JPH0634056B2 - Manufacturing method of nuclear fuel sintered body - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for producing a nuclear fuel sintered body having excellent fuel characteristics.

[Background of the Invention]

Nuclear fuel commonly used in nuclear reactors is uranium dioxide (UO).
This powder is crushed, granulated or mixed with a metal oxide such as ₂ ) as a constituent, molded, and then sintered in a reducing atmosphere to be used as a sintered pellet. Further, conventionally, an organic compound containing any one of carbon, oxygen, hydrogen, and nitrogen as a pore-forming agent as a constituent element, or uranium trioxide (U ₃ O ₈ ) having the same degree of enrichment as the UO ₂ powder to be mixed. There is also known a method in which the powder is mixed with UO ₂ powder and then molded and sintered. By the way, generally, the pores in the pellet generated by the pore-forming agent are such that the gaseous fission product (FP gas) generated when the pellet is sintered in the reactor is stored in that portion and is not discharged to the outside of the pellet. It is used to

However, in the conventional sintered pellet, the distribution of these pores in the pellet is merely produced so as to have a spatially uniform distribution in the pellet, and the amount of FP gas released There is a problem that the reduction effect is not always sufficiently satisfactory.

[Outline of Invention]

The present invention provides a method for producing a nuclear fuel sintered body having excellent combustion characteristics by enhancing the trapping property of FP gas into the pores inside the pellet to further reduce the amount of FP gas released outside the pellet. It is intended to be provided.

In order to achieve the above object, the method for producing a nuclear fuel sintered body according to the present invention is (a) a nuclear fuel metal oxide powder, and (b) a nuclear fuel having a higher oxidation state and a higher enrichment than (a). It is characterized in that a nuclear fuel raw material powder made of a mixture with a metal oxide powder for use is shaped, and the shaped body is further sintered in a reducing atmosphere.

When nuclear fuel oxide powder, especially UO ₂ powder, was added and mixed with U ₃ O ₈ having the same degree of enrichment, and molded, and sintered in a reducing atmosphere, it was spatially and uniformly distributed in the molded body for practical use. It is already known that U ₃ O ₈ is reduced to UO ₂ during sintering and its volume is reduced to generate fine pores (for example, HHDavis and LJ Ferrell: MICROSTRUCTU
RES OF AS-FABRICATED UO ₂ FUEL PELLETS; Ceramic Micr
ostructures'76 (1976)). Then, the FP gas generated when the pellets are burned in the nuclear reactor is diffused and captured in the pellets by the pores thus generated, thereby preventing the FP gas from being released to the outside of the pellets. It is also known.

However, according to the knowledge of the present inventor, the sintered body obtained by the above method is not always satisfactory in terms of the FP gas release prevention effect. The present inventor uses a mixture of UO ₂ powder and U ₃ O ₈ powder having a higher degree of enrichment as a raw material for the sintered body to determine the state of pore formation of the sintered body in the FP gas. It has been found that the optimal state can be achieved in order to prevent the emission of

[Specific Description of the Invention]

Hereinafter, the present invention will be described in more detail. In the following description, “%” representing an amount is based on weight unless otherwise specified.

The nuclear fuel powder used in the present invention is uranium dioxide powder (UO ₂ ), and the particle size of this powder is preferably in the range of about 10 to 800 μm before mixing.

As a powder for nuclear fuel having a high oxidation state, which is preferably used in the present invention, uranium trioxide powder (U ₃ O ₈ ) can be mentioned, and the enrichment of this powder is higher than that of the UO ₂ powder to be mixed. It is essential that Further, the particle size of this powder is preferably at least 150 μm or less, preferably 44 μm or less before mixing, and the addition amount of this powder is preferably in the range of more than 0% and 20% or less. Although there is no problem in terms of pellet manufacturing technology as far as the addition rate is low, it is more preferable that the addition rate is high in the above range in order to further exert the effect of the present invention.

However, if it exceeds 20%, the surface of the pellet ground after sintering is roughened, resulting in poor surface roughness and appearance, which is not preferable.

Further, in the present invention, the “concentration degree” means the weight ratio of ²³⁵ U to the weight of ²³⁸ U and ²³⁵ U, that is, Means

In the present invention, the greater the difference between the enrichment of U ₃ O _{8 and} the enrichment of UO ₂ , the greater the effect. However, the former is higher than the latter by 0.1 to 4%, which is the tolerance of enrichment. Also, it is preferable in terms of procuring enriched uranium.

