JPH01248092A - Manufacture of nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To enable actual manufacturing of fuel pellets having large crystalline grains, by specifying an adding amount of aluminum compounds to a nuclear fuel which is produced by sintering a green compact to be mixed with an added uranium dioxide or mixed compound of an uranium dioxide and a plutonium. CONSTITUTION:As a green compact, an aluminum compound is added and mixed with an uranium dioxide or a mixed compound of an uranium dioxide and a plutonium. In a manufacturing process of nuclear fuel pellets to produce nuclear fuels, in which process the green compact is sintered in a reducing atmosphere, one or more than two kinds of an oxidized aluminum, an aluminum hydroxide or an aluminum salt of fatty acid are added by amount of 0.001-0.07wt.%, as an aluminum element and then are mixed with a conventional mixing method and compressed to produce the green compact. This compressed powder mass is sintered in a reducing atmosphere and at a temperature of 1,700 to 1,800 deg.C therewith nuclear fuel pellets having large crystalline grain can be manufactured.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides oxides for nuclear reactor fuel (UO3 and U, P
This relates to a method for producing pellets (a) of mixed oxides of u, etc.).

[Conventional technology]

As a normal nuclear reactor fuel, sintered pellets of uranium dioxide filled in a cladding tube such as Zircaloy are used as a nuclear fuel element.

In this case, the above pellets are usually produced by mixing U02 powder with a lubricant, molding it into a compact, and sintering it in a reducing atmosphere at a temperature of 1700°C or higher. There is.

When nuclear fission is performed, the sintered fuel element, which is made by filling a cladding tube such as Zircaloy with the sintered pellets of uranium dioxide mentioned above, will undergo nuclear fission.
Nuclear fission generates fission product gas (hereinafter referred to as FP gas) such as xenon and krypton, and when generated inside the FPP2O cladding tube, high gas pressure is generated and there is a risk that the cladding tube will be damaged. Therefore, early replacement of the nuclear fuel element is required, and such early replacement of the nuclear fuel element impairs the economic efficiency of nuclear power generation using a nuclear reactor.

For this reason, various measures have been taken to suppress the release of the above-mentioned FP gas generated from nuclear fuel pellets, and the main methods include the following.

(A method of imparting FP gas holding ability to sintered pellets by growing and coarsening the crystal grains of 11UO2 ceramic fuel during sintering.

(2) A method of suppressing FPP2O release by immobilizing or bottling FPP2O using a chemical method.

In method (1) above, when the crystal grain size of U02 pellets becomes coarse, FP generated within one grain due to the large crystal grain size
This is a physical suppression method in which it takes a relatively long time for P2O to reach the grain boundaries where the diffusion rate increases, and as a result, FPP2O release is suppressed. Publication No. 54-148996, Japanese Patent Application Publication No. 1973
There are Japanese Patent Publications No. 8-44394 and Japanese Patent Publication No. 61-34079, and known documents showing the method (2) above include Japanese Patent Publications No. 50-5203 and Japanese Patent Publication No. 51-1983.
34079, and Japanese Patent Publication No. 54-8837.

Japanese Unexamined Patent Publication No. 54-148, which is a known document showing the method of (1) above.
No. 996 describes a method for suppressing FP gas release by creating pellets with an average grain size of 50 μm or more using activated U02 powder containing 20 to 1,000 ppm of S. In the above-mentioned Japanese Patent Application Laid-open No. 58-44394, it is stated that L1 compound is added to UO2 and Ll is 20 to 5000.
Add ppm.

A method is described in which large grain size pellets are created by low-temperature sintering at a temperature of 1200°C or higher to suppress FP gas release.

In the above-mentioned Japanese Patent Publication No. 61-29678, it is stated that UO2 has L1
20 as O, OO5~1. FP by creating pellets with large grain size by low-temperature sintering with Owt% loading of 1 l.
Methods for suppressing outgassing are described.

In addition, it is also known to add Nb2O5, Cr2O, etc. to UO2 powder to produce pellets with large crystal grain sizes.

Furthermore, in the above-mentioned Japanese Patent Publication No. 50-5203, which is a publicly known document showing the method (2) above, 002 particles are A! 20
This relates to high temperature gas reactor fuel coated with grade 3.

In the above-mentioned Japanese Patent Publication No. 51-340'79, U2O
5: Add 15 to 3.7 wt% of O, and add AA to the crystal.
203 is dispersed in fine particles to suppress FPP2O release (
It describes how to do this (stopping the gas bottle).

