JPS62206491A - Manufacture of nuclear fuel molded shape - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for manufacturing a molded body, and particularly to a method for manufacturing a ceramic nuclear fuel molded body that is free from defects such as cracks and has excellent mechanical properties.

[Technical background of the invention]

Nuclear fuel used in nuclear reactors is composed of components such as uranium dioxide and uranium dioxide containing plutonium oxide, and is usually used as a sintered body obtained by molding and sintering these oxides. These sintered pellets are obtained by press-molding nuclear fuel oxide powder, sintering it, and then grinding the outer surface of the sintered body. However, if the pellets have defects such as chips or cracks, they cannot be used as nuclear fuel. It cannot be used.

Many of these defects occur because the strength of the compact is low during molding. Therefore, in order to improve this problem, a method of adding a binder to the nuclear fuel oxide powder is commonly used.

Conventional binders used for this purpose include starch, dextrin, alginic acid, sodium alginate, casein, polyvinyl alcohol, sodium polyacrylate,
There are aqueous solutions and organic solvent solutions of organic substances such as polyethylene glycol, urea, microcrystalline cellulose, glucose, lactic acid, and malonic acid, and the method used is to wet-mix these binder solutions with nuclear fuel powder and then mold the mixture. .

However, such wet mixing requires a process of dissolving the binder in a solvent and a process of drying the mixture, making the process complicated and unfavorable in terms of thermal energy. Furthermore, there is also the problem that the mixture adheres and remains inside the mixer.

In this respect, the above-mentioned problem of wet mixing can be solved if a sufficient bonding effect can be obtained by dry mixing the binder as a powder with the nuclear fuel powder and press-molding it.

However, it is difficult to obtain a molded product having sufficient strength by dry mixing using a binder as described above and molding.

[Summary of the invention]

The present invention has been made in view of the above-mentioned points, and it is possible to further simplify the molded product manufacturing process and improve work efficiency, and to produce a relamic nuclear fuel that is free from defects such as chips and cracks and has excellent mechanical properties. The object of the present invention is to provide a method for manufacturing a molded article.

As a result of repeated various experiments in order to achieve the above object, the inventor of the present invention determined that the characteristics of the nuclear fuel powder is to use a powder with a high water content, and to use a binder with a strong deliquescent property such as citric acid, which is As a binder with the property of decomposing and dissipating during the heat treatment process for nuclear fuel bodies, by allowing the mixed powder of both to stand and mature for a certain period of time, the molding work becomes easier and the mechanical properties of the resulting nuclear fuel compacts are improved. It has been found that the properties are significantly improved.

The present invention has been made based on the above findings, and more specifically, a binder that disappears in the heat treatment process for the compact is added and mixed with nuclear fuel powder having a water content of 0.5 to 5%, This mixture is characterized by being aged and then pressure molded.

[Detailed description of the invention]

The present invention will be explained in more detail below. In the following description, the amount "1%" is based on weight unless otherwise specified.

Nuclear Fuel Powder The nuclear fuel powder used in the present invention includes uranium dioxide, other uranium oxides, plutonium oxide, thorium oxide, etc., and these may be used alone or in combination of two or more. It is also used as a mixture with neutron absorbing substances such as gadolinium oxide. The aggregate particle size before mixing is preferably in the range of about 10 to 1500 μm.

As nuclear fuel powder, one with a high moisture content is particularly preferably used. This water content is preferably 0.5 to 5%,
More preferably, it is 1 to 3%.

If the water content is less than 0.5%, the binder effect cannot be fully exhibited. On the other hand, if the water content exceeds 5%, problems such as difficulty in transporting the powder and adhesion of the powder to the inside of the mixer may occur, which is not preferable.

Binder A fugitive binder is used as the binder. In the present invention, the "fugitive binder" refers to
It means a binder that is completely removed from the nuclear fuel material by pyrolysis vaporization before sintering begins, leaves no residue such as carbon in the sintered body, and does not affect the sintered body tEJsli.

Examples of substances used as fugitive binders include hydroxycarboxylic acids, and specific examples include citric acid, malic acid, and tartaric acid, among which citric acid is particularly preferably used.

In addition, the amount added is 0.5 to 5
% is desirable. If the amount added is less than 0.5%, the effect of improving the physical properties of the molded product is small;
If this value is exceeded, the density of the sintered body decreases, depending on the shape of the binder, and it may become impossible to use it as a nuclear fuel product, and there will also be economic disadvantages due to the increased amount of binder used.

Production (1) First, nuclear fuel powder having a water content of, for example, about 0.5 to 5% is prepared, and a binder is added thereto.

