JP2813926B2 - Uranium dioxide powder for nuclear fuel and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a uranium dioxide powder for nuclear fuel and a method for producing the same, and more particularly, to a method for producing nuclear fuel pellets having large crystal grains by using a low-temperature redox sintering method. The present invention relates to a raw material uranium dioxide powder and a method for producing the same.

[0002]

2. Description of the Related Art Nuclear fuel pellets are packed in fuel rods and used as nuclear fuel in a nuclear power plant. In the manufacture of the nuclear fuel pellets, uranium dioxide powder is sintered to increase the crystal density to produce ceramic fuel. used.

Conventionally, uranium dioxide powder for nuclear fuel has been mainly used for pellet production by high-temperature reduction sintering, and the O / U ratio of the powder is determined by any powder production method including low-temperature reduction sintering. Even if it is a thing, it is set to about 2.05 to 2.15. This is because uranium dioxide has the property of being easily associated with oxygen, and in particular, as the O / U ratio is closer to 2.0, this property is stronger. Therefore, in order to obtain powder stability during handling during pellet production, The powder is appropriately oxidized at the time of finishing the production of the powder to have an O / U ratio of about 2.05 to 2.15.

[0004] Further, in the powder sintering step, an empirical O / U ratio of about 2.10 gives an appropriate pellet density, which is another reason for setting the O / U ratio as described above. One.

In the ADU-UO ₂ production method for producing uranium dioxide (UO ₂ ) from, for example, ammonium biuranate (ADU), the powder having the above O / U ratio is roasted to produce U ₃ O _8. After heating in a reducing atmosphere containing hydrogen to form UO _{2 · 0} , the UO _{2 · 0} is exposed to N ₂ gas containing a trace amount of oxygen to prevent re-oxidation during the handling. It is stabilized as UO _{2 + x} (where X = 0.05 to 0.15).

[0006]

When the crystal grain size is increased by the low-temperature oxidation-reduction sintering method,
The conventional uranium dioxide powder, UO _{2 + x,} where X = 0.
If the 05-.15, a U ₃ O ₈ should be added 25wt% ~45wt% are pointed out.

However, the specific amount of the U ₃ O _8, said U ₃ O ₈ before the uranium dioxide material powder added O /
It varies depending on the U ratio, the activity, the crystal grain size of the pellet to be obtained, the density, and the like. Therefore, in determining the amount of U ₃ O ₈ to be added, the O / U ratio of the uranium dioxide raw material powder is determined. It is necessary to know the activity first, and it is necessary to determine the amount of U ₃ O ₈ to be added from the general relationship between the amount of U ₃ O ₈ added and the crystal grain size. No raw material powder could be obtained.

According to the present invention, as a result of repeating various experiments in order to deal with the above-mentioned situation, when the same gold phase is obtained in the low-temperature redox sintering method, the final uranium dioxide powder O / U is obtained. The inventors have found that the ratio is an important parameter, and as a result, succeeded in obtaining an optimum uranium dioxide raw material powder for large-diameter pellets in the low-temperature sintering.

[0009]

That is, the present invention provides U ₃ O ₈
Is integrated by low-temperature redox sintering
Nuclear fuel to form nuclear fuel pellets of the required size
A method of manufacturing a use uranium dioxide _powder, U 3 _{O 8}
In reducing U , the contact time between U ₃ O ₈ and the reducing atmosphere is
Required for uranium dioxide powder reduced by adjustment
Leaving the amount of _U 3 _{O 8,} the by _U 3 _{O 8} that this remaining
1. O / U ratio of uranium dioxide powder after reduction as a whole
It is characterized in that it is adjusted to 26 to 2.38. In addition,
Grinding and mixing the remaining U ₃ O ₈ in the above method
Measures to homogenize the uranium dioxide powder after reduction.
Both are effective. In addition, the low temperature produced by the above method
Nuclear fuel pellets of the required size integrated by redox sintering
It also features uranium dioxide powder for nuclear fuel that forms
Uranium dioxide powder reduces U ₃ O ₈
When returning, the reduction must be stopped halfway and at least
The remaining reduced U ₃ O ₈ remains, and its O / U ratio is 2.26.
It is characterized by being adjusted to a range of up to 2.38.

[0010]

When the uranium dioxide powder for a nuclear fuel of the present invention having the above O / U ratio is used to produce the fuel pellets by a low-temperature oxidation-reduction sintering method, a suitable fuel pellet having a large crystal grain size is used. It is possible to obtain it reliably. Further, several percents of U ₃ O ₈ can be added in order to make effective use of the scrap and adjust the density and the crystal grain size. Further, according to the production method of the present invention, uranium dioxide powder having the above O / U ratio can be produced simply and efficiently.

