JPH04166800A - Fabrication method for sintered nuclear fuel and its sintering furnace - Google Patents

Abstract

PURPOSE:To omit necessity of using expensive CO2 gas, attain lowering cost of materials and prevent complication of the equipment by using N2 gas of industrial purity for main atmosphere gas in sintering in oxidation atmosphere. CONSTITUTION:To sinter pellets with using this sintering furnace, the forming material is heated firstly in a preliminary heating part 1 for 1 to 6 hours in an atmosphere of 150 deg.C to properly disperse the inside oxigen density. Then it is heated at temperature of 1,100 deg.C to 1,300 deg.C in an atmosphere of N2 or N2/air added with proper amount of H2O in a sintering part 2. The gas used here is N2 gas of industrial purity and to the use of N2 gas instead of CO2 gas enables simplifying the equipment to be compact.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing a nuclear fuel sintered body by a low-temperature sintering method of 1100°C to 1300°C, and a sintering furnace thereof. The present invention relates to a method for manufacturing a nuclear fuel sintered body that uses N2 as the main gas for maintaining an oxidizing atmosphere to reduce costs, and a sintering furnace for the same.

(Prior Art) The low-temperature sintering method allows sintering to be performed at a relatively low temperature of 1100°C to 1300°C, compared to normal nuclear fuel body sintering which is performed at 1600°C to 1750°C. - In general, two-stage sintering is known: sintering in an oxidizing atmosphere and reductive heating.

In this low-temperature sintering method,
635 and Japanese Patent Publication No. 61-19952, specific manufacturing methods are disclosed, and both of these are C02
The basic method is to perform sintering in an atmosphere and then perform reductive heating in a reducing atmosphere. .

On the other hand, as an apparatus for carrying out the above-mentioned low-temperature sintering method, a sintering furnace described in Japanese Patent Application Laid-Open No. 64-91094 by the applicant of the present invention is known, and this sintering furnace has a preliminary sintering section and a main sintering section. The sintering section has a continuous sintering section and a reducing section, and further has a gas curtain section at the boundary between each section that uses N, gas, or CO□ gas as a purge gas.The main sintering section has a 1000-
This is a heating zone of 1400°C.

However, in the above manufacturing method and apparatus, relatively expensive CO2 is used as the main atmosphere gas during sintering, which increases the cost of raw materials, and CO2 becomes solid under pressure (30 kg/cIlt). Therefore, there is a problem in that it is difficult in terms of equipment, and it is also difficult to stably gasify CO□ solid.

In addition, there is a manufacturing method described in JP-A-1-304391 that partially uses N2 or Air as the sintering gas, but high-temperature sintering requires a temperature of 1300°C to 1600°C for main sintering. This method is not an energy-saving, low-cost, low-temperature sintering method;
There is a lot of room for improvement in practical terms, such as requiring preliminary sintering at 00°C and controlling the oxygen partial pressure in the oxygen atmosphere.

(Problem to be Solved by the Invention) By the way, as mentioned above, CO□ is normally used as the main gas to maintain an oxidizing atmosphere. This was to make it easier to maintain a constant value depending on the conditions.

On the other hand, there is a low-temperature sintering method in which U3O8 is added to UO□ as typified by the above-mentioned Japanese Patent Publication No. 19952/1982, or a method described in Japanese Patent Application Laid-open No. 2-95298 filed by the applicant of the present application (7) LlOz niUOz-x (However, X=0.25
Low-temperature sintering methods have been proposed in which 25 to 45% by weight of UO2
U required for particle size increase from ゜8, 0. Because the oxygen necessary for the change to uranium oxide is supplied from within these uranium oxides,
The oxygen concentration in the sintering atmosphere is not very important.
Furthermore, it was found that the same effect was obtained when sintering only UO without adding the above oxide.

That is, the sintering atmosphere may be at least an inert atmosphere that is not a reducing atmosphere, and the oxygen concentration may be 400 ppm or less, which is said to cause excessive oxidation.

The present invention is based on understanding this fact, and by using N2 as the sintering gas in the first stage of the low-temperature two-stage oxidation sintering method,
The purpose is to reduce the manufacturing cost of nuclear fuel sintered bodies and to simplify and downsize the equipment.

(Means for Solving the Problems) That is, the feature of the method for manufacturing the nuclear fuel sintered body of the present invention that meets the above-mentioned purpose is that two nuclear fuel pellets are placed in a sintering furnace and the processing temperature is 1100°C to 1300°C. In a low-temperature sintering method for nuclear fuel sintered bodies in which sintering in an oxidizing atmosphere is followed by reductive heating at a treatment temperature in the same range, industrial-purity N2 gas is used as the main gas in the atmosphere in the sintering in an oxidizing atmosphere. There is a particular thing.