The relationship between the amount of U ₃ O ₈ added and the concentration of the mixed powder is UO ₂
Concentration of powder is A%, mixing ratio is C%, U content is E%
And the concentration of U ₃ O ₈ is B%, the mixing ratio is D%, and the U content is F% (C + D = 100), the concentration K% of the mixed powder is K = (A ・ C ・ E + B・ DF ・ / C ・
E + DF / F)%.

Specifically, the U content of UO ₂ powder is usually about 87.8.
%, And the U content of the U ₃ O ₈ powder is about 84.8%. Further, the addition amount of U ₃ O ₈ powder, that is, the mixing ratio is 10
%, And the concentration is 4.00%, UO ₂ powder having a concentration of 3.00% (mixing ratio 90%) was mixed with a conventionally known dry mixing device such as a ball mill or a blender. In this case, the concentration of the mixed powder will be 3.10%.

Then, the mixed powder (nuclear fuel raw material powder) thus obtained is charged into a mold of a molding machine having a desired shape by a conventional method, and molded under a pressure of, for example, about 1 to 4 ton / cm ^2. ,
40 of theoretical density of uranium dioxide 10.95 g / cc
A molded product of -60% TD is obtained. Then, the compact is sintered in a reducing gas atmosphere such as hydrogen or decomposed ammonia gas at, for example, about 1600 to 1800 ° C. for about 1 to 10 hours.

The obtained sintered body is ground to, for example, a desired diameter, filled in a combustion coating, replaced with an inert gas and enclosed, and used as a fuel rod for operation of a nuclear reactor.

The fuel pellets produced by the above-mentioned method not only have a substantially uniform spatial distribution of pores and ²³⁵ U concentration in the pellet, but also microscopically have a high ²³⁵ U atomic concentration. Since the pores are unevenly distributed in the FP, the FP gas is easily trapped in the pores, and it is possible to effectively prevent the reduction of the heat transfer in the gap between the cladding tube and the pellet,
This can improve the fuel characteristics.

The reason why the above-mentioned excellent effects are exhibited is not always clear, but it can be considered as follows.

That is, the _U 3 _{O 8} powder higher enrichment than UO ₂ powder was added and mixed into UO ₂ powder and sintering in a reducing atmosphere After molding, _U 3 _{O 8} during powder mixing uniformly distributed in the UO ₂ However, the pores generated after sintering are spatially and practically evenly distributed, and the concentration is also spatially and practically uniform, and the pores are formed in the portion where U ₃ O ₈ was present. However, stomata are locally formed in a highly concentrated portion, that is, in a portion where the ²³⁵ U concentration is high. On the other hand, when the pellets are exposed to the same neutron flux distribution, the FP gas generation amount increases in proportion to the concentration thereof, so that pores are formed at the site where the FP gas generation amount is large. That is, considering the pellet as a whole, it can be produced by the above-mentioned method and the conventionally known method, that is, UO ₂ powder and U ₃
Comparing the case where the O ₈ powder was produced with the same concentration, the number of pores formed in the pellets was the same and the volume was the same, and the spatial distribution was the same. Since the obtained pellets have pores arranged locally where the amount of FP gas generated is relatively large, it is considered that the trapping property of FP gas into the pores becomes extremely good.

Example 1 UO ₂ powder as a raw material had a concentration of 2.00% and a U% of 8
450 g of 7.7% is adjusted in advance to a particle size of 720 μm or less by sieving.

On the other hand, U ₃ O ₈ powder was obtained by oxidizing a sintered body in the air at 500 ° C. for 1 hour, and sieving the particles to a particle size of 44 μm or less. Concentration of 3.90% and U% of 84.8% 25 g To use.

After uniformly mixing both powders with a rotary mixer, the molding pressure 2
After molding at Ton / cm ² , sintering is performed in a hydrogen atmosphere at 1750 ° C. for 3 hours and grinding is performed.

The obtained pellets had a concentration of 2.10% and contained 1
Pores having a size of 0 to 40 μm were uniformly formed.

Claims (3)

[Claims] 1. A metal oxide powder for nuclear fuel, and (b)
Characteristic is that a nuclear fuel raw material powder composed of a mixture with a metal oxide powder for nuclear fuel having a higher oxidation state and a higher degree of enrichment than the above (a) is molded, and the molded body is sintered in a reducing atmosphere. And a method for producing a nuclear fuel sintered body. 2. The metal oxide powder for nuclear fuel according to (a) above,
The method according to claim 1, which is uranium dioxide (UO ₂ ). 3. The metal oxide for nuclear fuel of (b) above is UO ₂
The production method according to claim 2, comprising uranium trioxide (U ₃ O ₈ ) having a higher degree of enrichment.