[0011]

EXAMPLES Specific examples of the present invention will be described below together with details of the present invention. First, the uranium dioxide powder for a nuclear fuel of the present invention is integrated by low-temperature oxidation-reduction sintering as described above to form nuclear fuel pellets of a certain size. For example, ADU (ammonium biuranate) or A
In the production process of reducing U ₃ O ₈ obtained by roasting UC (ammonium uranyl carbonate) to obtain uranium dioxide powder, by adjusting the contact time between U ₃ O ₈ and a reducing atmosphere, part of the carbon dioxide is reduced. U ₃ O ₈ is left in the uranium, and the O / U ratio after reduction is reduced by the remaining U ₃ O _8.
It is adjusted to 2.26 to 2.38, preferably 2.30 to 2.34. One embodiment of the present invention according to the above method will be described below.

Ammonia water was added to the uranyl nitrate solution to obtain an ADU precipitate. The ADU was recovered, dried and crushed to obtain an ADU powder of about 100 μ or less. Next, this ADU powder was heated in air at about 600 ° C. to make U ₃ O ₈
The powder was obtained, and the U ₃ O ₈ powder was formed into a bed (layer) having a thickness of about 1 cm and heated at 600 ° C. in a mixed gas stream of 3H ₂ / N ₂ in a batch furnace.

The reduction of U ₃ O ₈ progressed from the surface of the bed to the inside with time, although a slight difference was recognized between the upstream side and the downstream side of the airflow. After the obtained powder was pulverized and mixed by a cutter mill for homogenization, the O / U ratio was measured. FIG. 1 shows the relationship between the heating time and the O / U ratio. That is, in the case of the manufacturing method of this embodiment, the optimum O / U
The heating time (T) at which the ratio becomes 2.26 to 2.38 may be selected. In general, the curve in FIG. 1 varies depending on the thickness of the bed layer and the microscopic characteristics of U ₃ O ₈ obtained from ADU and its heating.
It is preferable to set the optimal heating time by a small amount of a preliminary test each time.

In the embodiment, the reduction is in the form of a bed, but it is basically possible to use a rotary kiln (a type of gas flow mixer) or a fluidized bed. That is, ADU
And U ₃ O ₈ obtained by roasting AUC have crystals of 1 μm or less densely gathered to form a lump of 100 μm or less. UO ₂ of U ₃ O ₈ is formed from a U ₃ hydrogen diffusion into the O ₈ → reaction → generated of H ₂ O diffusional release, H ₂
O diffusion becomes the rate-limiting factor of the reaction, and not all U ₃ O ₈ is converted into UO ₂ instantaneously due to a temporal reaction. Therefore, even in a rotary kiln or a fluidized bed, the O / U ratio can be adjusted by controlling the residence time of U ₃ O ₈ in the reactor.

On the other hand, the uranium dioxide powder of the present invention having an O / U ratio of 2.26 to 2.38 can be produced by another method, that is, a method of adding U ₃ O ₈ provided separately, and will be described in detail below. When the crystal grain size is increased by the low-temperature oxidizing atmosphere sintering method, the conventional powder, that is, the O / U ratio of UO _{2 + x}
In the case where _x is set to 0.05 to 0.15, U ₃ O ₈
% To 45 wt%. However, the specific amount of addition varies depending on the O / U ratio and activity of the raw material powder, the crystal grain size and density of the pellet to be obtained.

For example, FIG. 2 shows that raw material powder having an O / U ratio of 2.13 is molded at a molding density of 5.5 g / cm ³ , at 1150 ° C. × 2 hours, and oxygen concentration.
Heat in nitrogen at 30 ppm and continue at 1150 ° C x 1 hr, 2
The sample was heated in H ₂ + N ₂ and had a density of 96% TD.

FIG. 3 shows that U ₃ O ₈ is added to the raw material powder.
This is a sample manufactured under the same conditions as above after adding and mixing 20 wt%, and has a density of 97% TD. FIG. 4 shows the case where the added amount of U ₃ O ₈ is 25 wt%, FIG. 5 shows the case where the amount is 30 wt%, and FIG. 6 shows the case where the amount is 35 wt%.
TD, 96% TD, 95% TD. That is, as is apparent from FIGS. 2 and 3, no significant increase in the particle size occurs when the addition is 20 wt% or less, and a remarkable increase in crystal grains is observed when the addition is 25 wt% or more shown in FIGS.

The above is summarized by the O / U ratio after the addition of U ₃ O _{8. In} the case of FIG. 2, 2.13 × 100/100 + 8/3 × 0/100 = 2.13 In the case of FIG. 3, 2.13 × 80 / 100 + 8/3 x 20/100 = 2.24 In the case of Fig. 4, 2.13 x 75/100 +8/3 x 25/100 = 2.26. In the case of Fig. 5, 2.13 x 70/100 +8/3 x 30/100 = 2.29 In the case of FIG. 6, 2.13 × 65/100 + 8/3 × 35/100 = 2.32.

Therefore, in order to obtain large-diameter pellets by low-temperature sintering, even if U ₃ O ₈ is added as a powder, an O / U ratio of 2.26 or more is essential, and the upper limit of the pellets is In order to maintain the sintering density, it is preferably 2.38 or less as described above. If the green compact density of the uranium dioxide powder is increased to 6.1 g / cm ³ , the U ₃ O ₈ becomes 45 wt%.
Even if added, a sample of 94% TD at the current level can be obtained, but this is not practical.