In this case, an appropriate amount of air may be added to the N2 gas to adjust the oxygen concentration, and N2 gas may be used as a reducing gas for reduction heating.
2, or a mixed gas of N2 and N2, and 0.01% or more of 820 may be added to this reducing gas as a volume ratio. Furthermore, in the above manufacturing method, 25 to 45% by weight of 002.
Added y (however, X <0.25≦y≦1.O
) is preferably used.

On the other hand, the sintering furnace of the present invention for carrying out the above manufacturing method has a preheating section in the front section, a main sintering section in the middle section, and a reduction section in the rear section, each section being connected continuously, and N2 at the boundary between each section. In a nuclear fuel assembly sintering furnace with a gas curtain region,
The preheating section is a heating zone where the main atmosphere gas is N2 or Nz/Air, and the main sintering section is a heating zone where the main gas is N2 or Nz/Air.
1100℃~1300℃ with /Air as the main atmosphere gas
heating zone, and the reduction section is N2 or H2/N2
Atmosphere: 1100°C to 1300°C with 8.0 added to the main gas
It is characterized by a heating zone of ℃.

(Function) When producing a nuclear fuel sintered body by the production method of the present invention using the above-mentioned sintering furnace of the present invention, first, the preheating section is
02 mixed from Air to prevent □ from becoming too high
The role of the gas replacement part is to expel the N2 with N2, and by keeping the temperature preferably below 150°C, it fulfills its original function of keeping the added U3O8 and the oxygen in the base material N02 in equilibrium. , and then proceed to the sintering process to reduce N40. This allows the diffusion of oxygen into the formation to be uniform within the pellet.

Next, the main sintering part has N2 gas as the main gas, and the amount of Air added to adjust the oxygen concentration can be adjusted within a range of 400 ppan or less depending on the purity of the N and gas and the amount of N2 flowing from the reduction region. Adjust (N2 gas has almost no impurities.
□ Therefore, its purity can be considered as the 02 content ratio). Further, the flow of the main N2 gas in the main sintering section flows homogeneously over the entire area of the main sintering section, and part of the main gas is discharged through the N2 curtain at the entrance side of the main sintering section, and the rest through the N2 curtain at the entrance of the reduction section. Therefore, the oxygen concentration in the atmosphere becomes uniform except for the N2 curtain section at the entrance of the reducing section.

In addition, when N2 is used as the main gas in the atmosphere, if a carbohydrate is used as a lubricant to improve powder moldability, a large amount of carbon-mediated reaction occurs due to the decomposition of the lubricant.
+C=UC,IJC+%N2=IN+C
However, it has been found that nitrogen can be removed by supplying an appropriate amount of 8.0 during the subsequent reduction heating.

(Embodiments) Hereinafter, embodiments of the method for producing a nuclear fuel sintered body of the present invention and its sintering furnace will be described with reference to the accompanying drawings.

FIG. 1 shows an example of a sintering furnace according to the present invention, in which nuclear fuel pellets are transported from the left to the right in the figure.

In the figure, (1) is a preheating section provided at the front of the furnace;
(2) is the main sintering section in the middle of the furnace, (3) is the reduction section provided at the rear, and N2 gas curtain sections (4) and (51) are provided at the boundaries of these sections.

Here, the preheating part (11 is a heating zone of around 150°C with N2 or Nz/Air as the main gas in the atmosphere,
The above-mentioned main sintering section (2) is a heating zone of 1100 to 1300 °C with the same <N2 or NZ/^ir as the main gas atmosphere, and the above-mentioned reduction section (3) is a heating zone with the main gas atmosphere being N2 or N2/NZ. 1100℃~1300℃ with H2O added
heating zone.

Therefore, when sintering pellets using the above furnace, first, the oxygen concentration in the molded body is reduced by heating it in the above atmosphere at 150°C for 1 to 6 hours in the preheating section +11. After proper dispersion, the main sintered part (2)
at N2 or Nz/Air atmosphere, 1100℃~1
Heating at 300°C for 1 to 4 hours and reducing N in the reduction section (3).
2 or N2/NZ atmosphere) by adding an appropriate amount of 120,
Heat at 00°C to 1300°C. Note that the N2 gas used here is of industrial purity.

Next, an example will be shown in which fuel pellets were sintered by the production method of the present invention using the above-mentioned sintering furnace of the present invention.

(Example) 3 U, O, (particle size -120 mesh) obtained by heating in air at 410°C was added to UO□ powder with 0/U = 2.09.
After adding 5% by weight and mixing, the density was 5.8 using the mold lubrication method.
A molded body of g/cat was obtained.

This was loaded in the continuous sintering furnace at 20 cm/Ilr.

Preheating temperature setting value is 150°C, residence time 1.58r.

