KR101415171B1 - Porous UO2 sintered pellet with improved electro reduction efficiency, and the preparation method thereof - Google Patents

Abstract

The present invention relates to a porous UO ₂ sintered pellet having improved electrolytic reduction efficiency and a method for producing the same, and more particularly, to a porous UO ₂ sintered pellet having porous UO ₂ A sintered pellet characterized by comprising at least one hollow space formed from the surface of the sintered pellet so as to face the inside of the sintered pellet. The porous UO ₂ having improved electrolytic reduction efficiency The sintered pellets are provided. The porous UO ₂ sintered pellet having improved electrolytic reduction efficiency according to the present invention has an increased surface area capable of reacting with the outside as compared with the porous sintered pellet of the conventional cylindrical type and the number of pores present on the surface is increased due to the increase of the surface area The volatile and quasi-volatile fission products present in spent fuel can be readily released during sintering and reduction of the atmosphere. Also, due to the increased surface area, the sintering density required in the electrolytic reduction step can be achieved at a lower sintering temperature, and the temperature and time at which the O / U ratio becomes 2.00 at the time of reduction can be shortened.

Description

TECHNICAL FIELD [0001] The present invention relates to a porous UO2 sintered pellet having improved electrolytic reduction efficiency and a manufacturing method thereof,

The present invention relates to a porous UO ₂ sintered pellet having improved electrolytic reduction efficiency and a method for producing the same.

Uranium dioxide (UO ₂ ), a spent fuel in PWR, contains uranium (U), which is an unburned fissile material, and fission products, in addition to uranium (TRU) exist. The pyro processing process is a recycling technology that produces metal fuel, which is a fuel at high speed, by treating the uranium dioxide (UO ₂ ) burned in the light water reactor with high temperature, and is excellent in nuclear diffusion resistance and nuclear nonproliferation. The pyrolytic process used to recover the fission material, UO ₂ powder in U ₃ O ₈ powder The pretreatment process for producing the sintered pellets and the UO ₂ The sintered pellets, that is, the subsequent process of converting the ceramic fuel into a metal fuel, and the existing fission products are preferably removed in the pretreatment process since they may have a significant effect on the subsequent process of converting the ceramic fuel into metal fuel.

At this time, the pretreatment process includes disassembling / cutting and decladding of fuel rods, molding and sintering processes, and the subsequent processes include electrolytic reduction, electrolytic refining, and electrolytic refining. The removal process in the pretreatment process is a process of removing uranium dioxide (UO ₂ ) sintered pellets in the disused / cut fuel rods. Generally, when the uranium dioxide sintered pellets in the fuel rods are oxidized at a temperature of 350 to 700 ° C U ₃ O ₈ , and it becomes powder due to the volume expansion due to density reduction and exits from the fuel rod. At this time, as uranium dioxide sintered pellets are oxidized, iodine (I) and bromine (Br), which are volatile products on the gas phase, are volatilized.

The U ₃ O ₈ powder formed through the recycling process is formed into a desired shape and size by using a molding machine such as a press and then sintered at a suitable temperature in a desired atmosphere (oxidizing, inert, nitrogen and reducing) A sintered pellet having porosity suitable for volatilization and suitable for handling can be produced. Porous UO ₂ sintered pellets are easy to volatilize the fission product and when treated with UO ₂ than in U ₃ O ₈ in the subsequent electrolytic reduction process, the O / U ratio decreases from 2.67 to 2.00, The processing speed can be greatly increased and the throughput can be increased, so that the productivity of the process can be improved.

As one example of a method for producing the porous UO ₂ sintered pellet as described above, Korean Patent No. 10-0293482 discloses a method for producing a porous UO ₂ sintered pellet by adding various kinds of sintering accelerator to U ₃ O ₈ powder oxidized using spent fuel then, the bar, and by sintering it in a reducing atmosphere above 1500 ℃ disclosed a process for producing UO ₂ sintered body is shown to be effective to produce a UO ₂ sintered body exhibiting a high sintering density.

