JPH0458192A - Production of oxide nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To decrease the amt. of the gaseous fission products to be released under high burnup of fuel and to mitigate the mechanical interaction between pellets and a cladding pipe by respectively pelletizing two kinds of uranium oxide powders which are different in activity, then mixing these powders, molding the mixture to a required size and sintering the molding under specific heating conditions. CONSTITUTION:Two kinds of the uranium oxide powders which are different in activity are respectively pelletized and thereafter, the pellets are mixed and molded to circular cylindrical bodies of a required size. The moldings are sintered under >=5 minute heating conditions in a 1000 to 2000 deg.C range in a reducing or oxidative atmosphere. The degrees of the density, grain size distribution, thermal dimensional stability, and mechanical characteristics of the sintered pellets are freely changed by arbitrarily changing the mixing ratios of two kinds of the uranium oxide powders which are different in activity.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention improves the retention force of fissile product gas (FP gas) by increasing the particle size of U02 pellets, and also improves the mechanical strength between the fuel pellets and the cladding tube. This invention relates to a method for producing oxide nuclear fuel pellets mainly for light water reactors, which has been improved to alleviate the effects of interaction (PCMI).

[Prior art] Oxide nuclear fuel pellets conventionally used for light water reactors are UO as shown in the manufacturing process shown in Figure 2.
After granulating the two powders, adding and mixing lubricant, and compressing them into a cylinder shape with a diameter and height of about 1 cm, sintering in a hydrogen atmosphere, grinding the outer periphery of these sintered bodies, and inspecting them. The sintered pellets are uniform in crystal grain size in the range of 5 to 10 gm.

Furthermore, it is known to produce oxide nuclear fuel pellets with coarser crystal grains in order to reduce FP gas release, which is a problem when using fuel with high burnup. Also,
According to Japanese Patent Application Laid-Open No. 55-87089, it is known that similar sintered pellets can be obtained by sintering a similar compression molded product at 1000 to 1400°C in an oxidizing atmosphere. In addition, by adding U3O8 powder to the raw material powder, 20~20~
It is known that sintered pellets with a bimodal grain structure embedded in a grain fraction of 50 Bya can be obtained. Such a bi-monograin structure has two improving effects: the fine grain region constitutes the skeleton of the sintered body and takes up the mechanical load, and the coarse grain region reduces FP gas release. It is known that it is a pellet.

[Problem to be solved by the invention] Incidentally, the oxide nuclear fuel pellets produced by the method described above have problems in reducing the release of fission product gas at high burnup and in mechanical interaction between the pellets and the cladding tube. (PCMI)
This may be due to the improvement effect of uranium oxide powder or UO2 pellets recycled during the manufacturing process in order to obtain a bimodal particle structure.
．． Since powder is used, a certain amount of U306 powder is always required, and there is a limit to controlling the structure of a wide range of fine grain structure regions. Further, as the crystal grain size becomes coarser and the density increases, it becomes difficult to control the predetermined density and its thermal dimensional stability.

The present invention has been made in view of the above circumstances, and provides an oxide nuclear fuel pellet that solves the above two problems without impairing the effects of reducing FP gas emission under high burnup and improving PCMI mitigation. The purpose of this invention is to provide a method for manufacturing.

[Means for Solving the Problems] In order to achieve the above object, the method for producing oxide nuclear fuel pellets of the present invention involves granulating two types of uranium oxide powders having different activities, and then pulverizing them. Large particle size and small particle size are obtained by mixing the mixture, forming it into a cylindrical body of the desired size, and sintering it under heating conditions of 5 minutes or more at a temperature range of 1000 to 2000 °C in a reducing or oxidizing atmosphere. The method is to produce oxide nuclear fuel pellets in which each region is mixed at a weight ratio of 9:1 to 1.9.

[Function] According to the manufacturing process of the present invention, the area in which the highly activated uranium oxide powder is granulated in the sintered pellet is 20 to 100
It constitutes crystals with a large grain size of gm, and can play an improving effect in reducing FP gas release. On the other hand, the region in which the low-activated uranium oxide powder is granulated has a crystal grain size in the range of 1 to 20 gm, and constitutes a region that has the same amount of densification and mechanical properties as conventional pellets. In this method, the density, particle size distribution, thermal dimensional stability, and mechanical properties of the sintered pellets can be freely changed by arbitrarily changing the mixing ratio of uranium oxide powder having two types of activity. be able to. In addition, in the granulation process of each raw material powder, by controlling the density and size of the granulated powder, it is possible to control the distribution state of the two crystal grain size regions in the two-moist particle structure over a wide range. can.

