated in oxygen at a temperature of 500° C. or lower, U O 3 may be obtained.

Therefore, the triuranium octoxide used in the method of the present invention includes triuranium octoxide (U30&
) powder, for example, tetrauranium nineoxide (U409
) powder, triuranium heptoxide (U307) powder, uranium trioxide (UO3) powder, etc.

Furthermore, in the method of the present invention, when the uranium dioxide raw material powder contains the mixed oxide nuclear fuel powder,
That is, 1. For example, if the uranium dioxide raw material powder is (U,
When the uranium oxide powder is Pu)02 powder, (U,Pu)30B powder can be used as the uranium oxide powder, and when the nuclear fuel powder is (U,Cd)Oz powder, the uranium oxide powder can be used as the uranium oxide powder. (U,Gd)aOs powder can also be used as the material powder.

In this invention, the crystal density and crystal grain size of the nuclear fuel sintered body are adjusted to desired values by changing the roasting temperature during the production of triuranium octoxide.

The roasting temperature during the production of triuranium octoxide is normal.

The temperature is 300℃ or higher, but as shown in Figure 2, the temperature is 350℃ or higher.
By appropriately selecting a temperature within the range of 850°C,
The nuclear fuel crystal density can be set in the range of 93 to 98% TD, and as shown in Figure 3, the roasting temperature can be set to 35%.
By selecting an appropriate temperature within the range of 0 to 850°C, the average crystal grain size of the nuclear fuel sintered body can be set within the range of 20 to 807 zm.

Note that even if this roasting temperature range is not specifically selected within the above range, it can be achieved by selecting the particle size of triuranium octoxide and/or selecting the timing and amount of triuranium octoxide to be blended with uranium dioxide, which will be described later. Adjustment of the grain size and grain size of the nuclear fuel sintered body can also be carried out.

Mixing treatment The average particle size of triuranium octoxide obtained by roasting uranium dioxide at a roasting temperature within the range of the roasting temperature mentioned above is usually 30~
It is within the range of 500 lm.

In the present invention, triuranium octoxide and uranium dioxide having the above particle size range may be mixed, and then the resulting mixture may be subjected to a molding treatment.

If further particle size adjustment is required, the particle size of the input triuranium oxide may be adjusted, or as shown in FIG. The crystal density and crystal grain size of the nuclear fuel sintered body can also be adjusted by mixing the resulting mixture A with triuranium octoxide B having a predetermined grain size. In this case, the first mixing process is a process of pulverizing and mixing the triuranium octoxide with the uranium dioxide raw material powder, and the subsequent mixing process is a process of uniformly mixing them.

In the mixing process, the blending amount of triuranium octoxide in the mixture of uranium dioxide raw material powder and triuranium octoxide is 25 to 25%.
It is preferable to mix uranium dioxide raw material powder and triuranium octoxide at a ratio of 45% by weight, especially 30 to 40% by weight. By mixing triuranium octoxide under these conditions, nuclear fuel sintering can be achieved. The crystal grain size and crystal density of the body can be adjusted.

When the amount of triuranium octoxide in the mixture increases, specifically, when the amount of triuranium octoxide is greater than 40% by weight and less than 45% by weight, large-diameter bores ( If the blended amount of triuranium octoxide in the mixture is small, specifically, the blended amount of triuranium octoxide will be 25% by weight or more.
When incorporated at less than % by weight, the particle size distribution tends to be extremely non-uniform.

For the mixing process, a known mixing amount such as a V-type blender, or a mixing pulverizer such as a ball mill or a mixer mill can be used.

In addition, in the method of the present invention, as a selection of the timing of blending triuranium octoxide, the uranium oxide produced by the roasting is pulverized and mixed with uranium dioxide for 8 times, and then the mixture obtained is mixed with the triuranium octoxide. The density of the nuclear fuel sintered body can be adjusted by mixing triuranium octoxide with an average particle size of 250 to 5004 m in a proportion of 5 to 10% by weight based on the mixture. can.

The amount and particle size of triuranium octoxide added to adjust the crystal grain size and density of the sintered body vary slightly depending on the loft of the uranium dioxide raw material powder.
) and the amount added, the following relationship is obtained.

p = p, −0,3 Is the amount of trian octoxide added in the post-assembly 2x? % (Average particle size of triuranium octoxide: 250-350 gm)
On the other hand, the crystal grain size of the sintered body can be determined by the total amount of triuranium octoxide added and/or the roasting temperature.

In the method of the present invention, the adjusted mixture obtained in the mixing process is then compression molded to form a molded body.

The molding pressure during compression molding is usually in the range of 1 to 5 t/cm2, preferably in the range of 1.4 to 2.8 t/cm2. If the molding pressure is less than 1 t/c layer 2, the resulting molded product will easily collapse. On the other hand, 5t/c■
When it exceeds 2, cracks are likely to occur in the obtained molded body and pellets to be sintered.

Award 1 In the method of the present invention, the molded body is then sintered in an oxidizing atmosphere, particularly in a slightly oxidizing atmosphere.

