JPS6246501B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing uranium dioxide powder with a low fluorine content from uranium hexafluoride by a wet method.

In the case of a wet process, uranium dioxide powder, which is a raw material for nuclear reactor fuel for power generation, is usually produced by the following process. That is, solid uranium hexafluoride (UF ₆ ) is heated and vaporized, and this uranium hexafluoride gas is brought into contact with pure water and hydrolyzed to form a dilute aqueous solution of uranyl fluoride (UO ₂ F ₂ ). Ammonia aqueous solution was added to form a precipitate of ammonium deuterate (ADU), this slurry solution was filtered, and the resulting cake was washed and dried.
The ADU powder obtained by pulverization is roasted in a reduction furnace (600 to 700
℃) to produce uranium dioxide powder. The above-mentioned uranium hexafluoride (UF ₆ ) gas is reacted with an aqueous ammonium carbonate solution to form a precipitate of uranyl ammonium carbonate (AUC), and the slurry solution is filtered. After washing and drying the resulting cake,
The AUC powder obtained by pulverization is roasted in a reduction furnace (600 to 700
℃) to produce uranium dioxide powder. Generally, the former is called the ADU method and the latter is called the AUC method.

These uranium dioxide powder manufacturing methods all use uranium hexafluoride (UF ₆ ) as a raw material, so the resulting uranium dioxide powder usually contains 50 to 150 ppm.
Some amount of fluorine remains. Therefore, in order to reduce the residual amount of fluorine as much as possible, the fluorine content in ADU powder or AUC powder obtained by washing the cake with pure water or alcohol, then drying and pulverizing it in the above-mentioned cake washing process. Approximately 0.5~
Reduced to 1.5% by weight and the ADU powder or AUC
The amount of fluorine remaining in uranium dioxide has been reduced by raising the temperature to a level that does not cause the sinterability of uranium dioxide to be lost during roasting and reduction of powder, and by flowing steam. However, even with these methods, uranium dioxide powder still contains 50~
It was normal for about 150 ppm of fluorine to remain. Most of these residual fluorine are removed during the pellet sintering process, but uranium dioxide powder with as low a fluorine content as possible is required for pellet production, and therefore there is a strong need for technology to reduce the fluorine content of uranium dioxide powder. ing.

The present inventors solved the drawbacks of the above-mentioned conventional methods and investigated to provide a method for producing uranium dioxide powder with a low fluorine content from uranium hexafluoride by a wet method. The inventors have discovered that the object can be achieved by adding an ammonium salt that thermally decomposes with heat generation and gas generation to an ammonium cake or an ammonium carbonate cake, and have arrived at the present invention. That is, the gist of the present invention is to provide a method for producing uranium dioxide powder from uranium hexafluoride via an ammonium deuterate cake or a uranyl ammonium carbonate cake, in which the ammonium deuterate cake or the uranyl ammonium carbonate 300℃ for cake
The ammonium salt that thermally decomposes with heat generation and gas generation is dried in a state containing 4 to 9% by weight (dry weight basis), and then this is heated at a temperature of 550 to 700°C to a reducing gas containing a water vapor flow. A method for producing uranium dioxide powder, characterized by roasting and reducing it in an atmosphere.

As the ammonium salt added in the present invention, which irreversibly thermally decomposes with heat generation and gas generation at a temperature of 300° C. or higher, ammonium fluoride, ammonium nitrate, ammonium carbonate, ammonium oxalate, etc. are suitable. In the present invention, ammonium deuterate cake (hereinafter referred to as ADU cake) and uranyl ammonium carbonate cake (hereinafter referred to as ADU cake)
without cleaning the AUC cake (called AUC cake).
One or two of the above-mentioned ammonium salts are uniformly added to these cakes at a ratio of 4 to 9% by weight (dry weight basis), and the cake is dried in a state in which it is contained, and then this is dried.
When roasted and reduced in a reducing gas atmosphere containing steam flow in the temperature range of 550 to 700°C, the ammonium salts uniformly contained in these dried cakes generate heat and gas due to thermal decomposition. ,
This generated gas and the steam flow drive out the fluorine contained in the ADU powder or AUC powder, and raise the roasting reduction temperature by generating heat, thereby promoting the expulsion of fluorine. This makes it possible to significantly reduce the amount of fluorine remaining in the obtained uranium dioxide powder compared to conventional methods.

