JP4007508B2 - Method for producing Gd2O3-added UO2 pellets using co-grinding and spheronization (SACAM) process - Google Patents

Description

The present invention relates to a method for producing a Gd ₂ O ₃ -added UO ₂ pellet. More specifically, the present invention relates to a method for producing a pellet using a co-grinding spheroidization (SACAM) process capable of effectively controlling the sintered density and grain size of UO ₂ pellets.

FIG. 1.1 shows a conventional general manufacturing process in which gadolinium oxide (Gd ₂ O ₃ ) is added to manufacture UO ₂ pellets. As in Figure 1.1, the conventional after mechanically mixing a UO ₂ powder and Gd ₂ O ₃ powder, preformed, granulation, molding, and to produce UO ₂ pellets through a step of sintering In this case, as a method of mechanically mixing the UO ₂ powder and the Gd ₂ O ₃ powder, the V-type mixing method or the conical mixing method shown in FIGS. 2.1 and 2.2, respectively, may be used. It was general.

Factors controlling the sintering density and grain size of UO ₂ pellets to which Gd ₂ O ₃ powder has been added include molding pressure, sintering temperature, and sintering atmosphere. However, it is almost impossible to change the powder characteristics and the sintering characteristics due to the mixing state itself only by the factors of the molding pressure and the sintering temperature, and it is difficult to produce sintered pellets having a fine microstructure. Therefore, in order to solve this problem, in some cases, the sintering atmosphere is changed from a dry hydrogen atmosphere to a moisture-containing atmosphere.

In this way, controlling the molding pressure and sintering factor (temperature and atmosphere) to control the sintering density and grain size is accompanied by certain restrictions, and there is a problem in using the equipment used at the same time. It can be. Therefore, only a limited range of molding pressure and sintering conditions can be used in the manufacture of UO ₂ pellets to which Gd ₂ O ₃ has been added, and quality is also limited.

The mixed oxide of Gd ₂ O ₃ and UO ₂ has lower sinterability than UO ₂ and under the same sintering conditions, the density and grain size of the sintered pellets are all converted into UO ₂ sintered pellets. It is known to be smaller than that. In addition, when sintered in a hydrogen stream, there are problems that a large number of fine cracks are generated in the sintered pellet and that the microstructure is not uniform. Therefore, in order to solve such a problem, UO ₂ to which Gd ₂ O ₃ is added is usually sintered in a hydrogen atmosphere containing moisture or a mixed gas atmosphere of carbon dioxide and carbon monoxide.

Therefore, the present invention has been devised to solve the above-described problems of the prior art, and in the process of manufacturing UO ₂ pellets by adding Gd ₂ O ₃ , the sintering density and the grain size are controlled. The objective is to provide a method for producing UO ₂ pellets having a uniform sintering density and grain size by optimizing the mixing method as well as the molding pressure, sintering temperature, and atmosphere as factors.

In order to achieve the above object, the present invention provides a mixer in which a cylindrical container is loaded with 1 to 10% by weight of Gd ₂ O ₃ powder in UO ₂ powder in the manufacturing process of UO ₂ pellets to which Gd ₂ O ₃ is added. In the preliminary mixing using the fluidity of the powder itself, the mixed powder is charged into a single-stage multi-pass or multi-stage single-pass continuous attrition mill mixing and grinding machine and mixed and ground. stages; stage to spheroidizing the powder particles charged with milled powder in a cylindrical vessel mixed-machine; and the step of sintering after producing a molded body by using the spheroidized powder A co-grinding and spheroidizing (SACAM) process including a Gd ₂ O ₃ added UO ₂ pellet.

As described above, the present invention is in a process of manufacturing the UO ₂ pellets with the addition of Gd ₂ O _3, to adjust and subsequent milling times by mixing ground using a continuous-type Atorisshonmi Le mixing grinder powder spherical The homogenization of the powder and fluidity can be improved by the conversion treatment, and the lubricant can be added and the Gd ₂ O ₃ added UO ₂ pellets can be directly sintered and molded without pre-forming and granulating steps. In addition, in order to obtain a homogeneous sintered body with large crystal grains without microcracks even in a small molding pressure and a dry reducing sintering atmosphere, if a desired sintering density is to be obtained, a pore forming agent or the like is added. It is possible to increase the margin for manufacturing the bonded body. Therefore, this margin improved the quality of the Gd ₂ O ₃ added UO ₂ pellets and improved the productivity. Furthermore, the mixed powder produced by the method of the present invention can produce a sintered body having no defects even when it is directly molded by die wall lubrication without adding a lubricant during molding.