The oxidizing atmosphere is realized, for example, by the presence of carbon dioxide, a mixed gas of nitrogen and oxygen, a mixed gas of carbon dioxide and carbon monoxide, or the like. A particularly preferable atmosphere is an inert gas with a concentration of I x 10-3 to 2
X is a gas atmosphere containing 10-2% by volume of oxygen gas.

The sintering temperature in the oil temperature sintering process is usually 1,100 ~
It is preferable to set the temperature within the range of 1,400°C.

If the sintering temperature is lower than 1,100°C, the sintered density of the obtained uranium dioxide nuclear fuel sintered body may decrease,
In addition, in the above temperature range, when the sintering time is 2 hours, for example, the temperature on the low temperature side is 1.100 to 1.1
Sintering at a temperature of 50°C increases the particle size distribution of the nuclear fuel sintered body, while sintering at a higher temperature in the range of 1,200 to 1.250°C increases the particle size distribution of the nuclear fuel sintered body. The particle size distribution tends to become smaller.

Therefore, the average particle size distribution of the nuclear fuel sintered body can be adjusted by adjusting the sintering temperature.

The time required for sintering is usually 10 minutes to 4 hours.

In the method of the present invention, after the sintering is performed, reduction treatment is performed by heating in a reducing atmosphere.

That is, by this reduction treatment, the molded body that has undergone the sintering treatment is reduced.

The reducing atmosphere is realized, for example, by the presence of hydrogen, a mixed gas of hydrogen and nitrogen, a mixed gas of hydrogen and argon, or a gas in which these and water vapor coexist.

The heating temperature in the Jingji reduction treatment is usually 1,100 to 1
．． Set the temperature to 400℃.

If the heating temperature is lower than 1,100°C, the heating time described below will be significantly longer in order to make the 07U ratio of the uranium dioxide nuclear fuel sintered body obtained by the method of the present invention in the range of 1.88 to 2.02. Become. On the other hand, even if the temperature is higher than 1.400°C, no commensurate effect will be achieved, which is disadvantageous in terms of energy efficiency. Note that the heating conditions are usually set to be the same as the sintering conditions in consideration of furnace design and balance.

That is, the heating time is usually 10 minutes to 3 hours.

In the method of this invention, the 0/U ratio of the uranium dioxide nuclear fuel sintered body obtained by performing the above treatment is 1.98.
The density of the obtained uranium dioxide nuclear fuel sintered body is adjusted to be in the range of 93 to 98% TD.

If this O/U ratio and density are out of the above range, the melting point and strength of the obtained uranium dioxide nuclear fuel sintered body will decrease, and there is a risk that it will deviate from the design value, which may be unfavorable in terms of fuel design. be.

As described above, it is possible to produce a nuclear fuel sintered body having an arbitrary average particle size and average crystal density within the range of 20 to 804 m and an average crystal density of 93 to 98% TD. This nuclear fuel sintered body can be suitably used as a nuclear fuel material for a light water reactor, for example.

[Example] Next, Examples of the present invention will be shown and the present invention will be explained in more detail.

This example was carried out by the method shown in FIG.

(Example 1) Mixture of 70% by weight of uranium dioxide raw powder with a 0/U ratio of 2.12 and 1% by weight of triuranium octoxide (average particle size 125pm) roasted at 400°C The mixture obtained by the treatment was molded and heated to 1,10% in a slightly oxidizing atmosphere.
A sintering process was performed at 0°C for 3.5 hours, and then a reduction process was performed by heating in a reducing atmosphere to produce a uranium dioxide-based nuclear fuel sintered body.

The density and 0/U ratio of this uranium dioxide-based nuclear fuel sintered body were as follows.

Density, 97.4% T, D.

0/U ratio: 2.00 Also, when observed with a metallurgical microscope, as shown in Figure 4, a large particle size part with an average particle size of 3071 m and a large particle size part with an average particle size of 2 gm.
It was confirmed that the small particle diameter part was a healthy uranium dioxide-based nuclear fuel sintered body with a nearly uniformly distributed gold phase.

(Example 2) Example 1 was repeated except that triuranium octoxide roasted at 600°C was used instead of triuranium octoxide roasted at 400°C. A uranium dioxide-based nuclear fuel sintered body was produced in the same manner.

The density and 0/U ratio of this uranium dioxide-based nuclear fuel sintered body were as follows.

1 gate 1: [: 95.3% T, I).

0/U ratio: 2.0G Also, when observed with a metallurgical microscope, the large grain size portion with an average grain size of 25 pm occupies most of the gold phase as shown in Figure 5, but there is also a small amount of small grain size portion. It was confirmed that it was a healthy uranium dioxide-based nuclear fuel sintered body with a

(Example 3) Instead of the mixing process in Example 1, Example 1
29% by weight of triuranium octoxide (the final mixture obtained by mixing uranium dioxide and triuranium octoxide)
The average particle size is 350μm triuranium octoxide 4% by weight (
Sintering of uranium dioxide-based nuclear fuel was carried out in the same manner as in Example 1, except that the mixing treatment was carried out in which the final mixture obtained by mixing uranium dioxide and triuranium octoxide was taken as 100%). manufactured a body.