In the above-mentioned Japanese Patent Publication No. 54-8837, U02 powder contains
Aluminum silicate, silica-alumina-magnesia,
A method for immobilizing FPP2O and suppressing its release by adding silica-alumina-calcia, aluminum titanate, etc. is described.

Other than the above (1) and (2), there are other bale fuel pellet manufacturing technologies such as JP-A-55-2'7942 and JP-A-55-104791. In addition, an oxide t containing 0.2 to 0.5 weight of M2O3 is placed on the outer shell of the pellet, and the hardness of the pellet is treated with metal, and the difference in thermal expansion T and the metal clad tube as a ceramic nuclear fuel element is achieved. A method for reducing the mechanical interaction caused by this is described.

[Problem to be solved by the invention]

The above-mentioned conventional technique (1) of JP-A-54-148996, in which active UO2 powder containing 8 is used to create a large crystal grain size R, is difficult to handle because UO2 powder is active. There was also a problem with moldability. Also,
The above-mentioned Japanese Unexamined Patent Publication No. 58-44394 and Japanese Patent Publication No. 1988-61-
L1 compound k such as L102 described in 29'6'78 publication
The method of adding m to create large grain size pellets is difficult because Li with a large thermal neutron absorption cross section (neutron absorption cross section near Q, '7 burn) may remain in the sintered pellet. Furthermore, the above-mentioned Nb2O5,
When adding Cr2O3, etc., it is necessary to add a large amount of about 0.2 to 1 weight s, which poses problems such as lowering the melting point of the U○2 pellets.

In the method of conventional technology (2) above, &A! 203. The effect will not be obtained unless a large amount of aluminum AM acid, aluminum titanate, etc. is added, and adding a large amount lowers the melting point of the U○2 nuclear fuel pellets and lowers the purity, which is not preferable.

[Means to solve the problem]

Therefore, the present inventors proposed that the thermal neutron absorption cross section is small,
We have conducted extensive research into metal compounds that do not affect the melting point of U02 pellets and can increase the crystal grain size of UO3 pellets even with the addition of a small amount of i that is acceptable as an impurity.

One or more of aluminum oxide, aluminum hydroxide, and aluminum salts of fatty acids, M = 0.0
01 to 0.07% by weight was added, mixed and made into a green compact by a known method, and this was heated in a reducing atmosphere at a temperature of 70 liters.
They found that nuclear fuel pellets with large crystal grain sizes can be produced by sintering at 0 to 1,800°C.

The nuclear fuel referred to in this invention is mainly UO2, but
With mixed oxides of uranium and plutonium.

It may be called MOX, as will be recognized in theory by those skilled in the art.

In this invention, At203. AM(Of
When we confirmed the effect of the addition of aluminum salts of ()3° fatty acids on crystal grain size, we found that they were not affected by the chemical forms of these aluminum compounds and were due to the addition of A11ii.

The addition of the above M compound is 1% by weight of O, OO as AI (
loppm), the grains of the UO2 sintered pellets do not coarsen;
m) When added, crystal grains begin to coarsen.

When 0.003% by weight (30ppm) of M is added, U
The average grain size of the o2 sintered pellets is 30 to 40 μm, and the AL is 0.07% by weight (γOOppm)'
Even if it is added in excess of fr, the crystal grain size will not increase further; on the contrary, if it is added in excess of 0.07% by weight ()OOppm), it will lower the melting point of the UO2 sintered tube fuel pellets and reduce the purity. Undesirable. Therefore, f
iJ, addition of compound as M'ii Q, OOl~
0,07 Megumi fk% (l Oppm ~)OOppm
).

In particular, the lower limit of the addition of the M compound as M is 0.003% (30 ppm), which makes the average crystal grain size of the U02 sintered pellets 30 to 40 μm.
Barrett's use of impurities raises the upper limit of M content to 250 ppm
Therefore, the addition of 5M compound as M is 0
．． A range of 0.003 to 0.025% by weight (30 to 250 ppm) is particularly preferred.

The addition of M compound to UO2 powder is *”2o3, u
In addition to (oH)3, aluminum salts of fatty acids can be added. The above-mentioned aluminum salts of fatty acids include aluminum stearate and aluminum oleate.

Aluminum valmitate is primarily used.

These aluminum salts of fatty acids can be added to UO2 powder as a lubricant during compaction.

This is advantageous because it serves as a lubricant while supplying AM.

