JPH0812264B2 - Manufacturing method of nuclear fuel sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION “Industrial field of application” The present invention produces UO ₂ powder having a large specific surface area from a uranyl chloride aqueous solution, and further uses it to produce a nuclear fuel sintered body having a large crystal grain size. A manufacturing method for obtaining the same.

“Conventional technology and problems to be solved” Recently, a so-called high burnup plan, which uses the reactor fuel for a longer period of time, has been studied. However, in order to realize it, the fission product gas (FP gas) ) Should be released to the outside of the nuclear fuel pellets as much as possible.

The mechanism by which the FP gas is released outside the pellet is generally considered as follows. First, within the crystal grains of the pellet, FP
Gas is generated, and this gas forms bubbles in the crystal grains or at the crystal grain boundaries. Of these, when the amount of bubbles generated at the grain boundaries reaches a certain amount, finally tunnels are formed along the grain boundaries and exist at the grain boundaries through these tunnels.
FP gas is released outside the pellet.

From this, even if it is not possible to suppress the generation of FP gas itself, by increasing the crystal grain size in the sintered pellets and increasing the reaching distance of the FP gas generated in the crystal grains to the grain boundary. , Trap the gas in the pellet,
As a result, it is considered that the amount of FP gas released can be reduced. Therefore, the idea of using pellets having a large crystal grain size as a nuclear fuel for high burnup is becoming common. In addition,
Although it is not clear yet about the optimum grain size,
According to the examination of the burnup and the FP gas release rate conducted by the applicant, 20 μm or more is considered to be suitable.

By the way, as a conventional method for producing large-sized pellets, niobium (Nb ₂ O ₅ ) or the like is added to the raw material UO ₂ powder, or the compact is molded in an oxidizing atmosphere such as CO _2. Have been already proposed, such as a method of sintering at 1, a method of using highly active UO ₂ powder having a high crystal grain growth rate as a raw material.

However, in the method using the additive, the influence on the physical properties such as the melting point of the nuclear fuel pellet is not always clear, and in the method of sintering in an oxidizing atmosphere, the manufacturing method is very complicated and costly. Have. Therefore, the method of forming pellets using highly active UO ₂ powder as the raw material has the least problems. From this point of view, the present applicants have previously disclosed Japanese Patent Application No. 61-142506 (Japanese Patent Application Laid-Open No. 62-297215), Japanese Patent Application No. 61-190079 (Japanese Patent Application Laid-Open No. 63-45594), and Japanese Patent Application No. Sho 63-127934 (JP-A 1-298026), JP-A 63-127935 (JP-A 1-298027)
And in U.S. Patent Application No. 139447, we have proposed a method for manufacturing a large grain size pellets using highly active UO ₂ powder.
Both of these production methods use UO ₂ F ₂ aqueous solution or UO ₂ (NO
₃ ) The feature is that highly active UO ₂ powder is obtained by using ₂ aqueous solution as a raw material.

By the way, in recent years, the concentration method utilizing the chemical exchange method has been attracting attention in the attempts of technological innovations relating to the concentration of uranium. This method uses tetravalent uranium (U ⁴⁺ ) and hexavalent uranium (UO ₂
^In the isotopic equilibrium of ( ²⁺ ), U ²³⁵ is on the hexavalent side and U ²³⁵ is on the tetravalent side.
Utilizing the property that U ²³⁸ is concentrated respectively, uranium enrichment is performed by further combining the separation of U ⁴⁺ and UO ₂ ²⁺ by the ion exchange method.

The concentrated product obtained by this chemical exchange method is generally UO ₂ Cl ₂
Although it is a solution, conventionally it is a nuclear fuel raw material from UO ₂ Cl ₂ solution
How to obtain UO ₂ powder has not been studied almost industrial process to obtain a much less highly active UO ₂ powder is not established.

The present invention has been made in view of the above circumstances, UO ₂ Cl ₂ solution to prepare a UO ₂ powder of highly active specific surface area is large as a raw material for nuclear fuel sintered large grain using this highly active UO ₂ powder It is an object to provide a method capable of producing a bound body.