Hereinafter, the present invention will be described with reference to the accompanying drawings. FIG. 1.2 is a production process diagram of Gd ₂ O ₃ added UO ₂ pellets in the SACAM (Spheroidizing After Continuous Attrition Co-Milling) process of the present invention. The UO ₂ pellet to which Gd ₂ O ₃ is added forms a solid solution, and the sintering density and the grain size change according to the change in the solid solution state of Gd ₂ O ₃ with respect to UO ₂ . Factors that change the solid solution state include molding pressure, that is, molding density, sintering temperature, atmosphere, powder (UO ₂ and Gd ₂ O ₃ ) particle size, mixed state of UO ₂ powder and Gd ₂ O ₃ powder, etc. Is mentioned.

In order to control the sintering density of the UO ₂ pellets to which Gd ₂ O ₃ is added, the present inventors have used powders (UO ₂ and UO _{2) in} addition to conventionally used molding pressure, sintering temperature, and atmospheric conditions. We focused on Gd ₂ O ₃ ) characteristics and the pulverized and mixed state of UO ₂ powder and Gd ₂ O ₃ powder as control factors. In particular, as a method of mixing UO ₂ powder and Gd ₂ O ₃ powder, the fluidity of the powder is simply used instead of the conventional mechanical mixing method such as V-type mixing method and conical type mixing method. After mixing, the continuous attrition mill method was found to improve the control of the sintering density and the crystal grain size, and the present invention has been presented.
First, in the present invention, UO ₂ powder and Gd ₂ O ₃ powder are preliminarily mixed using the fluidity of the powder. At this time, it is preferable to limit the added Gd ₂ O ₃ powder to a range of 1 to 10% by weight. This is because if the addition amount is less than 1% by weight, the effect of the addition is insignificant, and if it exceeds 10% by weight, the UO ₂ solid solution limit may be exceeded. More preferably, it is added in the range of 2 to 8% by weight.

  In the present invention, such two kinds of powders are premixed by a mixer using only the fluidity of the powders. In other words, the powders are mixed using the flow of the two types of powders without using any mediator. Usually, the two types of powders are charged into a cylindrical container and then mixed for a certain period of time. It can be easily accomplished by rotating and mixing in the machine.

The initial particle size ranges of UO ₂ and Gd ₂ O ₃ powders are in the range of 0.5 to 103 μm and 1.16 to 93.6 μm, respectively, and the average particle sizes are also very uneven, such as 4.75 μm and 9.8 μm, respectively. Therefore, when UO ₂ and Gd ₂ O ₃ powder having such a particle size distribution are mixed at the mixing ratio described above and mixed by the conventional mechanical method, a powder having poor uniformity due to non-uniform particle size and mixing ratio difference can be obtained. In a pellet produced from such a mixed powder, it is difficult to obtain a fine structure with uniform crystal grains. FIG. 2 shows a conventional mechanical mixer, FIG. 2.1 shows a V-type mixer, and FIG. 2.2 shows a conical mixer.

Therefore, in the present invention, in view of the above problems, a mixing ground using a continuous-type Atorisshonmi Le mixed-grinding machine as shown in FIG. 3 UO ₂ and Gd ₂ O ₃ powder mixed as above Features. Using such continuous Atorisshonmi Le mixed-grinder, or by changing the milling times in the equipment of FIG. 3.1, or by using a multi-stage single equipment in Figure 3.2, the particle size and mixed state of powder Fine control can be performed, and the sintered density and crystal grain size can be controlled within a wide range within a desired range. It is also possible to control the sintering density by adding a pore forming agent or the like in a state having crystal grains of a desired size.

In this case, powder charging amount charged to the Atorisshonmi Le mixed-grinding machine for mixing and grinding 10 to 30% of the mixing vessel volume, and a charged by ball charging amount to 50% to 70% Each is preferably restricted. The reason for setting the powder charge in this way is to maximize the mixing and grinding effect. More specifically, the mixing and grinding effect increases as the ball charge increases, but the amount of powder to be charged decreases. Conversely, when the ball charge decreases, the amount of powder to be charged increases but the mixing and grinding effect decreases. It is to do. More preferably, the size of the ball is limited to a range of 3 mm to 10 mm in diameter.

  Since this effect has a correlation with the number of rotations of the rotating blades, the pulverization effect can be maximized by a combination of the amount of charged balls and powder and the number of rotations of the rotating blades. In the present invention, it is preferable to limit the rotational speed of the rotary blade of the mixing and grinding machine to 30 to 200 rpm.

  Further, since the attrition mill is a continuous type, the passing time is the mixing and pulverizing time, and generally it takes 2 to 5 minutes to discharge almost 100% of the input amount.