The density and 0/U ratio of this uranium dioxide-based nuclear fuel sintered body were as follows.

Density: 95.7% T, D.

0/lJ ratio: 2.00 Furthermore, when observed with a metallurgical microscope, as shown in Figure 6, healthy dioxide was found to have a gold phase with an average grain size of 60 gm, in which the large grain portion with a grain size of 50 JLm or more occupied the majority. It was confirmed that it was a uranium-based nuclear fuel sintered body.

(Example 4) The roasting temperature in Example 1 was set to 500°C.

A uranium dioxide-based nuclear fuel sintered body was produced in the same manner as in Example 1 except that the sintering temperature and time were 1100° C. and 2 hours.

The density and 0/U ratio of this uranium dioxide-based nuclear fuel sintered body were as follows.

Density: 94.5% T, D.

0/U ratio 82.00 Furthermore, when observed with a metallurgical microscope, it was confirmed that it was a healthy uranium dioxide-based nuclear fuel sintered body with a homogeneous gold phase with an average particle size of approximately 20 pm as shown in Figure 7. did.

(Example 5) The roasting temperature in Example 1 was 450°C, the amount of triuranium octoxide was 35% by weight, and the sintering temperature and time were 1200°C and 2 hours. A uranium dioxide-based nuclear fuel sintered body was manufactured in the same manner as in Example 1 above.

The density and 0/U ratio of this uranium dioxide-based nuclear fuel sintered body were as follows.

Density: 95.0% T, D.

07U ratio: 2.0G When observed using a metallurgical microscope, it was confirmed that the uranium dioxide-based nuclear fuel sintered body had a gold phase with an average grain size of 20 #Lm as shown in FIG.

(Evaluation) By changing the roasting temperature according to Examples 1 and 2, the density and particle size distribution of the nuclear fuel sintered body can be changed. It is understood that the density and particle size distribution of the nuclear fuel sintered body can also be changed by changing the timing of addition of triuranium.

[Effects of the Invention] According to the present invention, at least one of the particle size of triuranium octoxide added to uranium dioxide, its addition timing, and the roasting temperature of uranium dioxide when producing triuranium octoxide can be adjusted. By doing so, it is possible to adjust the crystal density of the nuclear fuel sintered body to a desired value within the range of 93 to 88% TD and the crystal grain size within the range of 20 to 801 Lm.

[Brief explanation of drawings]

FIG. 1 is a flowchart outlining an example of the method of the present invention. FIG. 2 is a graph showing an example of the relationship between the roasting temperature of triuranium octoxide and the density (%TD) of a uranium dioxide-based nuclear fuel sintered body. FIG. 3 is a graph showing an example of the relationship between the roasting temperature of triuranium octoxide and the particle size of triuranium octoxide. FIG. 4 is a photograph substituted for a drawing obtained by observing the uranium dioxide-based nuclear fuel sintered body obtained in Example 1 at a magnification of 100 times using a metallurgical microscope. Figure 5 shows Example 2
This is a photograph substituted for a drawing taken at 100x magnification by observing the uranium dioxide-based nuclear fuel sintered body obtained in . FIG. 6 is a photograph substituted for a drawing taken by observing the uranium dioxide-based nuclear fuel sintered body obtained in Example 3 with a metallurgical microscope at a magnification of 100 times. FIG. 7 is a photograph substituted for a drawing obtained by observing the uranium dioxide-based nuclear fuel sintered body obtained in Example 4 at a magnification of 200 times using a metallurgical microscope. FIG. 8 is a photograph substituted for a drawing obtained by observing the uranium dioxide-based nuclear fuel sintered body obtained in Example 7 at a magnification of 100 times using a metallurgical microscope. Patent applicant Nuclear Fuel Industry Co., Ltd.

Claims (1)

[Claims] (1) A molded body obtained by molding a mixed powder of 75 to 55% by weight of uranium dioxide raw material powder and 25 to 45% by weight of triuranium octoxide is heated to 1,100 to 100% by weight in an oxidizing atmosphere.
After sintering at a temperature of 1,400°C, 1
In a method for producing a nuclear fuel sintered body, the nuclear fuel sintered body is heated at a temperature of 100 to 1,400°C and the oxygen/metal ratio (O/U) is adjusted to 1.98 to 2.02. The grain size of the nuclear fuel sintered body is adjusted by [1] adjusting the roasting temperature when roasting uranium oxide to produce the triuranium octoxide, and [2] mixing it with the raw material powder of uranium dioxide. The sintered body density of the nuclear fuel sintered body is adjusted by adjusting the amount of said triuranium octoxide, and the sintered body density of the nuclear fuel sintered body is adjusted by: [1] Roasting uranium oxide to produce said triuranium octoxide. [2] adjustment by the amount of the triuranium octoxide mixed into the uranium dioxide raw material powder; and [3] dividing the mixing period of the triuranium octoxide into an early period and a latter period; A method for producing a nuclear fuel sintered body, characterized in that adjustment is made by at least one of the following: adjustment by the particle size of the triuranium octoxide during late mixing.