[Example] Figure 1 is a flow diagram showing the manufacturing process of oxide nuclear fuel pellets according to the present invention.Compared to the conventional manufacturing process shown in Figure 2, in the manufacturing process of Figure 1, first, the raw material is Uranium hexafluoride is converted into uranium oxide powder with different activities by a conversion method that allows control of powder activity. Regarding these conversion methods, for example, ADU (Ammoniu
m diuranate) method.

Improved ADO method, A U C (=Ams+onium I
Methods such as the Jranyl Carbonate method and the modified AUG method are known. In these conversion methods, the activity of the obtained uranium oxide powder can be varied over a wide range, and the sintered particle size of the sintered pellets can be increased to more than 10 times that of the conventional method. The highly activated uranium oxide powder with a specific surface area of 3-27 g or more obtained by such a method and the low activated uranium oxide powder with a specific surface area of less than 3 m27 g are granulated, and then a lubricant is added and mixed. . however,
In this case, U30. Powder may be added in an amount of 10% or less. The subsequent molding and sintering steps are the same as in the conventional technology.

Next, the present invention will be specifically explained based on the embodiment shown in FIG. Low activated uranium oxide powder (
In the case of UO2゜8), when a compact formed by normal cold compression is heated and sintered at 1700℃ or higher in a reducing atmosphere,
Pellets with a grain size of approximately 5 to 10μ layers are produced.

Highly activated uranium oxide (UO, or When heated and sintered, pellets with a grain size of 20 to 1001 are produced.

Based on these facts, the oxide nuclear fuel pellets of the present invention are produced as follows.

That is, in the granulation step, highly activated uranium oxide powder is granulated into particles of about 100 gm or more, and low activated uranium oxide powder is granulated into particles of about 100 gm or less. Alternatively, the latter powder may be used without granulation. The proportion of highly activated U O2 powder in such granulated powder is 10 to 90.
Mix within the range of weight %.

After adding a lubricant (e.g. 0.2% by weight of zinc stearate) to the mixed powder obtained by the above method, it was made into a green pellet by a conventional known powder compaction press technique and heated to 1750 ml in a reducing atmosphere. Sintering was carried out under heating conditions of 5 hours at °C.

FIG. 3(a) is an optical microscope photograph showing the fine structure of the oxide nuclear fuel pellet obtained in this example, and FIG. 3(b) is an explanatory diagram of FIG. 3(a). As is clear from FIG. 3, the oxide nuclear fuel pellets of the present invention are
Large grain region l having small pore region 2 and multi-porous region 3
It can be seen that the small crystal grain regions 3 having .

Even if a molded body made of the above-mentioned granulated powders having different degrees of activity is subjected to sintering in an oxidizing atmosphere, an oxide nuclear fuel levette having a similar microstructure is produced.

[Effects of the Invention] As explained above, the oxide nuclear fuel pellets obtained by the method of the present invention have a continuous microcrystalline structure of about 5 to 10 g+ adjusted by the low activated uranium oxide powder. , a bimodal grain structure in which island-like isolated regions exist with a crystal structure with a large grain size (crystal grain size of about 20 to 1100 It) adjusted by highly activated uranium oxide powder (see Figure 3). Therefore, it is possible to control the amount of FP gas released under high fuel burnup and to alleviate mechanical interaction with the fuel cladding tube.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the steps of the method for producing oxide nuclear fuel pellets of the present invention, FIG. 2 is a block diagram showing the steps of the conventional method for producing oxide nuclear fuel pellets, and FIG.
FIG. 3(a) is a microscopic photograph showing the particle structure of the nuclear fuel pellet of the present invention, and FIG. 3(b) is an explanatory diagram of the microscopic photograph of FIG. 3(a). In the figure. l Two large crystal grain regions 3: Small grain regions 4: Multi-porous region Agent Patent attorney 1) Haru Kitatake 2: Low-porous regions Diagram

Claims (2)

[Claims] (1) After granulating two types of uranium oxide powders with different activities, they are mixed, molded to the required size, and granulated in a reducing or oxidizing atmosphere.
By sintering in a temperature range of 2000°C for 5 minutes or more, each region of large particle size and small particle size is
A method for producing oxide nuclear fuel pellets, comprising producing oxide nuclear fuel pellets in which oxide nuclear fuel pellets are mixed at a ratio ranging from 1:9 to 9:1. (2) Two different types of activated uranium oxide powders are highly activated uranium oxide powder with a specific surface area of 3 m^2/g or more and high sintering activity, and other low activated uranium oxide powders. The method for producing oxide nuclear fuel pellets according to claim (1).