Further, in the above-mentioned method of adding U ₃ O ₈ to uranium dioxide powder, it is possible to effectively utilize UO ₂ scrap which is inevitably generated in the pellet production process. That is, the UO ₂ scrap is converted into U ₃ O ₈ and mixed into the uranium dioxide powder. In this case, since the O / U ratio of the uranium dioxide powder increases due to the mixing of U ₃ O ₈ from the scrap, the uranium dioxide O / U ratio before mixing is appropriately 2.20 to 2.34.

For example, the O / U ratio of uranium dioxide powder is set to 2.
When 25 wt% of U ₃ O ₈ from the scrap was mixed, the total O / U ratio was 0.95 × 2.25 + 0.05 × 8/3 =
It becomes 2.27. That is, in this case, UO ₂ scrap is reduced by 5%.
This means that uranium dioxide powder having an O / U ratio of 2.26 to 2.38, which is an essential requirement of the present invention, was produced. In this case, the uranium dioxide powder having an O / U ratio of 2.20 to 2.34 before mixing can be produced by the method of the first embodiment.

[0022]

As described above, the urethane dioxide of the present invention is
The production method of the powdered powder is based on reduction of U ₃ O _8.
In the process of producing uranium oxide powder, the U ₃ O ₈ is
When reducing, the contact time between U ₃ O ₈ and the reducing atmosphere is adjusted.
Required amount of uranium dioxide powder reduced by
Leave _U 3 _{O 8,} instead of the by _U 3 _{O 8} that this remaining
The O / U ratio of the original uranium dioxide powder is 2.2 as a whole.
It is adjusted to 6 to 2.38, and has the O / U ratio.
Uranium dioxide powder that is simple and efficient to produce
It has a great effect. Further, it is produced by the method of the present invention.
Uranium dioxide powder for nuclear fuel is integrated by low-temperature sintering
Of uranium dioxide powder to form nuclear fuel pellets
/ U ratio adjusted to 2.26 to 2.38,
Low-temperature redox firing using uranium powder having a low O / U ratio
When producing nuclear fuel pellets by the sintering method, large crystal grain size
It is possible to reliably obtain suitable fuel pellets having
It has an effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the reduction time of U ₃ O ₈ and the O / U ratio of homogenized UO ₂ powder after pulverization and mixing.

FIG. 2 is a 200 × photomicrograph showing the metal structure of a fuel pellet obtained by sintering UO ₂ powder having an O / U ratio of 2.13 at a low temperature.

FIG. 3 shows a UO ₂ powder having an O / U ratio of 2.13 mixed with U ₃ O ₈ at 20 wt.
5 is a photomicrograph of 200 times showing a metal structure of a fuel pellet which was sintered at a low temperature after adding and mixing the same.

FIG. 4 shows a UO ₂ powder having an O / U ratio of 2.13 mixed with U ₃ O ₈ at 25 wt.
5 is a photomicrograph of 200 times showing a metal structure of a fuel pellet which was sintered at a low temperature after adding and mixing the same.

FIG. 5 shows a UO ₂ powder having an O / U ratio of 2.13 mixed with U ₃ O ₈ at 30 wt.
5 is a photomicrograph of 200 times showing a metal structure of a fuel pellet which was sintered at a low temperature after adding and mixing the same.

FIG. 6 shows a UO ₂ powder having an O / U ratio of 2.13 with U ₃ O ₈ at 35 wt.
5 is a photomicrograph of 200 times showing a metal structure of a fuel pellet which was sintered at a low temperature after adding and mixing the same.

Claims (3)

(57) [Claims] 1. U ₃ O ₈ Low temperature oxidation by reducing
Nuclear fuel pellets of the required size integrated by reduction sintering
Of uranium dioxide powder for nuclear fuel to form cuts
In a method, the U ₃ O ₈ In reducing ₃ O ₈
Is reduced by adjusting the contact time between
Of uranium dioxide powder ₃ O ₈ And leave this
Survived U ₃ O ₈ Of the uranium dioxide powder after the reduction
Adjust the O / U ratio as a whole to 2.26 to 2.38.
For producing uranium dioxide powder for nuclear fuel characterized by the following:
Law. 2. The method of claim 1, wherein said residual
U ₃ O ₈ By crushing and mixing,
Atomic fuel characterized by homogenizing uranium dioxide powder
Of producing uranium dioxide powder for use in foodstuffs. 3. U ₃ O ₈ Low temperature oxidation by reducing
Nuclear fuel pellets of the required size integrated by reduction sintering
Uranium dioxide powder for nuclear fuel that forms
Uranium dioxide powder is U ₃ O ₈ On the way to reduce
Stop reduction and at least required amount of unreduced U ₃ O ₈ Remains
The O / U ratio of which is 2.26 to 2.38.
Diacid for nuclear fuel characterized by being adjusted to a range
Uranium powder.