The added binder and nuclear fuel powder can be mixed using a suitable dry mixing device. As a device, V
Blenders, ribbon blenders, slab blenders, fluidized bed blenders, vibratory mills, ball mills, centrifugal mills, etc. may be used.

This mixed powder is then placed in a closed container and aged for at least about 1 day.

This aging step is important for developing excellent mechanical properties in the molded article. It is surprising to the present inventor that the mechanical properties of the obtained molded product are improved by preparing the raw material mixture and then allowing it to mature for a certain period of time.The reason for this is speculated as follows. be done.

In nuclear fuel powder with a high water content, water is uniformly adsorbed on the surface of the powder. On the other hand, since the binder is in solid form and is mixed with the nuclear fuel powder, the two can more easily be brought into a uniform state. When these are aged for a certain period of time, the binder surrounded by powder with surface adsorbed water absorbs the adsorbed water and deliquesces due to its deliquescent property. Next, the binder deliquesced by capillary action gathers appropriately at the joint between the powders and solidifies there. As a result, the nuclear fuel powders are strongly bound together by the binder, which is thought to lead to improvement in the mechanical properties of the compact.

After aging, the mixed powder is charged into the mold of a molding machine of a desired shape according to a conventional method, and is molded with a pressure horn of, for example, a 0.5-5 ton/cII culm, to obtain a 40-60% TD (theoretical density 40 of
~60%, theoretical density is 10.95g for uranium dioxide
/CIj) is obtained.

The compact is then sintered in an atmosphere of hydrogen, reducing hydrogen atoms, nitrogen, oxidizing carbon dioxide decomposition, or inert gas such as argon under conditions depending on the type of nuclear fuel powder (for example, in the case of ordinary nuclear fuel oxide powder). , about 1500-180
Sinter at 0° C. for about 1 to 10 hours). The obtained sintered body is, for example, ground to a desired diameter, loaded into a fuel cladding tube, replaced with an inert gas, and sealed to form a fuel rod, which is assembled into a fuel assembly for operation of a nuclear reactor. Serve.

Examples of the present invention will be described below, but the present invention is not limited to these Examples.

[Examples and comparative examples of the invention]

Four types of powders (a), (b), (c) and (d) shown below were prepared.

(a) Low water content (approximately 10001) l) 1111-120
) uranium dioxide powder, (b) low moisture content uranium dioxide powder with 2% citric acid added, (C) high moisture content (approx.
"OOOOppm H20) Uranium dioxide powder, (d)
High moisture content: 2% citric acid added to uranium dioxide powder.

Each of these powders was allowed to stand for one day for ripening.

These aged raw material powders are subjected to molding pressure 2. using a molding machine.
A cylindrical molded body with a diameter of about 13 mm was obtained at 0 ton/Ci. This molded body was placed on a pedestal of a molded body strength device with its axis horizontally, and the molded body was compressed with a pressurizer to measure the tensile strength coefficient. The tensile strength coefficient is based on the following formula.

Tensile strength coefficient -2P/π・DL Here, P: Maximum load (K9) D=diameter of molded body L: Length of molded body The elongation to break was also measured.

The measurement results for each sample are shown in Table 1 below.

As is clear from the table, in the case of low water content uranium dioxide powder, there is no change in the mechanical properties of the compacts even when citric acid is added or aged. Furthermore, even in the case of high water content uranium dioxide powder, if citric acid is not added, or if citric acid is added but the aging period after mixing is short, there is no change in the mechanical properties of the compact.

However, when citric acid is added to and mixed with high water content uranium dioxide powder and the material is subjected to an aging process, the tensile strength coefficient increases by about 2.2 to 2.5 times compared to other samples. The elongation up to this point is about 1.5 to 1.8 times, which is a remarkable increase.

As is clear from the results of the above examples, the method for producing a nuclear fuel compact of the present invention involves adding and mixing a fugitive binder to uranium dioxide powder with an increased water content, and molding after further aging. Therefore, it is possible to improve the mechanical properties of the molded body, to obtain a nuclear fuel molded body that has superior moldability compared to conventional methods and is free from defects such as chips and cracks.

Claims (1)

[Claims] 1. Nuclear fuel powder with a moisture content of 0.5 to 5% and a binder that disappears during the heat treatment process for the compact are added and mixed, this mixture is aged, and then pressure molded. A method for producing a nuclear fuel molded body, characterized by: 2. The method according to claim 1, wherein the binder is an organic acid. 3. The method of claim 1, wherein the organic acid is selected from citric acid, malic acid and tartaric acid. 4. The method according to claim 1, wherein the aging is carried out by allowing the mixture to stand for at least 24 hours.