Conventionally, in order to create a Uo2 powder compact, the above U02
Zinc stearate is generally known as a lubricant added to powder, but the above A! 20s, AM(OH)3
m and/or with aluminum salts of fatty acids.

The method of the present invention also includes a method of adding the above-mentioned known zinc stearate as a lubricant, molding and sintering a green compact.

Regarding the U○2 ceramic nuclear fuel to which M2O3 is added or coated as mentioned above, Japanese Patent Publication No. 50-5203 and Japanese Patent Publication No. 51-3407 mentioned in the above-mentioned conventional technology (2)
It is known from Publication No. 9.

However, in the above-mentioned publicly known Japanese Patent Publication No. 50-5203 and Japanese Patent Publication No. 51-34079.

The amount of Al2O3 added is as high as 0.15% or more, and F
While P gas is immobilized by a chemical method and its release is suppressed, the present invention has a 1M concentration of 0.071 kg or less (0.132 weight 1j when A1203 is added).
tts or less) by adding UO2-! By increasing the crystal grain size of l'let and prolonging the time it takes for the FP gas generated inside the crystal to reach the grain boundary through diffusion, FP
They are intended to suppress the release of gas, and the technical philosophy is different.

Also, JP-A-55-27942 and JP-A-55
-The method described in Publication No. 104791 is 1M20
3 is added in a large amount of 0.5% by weight or more to lower the hardness of the pellet and reduce the mechanical interaction caused by the difference in thermal expansion between the ceramic nuclear fuel element and the metal cladding tube. 1 This invention is completely different in purpose, structure, and effect from the claimed invention.

〔Example〕

Next, the present invention will be specifically explained based on examples.

UO2 powder with average particle size: 0.97 pm.

M2O3 powder with average particle size: 0.50 μm.

FiJ, (OH)3 powder with average particle size: 0.61 μm.

and further prepared aluminum stearate, aluminum palmitate, aluminum oleate, and zinc stearate, and blended them to have the composition shown in Examples 1 to 10 and Comparative Examples 1 to 4 in Table 1. Mix your friends,
The above mixture was filled into a mold and pressed at a pressure of 3t/c11L2 to form a green compact with a diameter of 10 pieces and a height of 15 u.

These green compacts were sintered in a hydrogen stream at 750°C for 5 hours, and the average crystal grain size and sintered density of the U02 pellets obtained were measured. The results of these measurements are shown in Table 1. Ta. In this example, aluminum stearate is used as the aluminum salt of fatty acid.

Although aluminum palmitate and aluminum oleate were used, aluminum salts of fatty acids are not limited to these.

〔Effect of the invention〕

Based on the results of the above examples, in this invention.

By adding and mixing a small amount of aluminum compound to U○2 powder, it is possible to easily obtain U○2 pellets with a large crystal grain size using conventional manufacturing methods, and it is possible to obtain U○2 particles with a large crystal grain size using the conventional manufacturing method. Since the added amount of M in the U02 pellets is 1 alk, it does not have a negative effect on the thermal properties and nuclear properties required for the 002 pellets.

Applicant: Mitsubishi Nuclear Fuel Co., Ltd.

Claims (1)

[Claims] 1. By adding an aluminum compound to uranium dioxide or a mixed oxide of uranium dioxide and plutonium to form a green compact, and sintering the green compact in a reducing atmosphere. In the method for producing nuclear fuel pellets, the aluminum compound is converted into aluminum and has a content of 0.
A method for producing nuclear fuel pellets, characterized by adding 0.001 to 0.07% by weight (10 to 700 ppm). 2. 0.003 to 0.025% by weight (30 to 250 ppm) of the above aluminum compound in terms of aluminum
2. The method for producing nuclear fuel pellets according to claim 1, further comprising adding: 3. The method for producing nuclear fuel pellets according to claim 1 or 2, wherein the aluminum compound is aluminum oxide. 4. The method for producing nuclear fuel pellets according to claim 1 or 2, wherein the aluminum compound is aluminum hydroxide. 5. The method for producing nuclear fuel pellets according to claim 1 or 2, wherein the aluminum compound is an aluminum salt of a fatty acid. 6. The method for producing nuclear fuel pellets according to claim 1 or 2, wherein the aluminum compound is two or more of aluminum oxide, aluminum hydroxide, and aluminum salts of fatty acids. 7. Claim 5 or 6, wherein the aluminum salt of the fatty acid is added as a lubricant during compacting.
The method for producing nuclear fuel pellets described.