The atmosphere is N2 gas. Main sintering has a heating rate of 250℃/llr
The temperature was maintained at 1150℃ for 3 hours, the atmosphere was Nz 10 air, the oxygen concentration was 50-70ppm, and the reduction was carried out at 1150℃ for 1 hour.
．． 5) A sample with a density of 95.3% was obtained under conditions of holding at 1r, atmosphere of 3 82 10 NZ obtained by decomposing ammonia, 0° and 2% by volume of H, and 0° or more.

The gold phase is as shown in Figure 2, and the CO as shown in Figure 3.
□There is no difference from the sintered one. In addition, C in the above sample
As shown in Table 1, the amount of N also does not pose any particular problem.

Table 1 (Effects of the Invention) As explained above, the method for producing the nuclear fuel sintered body of the present invention uses an energy-saving low-temperature sintering method in which sintering is performed at a low temperature of 1100 to 1300°C, and the atmosphere during sintering is mainly It uses industrial-purity N2 gas as the gas, eliminating the need to use the expensive CO□ gas that was previously used, reducing raw material costs, and reducing the complexity of COz, which becomes solid under pressure. Eliminating equipment and difficult gasification stability,
This makes it possible to simplify and downsize the equipment and provide a stable supply of gas using N2, which can be liquefied.

In addition, the sintering furnace of the present invention is a continuous furnace equipped with a preheating section, a main sintering section, a reducing section, and an N2 gas curtain section at the boundary thereof, and the atmosphere of the preheating furnace and main sintering section is N2 or N7 Air. This is a low-temperature sintering furnace that uses gas and N2 or llz/Nz+820 as the atmosphere in the reduction section, and is most effective in achieving the method of the present invention described above.
By using N2 gas instead of C0g gas, the equipment can be made simpler and more compact as mentioned above.
Moreover, the manufactured sintered body has a metal phase equivalent to that using CO□, and the amount of carbon and nitrogen does not change, making its usefulness in today's world where energy conservation and cost reduction are emphasized. It is extremely large.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an embodiment of the sintering furnace of the present invention, Fig. 2 is a micrograph showing the gold phase of the nuclear fuel sintered body obtained by the production method of the present invention at a magnification of 210 times, and Fig. 3 is a micrograph showing the gold phase of a nuclear fuel sintered body obtained by conventional CO□ sintering at a magnification of 210 times. (1) Preheating section, (2) Main sintering section, (3) Reduction section, (4), (5) N2 gas curtain section. Figure 1 March 1, 1991 Mr. Satoshi Uematsu, Commissioner of the Japan Patent Office ■, Indication of the case, 1990 Patent Application No. 293076 2, Title of invention, Method for manufacturing nuclear fuel sintered bodies and sintering furnace 3, Amendment Relationship with the case of a person who does the following Patent applicant address: 3-23-5 Nishi-Shinbashi, Minato-ku, Tokyo Name
Nuclear Fuel Industry Co., Ltd. Representative Shun Shimizu-4, Agent

Claims (1)

[Claims] 1. Nuclear fuel pellets are placed in a sintering furnace and the treatment temperature is 1100.
In a low-temperature sintering method for nuclear fuel sintered bodies in which sintering is performed in an oxidizing atmosphere at a temperature of 1,300°C to 1,300°C, and then reductive heating is performed at a treatment temperature in the same range, industrial A method for producing a nuclear fuel sintered body, characterized by using pure N_2 gas. 2. The method for producing a nuclear fuel sintered body according to claim 1, wherein an appropriate amount of air is added to the N_2 gas to adjust the oxygen concentration. 3. In the manufacturing method according to claim 1 or 2, H_2 or a mixed gas of H_2 and N_2 is used as the reducing gas for the reduction heating, and the volume ratio of H_2O to this reducing gas is 0.
A method for producing a nuclear fuel sintered body containing 0.01% or more. 4. In the manufacturing method according to claim 1, 2 or 3, the nuclear fuel pellet is a base material UO_2_+_x (where x<
0.25) by adding 25 to 45% by weight of UO_2_+_y (0.25≦y≦1.0). 5. The front part is the preheating part, the middle part is the main sintering part, and the rear part is the reduction part, and each part is connected continuously, and there is an N at the boundary of each part.
_2 In the sintering furnace of a nuclear fuel assembly with a gas curtain region, the preheating section is N_2 or N_2/Air.
The main sintering zone is a heating zone in which the main atmosphere gas is N_2 or N_2/Air.
Sintering of a nuclear fuel sintered body, characterized in that the heating zone is a heating zone of 00°C to 1300°C, and the reducing section is a heating zone of 1100°C to 1300°C in which H_2O is added to the main gas in an H_2 or H_2/N_2 atmosphere. Furnace.