The ammonium salt content in the ADU powder or AUC powder is 4 to 9% by weight (dry weight basis). If the ammonium salt content is less than 4% by weight, the defluorination effect will be small, and if it exceeds 9% by weight, the amount of uranium dioxide obtained will be relatively small, which is industrially disadvantageous.

FIG. 1 is a graph showing the relationship between the ammonium salt content in ADU powder and the amount of residual fluorine in UO ₂ powder after roasting and reduction. The curve shows that when NH ₄ F is included as an ammonium salt, the curve shows that NH ₄ NO ₃
+1% by weight of NH ₄ F was contained, and the roasting reduction temperature was 650°C in both cases.

As shown in the figure, as the content of NH ₄ F or NH ₄ NO ₃ increases, the amount of residual fluorine in the UO ₂ powder obtained by roast reduction of ADU powder or AUC powder decreases. are doing. In the case of NH ₄ F, the opposite phenomenon is shown in that the more _fluorine compounds are included, the better the removal of fluorine _is. Since the amount of heat generated by thermal decomposition of F increases, it is thought that the same effect as that of further increasing the roasting reduction temperature will appear as a result. Moreover, in this case ADU
Since the powder is heated from the inside rather than from the outside, it is thought to further improve the fluoridation removal effect. As the figure shows, in the case of NH ₄ F alone as shown in the curve, or when NH ₄ F is added to NH ₄ NO ₃ as shown in the curve, the defluorination effect with residual fluorine content of 50 ppm or less is seen at 4% by weight. In the above case, the higher the ammonium salt content, the lower the amount of residual fluorine, but the higher the ammonium salt content, the lower the amount of uranium dioxide obtained. The upper limit of ammonium content is 9% by weight. Although Figure 1 is for ADU powder, similar results can be obtained for AUC powder.

The roasting reduction temperature of AUD powder or AUC powder in the present invention is in the range of 550 to 700°C. If the roasting reduction temperature is lower than 550°C, the amount of residual fluorine in the obtained UO ₂ powder will be 50 ppm or more, and if it exceeds 700°C, the defluoridation effect will improve, but the activity of the obtained UO ₂ powder will decrease. is lost.

Figure 2 shows the roasting reduction temperature of ADU powder and the obtained
FIG. 2 is a graph diagram showing the relationship between the amount of fluorine remaining in UO ₂ powder.

The curve is for NH ₄ F3 wt% content;
In the case of containing 8% by weight of NH ₄ F, the curve shows the case of containing 3% by weight of NH ₄ NO ₃ and 5% by weight of NH ₄ F, and in the curve, cake washing is performed, and in the curve, cake is not washed.

As the curve shows, even when the ammonium salt (NH ₄ F) content is as low as 3% by weight, when the roasting reduction temperature is set to 700°C or higher, the amount of residual fluorine in the obtained UO ₂ powder can be reduced to 50 ppm or less. Although it is possible to reduce the roasting reduction temperature, the upper limit is 700°C because the activity of the UO ₂ powder will be lost and problems will arise in the sinterability of the pellets. Also, the curve,
As shown, a sufficiently good defluoridation effect can be obtained by roasting and reducing NH ₄ F at a temperature of 700° C. or lower. On the other hand, in order to suppress the amount of residual fluorine in the UO ₂ powder to 50 ppm or less, the roasting reduction temperature should not be lower than 550°C.

