JPH09127280A - Production of nuclear fuel particle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a large particle size fuel particle for high burnup by using an redox low temperature sintering method without adding U3 O8 . SOLUTION: By heating ADU particle produced with exterior gelation method in the air at 800 deg.C or lower, gelling agent such as polyvinyl alcohol is removal to make it UO3 or U3 O8 particle. This particle of UO3 or U3 O8 is first heated and sintered in slightly oxidizing atmosphere and then heated in reducing atmosphere to make the O/U ratio of the above UO3 or U3 O8 particle be 2.0.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear fuel particle used in a high temperature gas reactor or the like, and more particularly to a method for producing a high burnup fuel particle having a coarse crystal grain size.

[0002]

2. Description of the Related Art As a fuel for a high temperature gas reactor and a fuel for a vibration filling method, a particulate fuel in which a UO ₂ fuel core is coated in multiple layers with high density carbon, SiC or the like, which is called a coated particle fuel, is used.

In the case of these fuels for high temperature gas reactors and fuels of the vibration filling method, the high burnup which has been promoted in light water reactor fuels has not been discussed at present, and the grain size increase of crystals has not been examined, In the future, it will be necessary to increase the particle size in terms of FP retention characteristics.

In pellet fuels as opposed to particulate fuels,
(1) additives, (2) high temperature sintering method for increasing the particle size,
(3) Active powder usage method, (4) Redox high temperature sintering method,
(5) A method such as a redox low temperature sintering method is conceivable. Among these, the oxidation-reduction low-temperature sintering method (5) has many advantages because it does not rely on additives and does not require high-temperature heating.

On the other hand, the particulate fuel is usually produced by a gelation method by calcination and sintering a gelled solution of uranium-containing solution in a medium of ADU (ammonium diuranate). To be done. Therefore, the above (1)
The methods of (4) to (4) can be easily applied to the production of this particulate fuel, but the oxidation-reduction low-temperature sintering method of (5), in the case of UO ₂ powder starting from ADU, It is necessary to add U ₃ O ₈ in a required ratio before sintering for the purpose of increasing the grain size. From this, it is considered that such an operation is difficult in the above-mentioned gelatinization method.

[0006]

SUMMARY OF THE INVENTION The present invention addresses the above situation and performs the oxidation-reduction low temperature sintering method without adding the U ₃ O ₈ to obtain a large burnup for high burnup. It is intended to produce fuel particles having a particle size.

[0007]

That is, in the method for producing nuclear fuel particles of the present invention which meets the above-mentioned object, ADU (ammonium diuranate) particles produced by the external gelation method are heated to about 400 ° C in air. P by heating at 800 ℃
A gelling aid such as VA (polyvinyl alcohol) is removed to form UO ₃ or U ₃ O ₈ particles, and these UO ₃ or U ₃ O ₈ particles are first heated and sintered in a slightly oxidizing atmosphere. Then, by heating in a reducing atmosphere, the UO
The O / U ratio of ₃ or U ₃ O ₈ particles is 2.0 ± 0.05.

Further, in the above-mentioned manufacturing method of the present invention, the slightly oxidizing atmosphere is changed to an oxygen concentration of 10 ^{−3 to} 5 × 10 ² ppm.
Inert gas such as N ₂ or CO / CO _{2 with the} same oxygen concentration
As a gas, in the slightly oxidizing atmosphere, the UO ₃ or U
₃ O ₈ particles are heated at a temperature of about 1000 to 1400 ° C. for about 30
After heating / sintering for 3 minutes to 3 hours, it is also possible to heat at about 1000 to 1400 ° C. for about 30 minutes to 3 hours in the reducing atmosphere containing hydrogen.

Further, in each of the above-mentioned production methods of the present invention, UO ₃ or U ₃ O obtained by removing the gelling aid is obtained.
Before heating and sintering the ₈ particles in the above-mentioned slightly oxidizing atmosphere, they are heated in a reducing atmosphere at about 400 ° C to 900 ° C to obtain the above-mentioned UO.
It is also possible to adjust the O / U ratio of the ₃ or U ₃ O ₈ particles and the stability of the particles.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A general method for producing a particulate fuel by the external gelation method is as shown in FIG. Here, in the case of the external gelation method, since an organic binder such as PVA (polyvinyl alcohol) is required as an auxiliary agent for gelation, a step for decomposing and removing these is necessary before sintering. This is similar to the binder removal step during pellet production, the heating temperature is around 900 ° C in the reducing atmosphere, and the particle shape must be maintained in the air (in the oxidizing atmosphere) at approximately 700 ° C. Before and after.