"Means for Solving the Problem" Hereinafter, the method for producing the uranium dioxide powder according to the present invention will be specifically described.

In this method, the raw material uranyl chloride (UO ₂ C
l ₂ ) The concentration of the aqueous solution is 700 gU / or less, preferably 50 to 600 g
Adjust to U /.

Next, while stirring this aqueous solution, aqueous ammonia is rapidly added to precipitate ammonium diuranate (ADU). The addition amount of ammonia is set so that the NH ₃ / U molar ratio is 4 or more, preferably 6 to 12. This reaction is represented by the following equation.

UO ₂ CI ₂ ＋ 3NH ₄ OH → (NH ₄ ) ₂ U ₂ O ₇ ＋ 2NH ₄ Cl ＋ 3 / 2H ₂ O UO ₂ Cl ₂ Concentration of aqueous solution is more than 700gU /, or NH ₃ /
If the U molar ratio is less than 4, ADU will precipitate in both cases.
The particle size of the particles becomes large. From such ADU powder, only inert UO ₂ powder having a small specific surface area can be obtained, and pellets having a crystal grain size of 20 μm or more cannot be obtained.

On the other hand, the lower limit of U concentration and the upper limit of NH ₃ / U molar ratio are not limited in terms of product characteristics. However, if the U concentration is too low or the NH ₃ / U molar ratio is too high, the equipment capacity must be increased, and the amount of waste liquid increases and the treatment is costly, so it is desirable to avoid it. The above more preferable range satisfies such industrial demands.

Next, the produced ADU precipitate is filtered and dried, and this ADU powder is roasted / reduced to obtain highly active UO ₂ powder having a large specific surface area. Furthermore, this highly active UO ₂ powder is compacted and sintered in a hydrogen stream to produce nuclear fuel pellets. When pellets are produced from highly active UO ₂ powder in this way, the crystal growth rate during the sintering process is high, and the grain size of the obtained pellets is 20 μm or more. The conditions of each step after obtaining the ADU precipitate are the same as in the conventional method,
The description is omitted here.

"Examples" Next, the effects of the present invention will be demonstrated with reference to Examples.

UO ₂ Cl ₂ aqueous solutions were prepared with several U concentrations, and while stirring these aqueous solutions, ammonia water was rapidly added so that the NH ₃ / U molar ratio would be several values to form ADU precipitates. It was

Next, each ADU precipitate was filtered and dried, then roasted and reduced at 650 ° C. in a hydrogen stream to convert it into UO ₂ powder.

Further, the obtained UO ₂ powder was formed into pellets at a pressure of 4 t / cm ² , and then sintered in a hydrogen stream at 1750 ° C. for 4 hours to obtain nuclear fuel pellets.

Crystal grain size of these pellets, specific surface area of UO ₂ powder,
The precipitation conditions are shown in Table 1. In addition, for the measurement of specific surface area, BE
The T method was used, and an optical microscope was used to measure the crystal grain size.

From this result, the uranium concentration of the UO ₂ Cl ₂ aqueous solution was 700 gU /
It can be seen that when the NH ₃ / U molar ratio is 4 or more and UO ₂ powder having a large specific surface area and high activity is obtained when the NH ₃ / U molar ratio is 4 or more, nuclear fuel pellets having a crystal grain size of 20 μm or more can be produced.

"Effects of the Invention" As described above, according to the method for producing a nuclear fuel sintered body of the present invention, a highly active UO ₂ powder having a large specific surface area is produced from a UO ₂ Cl ₂ aqueous solution as a raw material. 20 μm
It is possible to obtain a nuclear fuel crystal body having the above-mentioned large crystal grain size and excellent in FP gas retention during combustion.

Claims (1)

[Claims] 1. A uranyl chloride (UO ₂ Cl ₂ ) aqueous solution having a concentration of 70
Ammonia diuranate (ADU) was precipitated by adjusting the amount to 0 gU / or less, and adding ammonia to this aqueous solution so that the NH ₃ / U molar ratio was 4 or more. The ADU precipitate was filtered and dried. Uranium dioxide (U
A method for producing a nuclear fuel sintered body, which comprises compacting and sintering this uranium dioxide powder after converting it into O ₂ ) powder.