As mentioned above, the ammonium salts contained in ADU or AUC are exothermically decomposed during torrefaction reduction, and NH ₄ F in these ammonium salts is the starting material of ADU or AUC.
It is the most desirable one in that it already exists in it. In addition, in order to impregnate ADU or AUC with these ammonium salts, by adding ammonia water in these precipitation generation steps,
NH ₄ F is also generated during ADU or AUC over time.
Impregnated with _NH4F . Furthermore, it is convenient to add aqueous ammonia after adding an aqueous solution of nitric acid or uranyl nitrate, since NH ₄ NO ₃ is impregnated into the precipitate.

Another effect of the present invention is that by omitting the cake cleaning step, 2 to 4
This makes it possible to significantly reduce the cost.

As described above, the present invention provides a method that enables the production of uranium dioxide powder from uranium hexafluoride using a wet method, which greatly reduces process waste liquid by omitting the cake cleaning step, and has an extremely low amount of residual fluorine. It is extremely useful for producing nuclear fuel.

Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to the following Examples unless the gist thereof is exceeded.

Example 1 Gasify UF ₆ and absorb water to produce 140g
Make a ₂ -solution UO ₂ F, add 28% by weight ammonia water to it, and stir until the pH becomes 10.
An ADU slurry was made, and slurry 1 was taken and passed through paper to obtain an ADU cake. The cake was dried without washing at a temperature of 120°C for about 5 hours. The resulting ADU powder contains 7% by weight of _NH4F .
This ADU powder is heated in a hydrogen-reducing atmosphere containing a stream of water vapor.
Roast reduction was performed at a temperature of 650°C for about 30 minutes to obtain UO ₂ powder. The residual fluorine in this UO ₂ powder was 16 ppm.

Example 2 Gasify UF ₆ and absorb it into water to produce 140g
Make U/UO ₂ F ₂ liquid 1 and add concentrated nitric acid 41 to this.
cc and then 28 wt% ammonia water to PH
Add and stir until the value reaches 10 to make ADU slurry, and pass this slurry 1 through paper.
Got ADU cake. This ADU cake was dried at a temperature of 120° C. for about 5 hours without washing. The obtained ADU powder contains 7% by weight of NH ₄ F and 1% by weight of NH ₄ NO ₃
including. This ADU powder was roasted and reduced under the same conditions as in Example 1 to obtain UO ₂ powder. The residual fluorine in this UO ₂ powder was 9 ppm.

Example 3 Gasify UF ₆ and absorb it into water to produce 140g
Make a _2- solution UO ₂ F of U/, add 20% by weight ammonium carbonate aqueous solution to it, and stir until the pH becomes 8.5 to make AUC ((NH ₄ ) ₄ UO ₂ (CO ₃ ) ₃ ) slurry. This slurry 1 was taken and filtered through paper to obtain AUC cake. This AUC cake was dried at a temperature of 120° C. for about 5 hours without washing. The resulting AUC cake contains 6% by weight of NH ₄ F. This AUC powder was roasted and reduced under the same conditions as in Example 1 to obtain UO ₂ powder. The residual fluorine in the UO ₂ powder in this case was 19 ppm.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the ammonium salt content in ADU powder and the amount of residual fluorine in UO ₂ powder after roast reduction, and Figure 2 shows the relationship between the roast reduction temperature of ADU powder and UO ₂ powder. It is a graph figure showing the relationship with the amount of residual fluorine inside.

Claims (1)

[Claims] 1. A method for producing uranium dioxide powder from uranium hexafluoride via an ammonium deuterate cake or a uranyl ammonium carbonate cake,
4 ammonium salts that thermally decompose with heat generation and gas generation at 300°C or higher are added to the ammonium diuranate cake or the uranyl ammonium carbonate cake.
A method for producing uranium dioxide powder, which is characterized by drying it in a state containing ~9% by weight (dry weight basis), and then roasting and reducing it at a temperature of 550 to 700°C in a reducing gas atmosphere containing a stream of water vapor. . 2. The method for producing uranium dioxide powder according to claim 1, wherein the ammonium salt is ammonium fluoride. 3. The method for producing uranium dioxide powder according to claim 1, wherein the ammonium salt is ammonium nitrate.