On the other hand, thermogravimetric analysis of ADU spheres produced by the gelation method gave the results shown in FIG. Furthermore, when UO ₃ particles obtained by heating at about 500 ° C. in an oxidizing atmosphere were heated in a reducing atmosphere, a weight change as shown by the broken line in the figure was shown.

When large particle size particles are obtained by redox low temperature sintering of UO _{2 +} x via ADU, carbon (C: carbon) becomes an atmospheric poison because it bonds with oxygen in the atmosphere. Therefore, in order to use this method, it is necessary to remove carbon in the material.

Further, in the case of oxidation-reduction low temperature sintering, the material is easily made to have a predetermined O / value in accordance with the oxygen potential in the atmosphere.
Although it is necessary to obtain a U ratio, UO ₃ and U ₃ O ₈ produced from ADU by the method of the present invention, and UO ₂₊ x (0 ≦ x ≦ 0.05) obtained by reducing UO ₃ are
It was confirmed that the O / U ratio defined by the temperature and the oxygen potential was easily obtained.

In the low temperature redox sintering method, U
It is said that it is important to pass through O _{2 .25} (U ₄ O ₉ ). Therefore, ADU particles obtained by the external gelation method are heated in air to form UO ₃ or U ₃ O ₈ particles at a predetermined O 2.
By heating in an atmosphere having a / U ratio (= 2.25), spherical fuel particles having large-sized crystals can be obtained.

The present invention clarifies the heating atmosphere and conditions.

[0016]

EXAMPLES Examples of the present invention will be further described below.

(1) Example 1 ADU particles containing about 4% PVA and having a particle size of about 1.3 mm obtained by the external gelation method were heated in air at 500 ° C. for 3 hours to give a particle size of about 1.2 mm. UO ₃ particles were obtained. Then, the particles were heated and sintered in a slightly oxidizing atmosphere, and the particles were heated to 1100 in N ₂ gas having an oxygen concentration of 40 ppm.
After heating up to 200 ° C. at a heating rate of 200 ° C./hour and holding at 1100 ° C. for 30 minutes, a hydrogen atmosphere (75
% H ₂ / N ₂ ) to heat UO ₂
I got a sphere. This UO ₂ sphere has a diameter of about 0.6 mm and a density of about 98.0% T. D, the average crystal grain size was about 80 μm.

(2) Example 2 UO ₃ particles produced in the same manner as in Example 1 were treated with 75% H
_{In a 2} / N ₂ atmosphere, heat at about 410 ° C for 1 hour, and
A ball with a U ratio of about 2.4 was obtained. Next, as heating / sintering in a slightly oxidizing atmosphere, the particles were held at a temperature of 1100 ° C. for 30 minutes in a nitrogen atmosphere having an oxygen concentration of 70 ppm,
The same reduction heating as in Example 1 was performed to obtain UO ₂ spheres. This UO ₂ sphere has a diameter of about 0.6 mm and a density of about 98.5% T.
D, the average crystal grain size was about 85 μm.

(3) Example 3 UO ₃ particles produced in the same manner as in Example 1 were treated with 75% H
UO ₂ particles were obtained by heating at about 500 ° C. in a ₂ / N ₂ atmosphere. However, the O / U ratio was about 2.1. It was speculated that this was partially oxidized at the time of extraction. The particles were heated in a slightly oxidizing atmosphere and a reducing atmosphere in the same manner as in Example 2 to give a diameter of about 0.6 mm and a density of about 98.7% T.S.
D, UO ₂ particles having an average crystal grain size of about 90 μm were obtained.

(4) Example 4 ADU particles produced in the same manner as in Example 1 were subjected to 6 in air.
U ₃ O ₈ particles were obtained by heating to 00 ° C. The particles retained perfect spheres and did not collapse. The particles were heated in the same slightly oxidizing atmosphere and reducing atmosphere as in Example 1 to obtain UO ₂ particles having a diameter of about 0.6 mm. Density is 97.5% T. D, average crystal grain size is about 57 μm
Met.