  On the other hand, the powder that has been subjected to a normal mechanical milling process is not finely pulverized while the powder is pulverized. As a result, the following powder processing operation was performed to improve the powder fluidity. That is, first, a powder having poor fluidity was charged, and then a preform (slug) was produced at a molding pressure of about 50 to 200 MPa (in this case, the green density was about 30 to 50 of the theoretical density). %). Then, the preform was sandwiched between a rotor and a sieve having a diameter of about 1 mm and granulated by impact and friction in a granulator. The granule thus obtained was excellent in powder flowability and was press-molded in the subsequent molding process, so that a green pellet having a constant length (diameter 10 mm, length 10 mm) could be quickly produced.

However, in the present invention for grinding the UO ₂ and Gd ₂ O ₃ powder were mixed using a continuous Atorisshonmi Le mixed-grinding machine such as a 3, preforming and granulation process described above is not required.

Alternatively, by rotating by charging the powder was ground in the continuous Atorisshonmi Le mixed-grinding machine in a predetermined mixer, and wherein effectively be spheroidized the powder particles in granular form To do. In the spheroidizing step, the powder is mixed and pulverized into a generally cylindrical container, and then the container is rotated for a certain period of time, similar to the process of mixing powders using only the fluidity of the powder described above. Is more easily accomplished.

  In general, if the rotation time is short, the spheroidization is not good, and if the rotation time is too long, the particles become too large, making it difficult to charge in the subsequent molding process, making it difficult to produce a molded body having a uniform length. Therefore, in the present invention, it is preferable to rotate the container for 30 to 90 minutes after charging the powder of 30 to 50% of the internal volume of the container.

  Thus, unlike the conventional method, the present invention does not require a preforming step or a separate granulation step, and the fine powder can be granulated through the above-described spheronization step. This is because it becomes hard like that and the surface is activated. Therefore, when the powder is charged into a commonly used cylindrical container and loaded into a mixer used for premixing, it is rotated for a certain period of time to obtain granules of a desired size.

The spheroidized UO ₂ and Gd ₂ O ₃ mixed powder as described above has very good fluidity. Therefore, in the present invention, by using the granules having excellent fluidity, only the lubricant is applied to the die wall without adding the lubricant instead of the conventional method of adding the lubricant during the production of the subsequent molded body. Thus, a direct compacting process capable of producing a good molded body can be realized.

Then, when a normal sintering process is performed on the molded body thus manufactured, UO ₂ pellets having excellent physical properties can be manufactured. At this time, the present invention is not limited to specific sintering conditions, but preferably the above molded body is sintered in a reducing atmosphere (generally a hydrogen atmosphere) at 1650 to 1750 ° C. for 2 to 6 hours. .

  Hereinafter, the present invention will be described in detail through examples.

8% by weight of Gd ₂ O ₃ powder was preliminarily mixed with UO ₂ powder using the flowability of the powder. Then, the mixed powder was mixed and pulverized by a mixing and pulverizing machine using a continuous attrition mill method. At this time, the number of milling was changed to five stages of 0, 3, 5, 7 and 10. Here, the number of milling times indicates the number of times of passing through the continuous attrition mill, and the meaning of 5 times of milling means that the same sample was repeatedly milled 5 times in the continuous attrition mill. At this time, the rotation speed of the rotary blade of the continuous attrition mill is 150 rpm, the ball to be charged is a zirconia ball having a diameter of 8 mm, the ball charging amount is 70 vol.%, And the amount of the powder sample is 20 vol. %.

  The finely pulverized powder was rotated by using the fluidity of the powders themselves for a certain period of time using a mixer to spheroidize the particles while coarsening the particles. At this time, the spheronization time was 60 minutes.

  For these samples, the number of milling times, the apparent density by the spheroidizing treatment, the molding density and the sintering density by the molding pressure, and the microstructure of the sintered body were examined. The results are shown in FIGS. 4 to 6 as the results of the sintering characteristic test.

FIG. 4 is a graph showing the number of attrition milling and the apparent density of the powder obtained by spheroidization after milling. As shown in Fig. 4, when 8% by weight of Gd ₂ O ₃ powder is added to UO ₂ powder and premixed, the average size of the powder particles is 4.7μm, but mixed 5 times in a continuous attrition mill It can be seen that the mixed pulverization property is improved by setting the particle size of the mixed powder after pulverization to 3.7 μm. That is, according to the method of the present invention, it can be seen that the apparent density of the powder is increased by the number of milling times and the spheroidizing treatment of the continuous attrition mill, thereby increasing the fluidity of the powder and thereby performing the preforming and granulating steps. It can be seen that the molded body can be manufactured directly without the omission.