(5) Comparative Example 1 ADU particles obtained by the same method as in Example 1 were kept in a reducing atmosphere at 900 ° C. for 1 hour, then heated to 1600 ° C. and kept at this temperature for 1 hour. UO _{2 with a} diameter of about 0.6 mm
The particles were obtained. Density is 98.2% T. However, the average crystal grain size was about 12 μm.

(6) Comparative Example 2 UO ₃ particles were obtained by keeping ADU particles obtained by the same method as in Example 1 in a reducing atmosphere at about 310 ° C. for 1 hour. This was heat-treated in a slightly oxidizing atmosphere and a reducing atmosphere in the same manner as in Example 1 to obtain UO ₂ particles. Density is 98.7% T. However, the average crystal grain size was 2 μm or less.

When UO ₃ or U ₃ O ₈ particles are produced from ADU particles by heating in air, if the temperature exceeds 800 ° C., the shape changes and the material becomes brittle, which is difficult to handle.

In a reducing atmosphere, from around 700 ° C.,
Decomposition / removal of organic substances such as PVA in the ADU particles starts and is almost completed at around 900 ° C. Using the UO ₃ or U ₃ O ₈ particles obtained in this way, almost the same results can be obtained by carrying out the oxidation, reduction and sintering as shown in the above Example (1), but it is economically disadvantageous. Is.

Furthermore, when the ADU particles are directly heated in a slightly oxidizing atmosphere, they pass through any of the shaded portions in FIG.
Particles having crystals of large particle size similar to those in Example (1) can be obtained, but it is considered economically disadvantageous because it is difficult to treat and adjust ammonia, CO _{2 and the} like generated by decomposition.

The sintering temperature and the reduction temperature can be 1000 ° C. or less, but the time becomes unnecessarily long and 140
Although it is possible to use it at 0 ° C. or higher, it is substantially the same as the high temperature sintering.

[0027]

As described above, according to the method for producing nuclear fuel particles of the present invention, the ADU particles produced by the external gelation method are heated in the air at 800 ° C. or lower to give a gelling aid such as PVA. The particles are removed to form UO ₃ or U ₃ O ₈ particles, and the UO ₃ or U ₃ O ₈ particles are first heated and sintered in a slightly oxidizing atmosphere, and then in a reducing atmosphere to obtain the above UO ₃ or U ₃ O ₈ particles. The O / U ratio of ₃ or U ₃ O ₈ particles is set to about 2.0, and the vibration filling method fuel in which the particles are packed in the cladding tube and the particle fuel for high temperature gas reactor are used as large crystals for high burnup. It has a remarkable effect that it can be economically produced as particles having a particle size.

[Brief description of the drawings]

FIG. 1 is a chart showing a method for producing a particulate fuel by a typical external gelation method.

FIG. 2 is a graph showing a thermogravimetric analysis result of ADU particles manufactured by an external gelation method.

Claims (3)

[Claims] 1. A gelling aid such as polyvinyl alcohol is removed by heating ammonium biuranate particles produced by the external gelation method in air at about 400 ° C. to 800 ° C. to remove UO ₃ or U _3. particles and none of the O _8, the UO
_By heating and sintering ₃ or U ₃ O ₈ particles in a slightly oxidizing atmosphere and then in a reducing atmosphere, the O / U ratio of the UO ₃ or U ₃ O ₈ particles is set to 2.0 ± 0.05. A method for producing nuclear fuel particles, characterized by comprising: 2. The oxygen concentration in the slightly oxidizing atmosphere is 10.sup.- ³ .
The UO ₃ or U ₃ O ₈ particles are heated at a heating temperature of about 1000 to 1 in this slightly oxidizing atmosphere as an inert gas such as 5 × 10 ² ppm of N ₂ or CO / CO ₂ gas having the same oxygen concentration.
After heating and sintering at 400 ° C for about 30 minutes to 3 hours, at about 1000 to 1400 ° C for about 3 in the reducing atmosphere containing hydrogen.
The method for producing nuclear fuel particles according to claim 1, wherein the heating is performed for 0 minutes to 3 hours. 3. UO ₃ or U ₃ O ₈ particles obtained by removing the gelling aid are heated at about 400 ° C. to 900 ° C. in a reducing atmosphere before heating and sintering in the slightly oxidizing atmosphere. The method for producing nuclear fuel particles according to claim 1 or 2, wherein the O / U ratio of the UO ₃ or U ₃ O ₈ particles and the particle stability are adjusted.