  Fig. 5 shows the green density by molding pressure (150MPa, 300MPa) after adding 0.3% by weight of lubricant to each continuous type milling cycles and spheroidized powder. 3 shows the sintered density of a pellet obtained by sintering a molded body manufactured under such conditions at 1750 ° C. for 4 hours in a dry air hydrogen atmosphere. As can be seen from FIG. 5, there is a general tendency for the molding density to increase as the number of milling increases and the molding pressure increases. In contrast, the sintered density tends to saturate after 3 or more milling cycles even if the molding density increases. That is, it can be seen that a high sintered density can be obtained even at a low molding pressure.

On the other hand, FIG. 6 is a microstructure photograph of a UO ₂ -8 wt% Gd ₂ O ₃ sintered body manufactured by the manufacturing method of the present invention (number of milling times: 5), and FIG. 6.1 shows the homogeneity of the mixed oxide nuclear fuel. Fig. 6.2 shows the grain size. As can be seen from Fig. 6.1, free UO ₂ (white part) appears in part, but it turns out that a homogeneous solid solution almost free of free UO ₂ and free Gd ₂ O ₃ is formed, and microcracks are formed. Is also absent. In FIG. 6.2, the crystal grain size is measured to be about 15 μm, and it can be seen that the dimensions are almost uniform.

As a production process diagram gd ₂ O ₃ added UO ₂ pellets is an explanatory view showing a conventional manufacturing process. It is explanatory drawing which showed the spheroidization after a co-grinding (SACAM) process which is a manufacturing process of this invention. It is explanatory drawing of a V-type mixing system. It is explanatory drawing of a cone type mixing system. It is explanatory drawing of the continuous attrition mill mixing pulverization system which shows the one step multipass system utilized for this invention method. It is explanatory drawing of the continuous attrition mill mixing pulverization system which shows the multistage one-pass system utilized for this invention method. It is the graph which showed the apparent density of the powder which changes according to the frequency | count of milling and the spheroidization process of the mixing grinder which concerns on this invention method. It is the graph which showed the correlation with the shaping | molding density according to the shaping | molding pressure of the powder manufactured by this invention method, and a sintered density. 6 is a photograph showing the homogeneity of a mixed oxide nuclear fuel in a fine structure of UO ₂ -8 wt% Gd ₂ O ₃ produced by the method of the present invention (number of milling times: 5 times). 6 is a photograph showing the crystal grain size of a mixed oxide nuclear fuel in a microstructure of UO ₂ -8 wt% Gd ₂ O ₃ produced by the method of the present invention (number of milling times: 5 times).

Claims (6)

In the manufacturing process of UO ₂ pellets with Gd ₂ O ₃ added,
Preliminarily mixing UO ₂ powder with 1 to 10% by weight of Gd ₂ O ₃ powder in a mixer equipped with a cylindrical container using only the fluidity of the powder itself;
Charging and mixing the mixed powder into a continuous attrition mill mixing and grinding machine of a single-stage multi-pass system or a multi-stage single-pass system;
Step to the milled powder the powder particles are charged into a cylindrical container of mixed-machine spheroidized; and,
Producing a molded body using the spheroidized powder and then sintering it;
A method for producing Gd ₂ O ₃ -added UO ₂ pellets using a co-grinding spheroidization (SACAM) process including: _{2. The} method for producing Gd ₂ O ₃ -added UO ₂ pellets according to claim 1, wherein ₂ to 8% by weight of Gd ₂ O ₃ powder is preliminarily mixed with the UO ₂ powder. To claim 1, characterized in that respectively control the 10 to 30 percent and 50% to 70% of the amount and the mixing pulverizer volume of the amount and the ball of the mixed powder to be charged into the Atorisshonmi Le mixed-crusher Gd ₂ O ₃ added UO ₂ method for producing pellets according. 2. The method for producing a Gd ₂ O ₃ -added UO ₂ pellet according to claim 1, wherein when the pulverized powder is charged into the mixer, the powder is charged at 30 to 50% of the internal volume of the mixer. _2. The method for producing a Gd ₂ O ₃ added UO ₂ pellet according to claim 1, wherein the molded body is sintered in a reducing atmosphere of 1650 to 1750 ° C. The above-mentioned continuous attrition mill mixing and pulverizing machine is a one-stage multi-pass system in which the number of milling is changed by one attrition mill mixing and pulverizing machine in accordance with fine control of powder particle size and mixing state, or a plurality of units The method for producing a Gd ₂ O ₃ -added UO ₂ pellet according to claim 1, wherein the attrition mill mixing and grinding machine is adapted to be adapted to a multi-stage single-pass system in which serially connected in series.

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