Also, Korean Patent No. 10-1020783 discloses a method for producing porous granules from spent nuclear fuel. The spent nuclear fuel is charged into a volatile oxidizer and then rotated, and oxidized at 450 to 600 ° C Treating the U ₃ O ₈ powder and the metal fission product by rotating the U ₃ O ₈ fine powder and the metal fission product at an oxidation temperature of 700 ° C. to 800 ° C. to produce a U ₃ O ₈ powder and a metal oxide Preparing a porous granule by preparing a porous granule of UO _{2 + z} by further heat-treating the powder of U ₃ O ₈ under inert conditions of rotation at 1000 ° C. to 1300 ° C. .

On the other hand, porous UO ₂ sintered pellets for electrolytic reduction for recovering existing metal fuels are generally in the form of cylinders, and such cylindrical sintered pellets require a sufficient time to volatilize the fission product during sintering and reduction of the atmosphere.

Further, in order to electrolytically reduce the cylindrical UO ₂ sintered pellet to metal fuel (U & TRU), molten salt, which is one of electrolytic materials, must penetrate through the pores existing on the surface of the UO ₂ sintered pellet. At this time, it takes a long time for the molten salt to penetrate into the sintered pellets, and it is difficult for the molten salt to penetrate into the inside of the electrolytic reduction process, so that the electrolytic reduction efficiency is not 100%.

Therefore, in order to solve these problems, the diameter of the UO ₂ sintered pellets must be small and the length thereof long. However, this type of sintered pellets requires considerable care during manufacturing and handling, and may limit the throughput, which may not be economically feasible.

Accordingly, the inventors of the present invention have been studying a method for producing a porous UO ₂ sintered pellet for electrolytic reduction in order to recover metal fuel from spent nuclear fuel (UO ₂ ) Porous UO ₂ sintered pellets having an improved electrolytic reduction efficiency by providing a hollow space, thereby completing the present invention.

It is an object of the present invention to provide a porous UO ₂ sintered pellet with improved electrolytic reduction efficiency and a method of manufacturing the same.

In order to achieve the above object,

Porous UO ₂ for input to the electrolytic reduction process for metallic fuel recovery The sintered pellet is characterized in that at least one hollow space is formed from the surface of the sintered pellet so as to face the inside of the sintered pellet, wherein the porous UO ₂ sintered pellet has improved electrolytic reduction efficiency.

In addition,

Oxidizing spent nuclear fuel containing uranium dioxide (UO ₂ ) to form a powder containing U ₃ O ₈ (step 1);

Molding the powder formed in step 1 into a shaped body having a hollow space (step 2); And

(3) a step of sintering the formed body produced in the step (2) under a reducing atmosphere and then reducing the reduced UO ₂ sintered pellet by a reducing atmosphere gas, thereby introducing the metal fuel into the electrolytic reduction step Porous UO ₂ with improved electrolytic reduction efficiency A method for producing a sintered pellet is provided.

The porous UO ₂ sintered pellet having improved electrolytic reduction efficiency according to the present invention has an increased surface area that can be reacted with the external porous sintered pellet as compared with the conventional cylindrical porous sintered pellet. The increase in the surface area increases the number of pores present on the surface The volatile and quasi-volatile fission products present in spent fuel can be readily released during sintering and reduction of the atmosphere.

Also, due to the increased surface area, the sintering density required in the electrolytic reduction step can be achieved at a lower sintering temperature, and the temperature and time at which the O / U ratio becomes 2.00 at the time of reduction can be shortened.

Further, the contact between the sintered pellet and the electrolytic material can be increased during the electrolytic reduction process, and the depth of the electrolytic material to penetrate into the sintered pellet can be shortened, thereby improving the rate of electrolytic reduction and shortening the depth at which the electrolytic material should penetrate , The diameter of the sintered pellet can be made larger than that of the conventional porous UO ₂ sintered pellet, and the weight reduction effect can be compensated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a porous UO ₂ sintered pellet of a general cylindrical shape; FIG.
2 is a view of a porous UO ₂ sintered pellet with a hollow space according to the present invention;
Figure 3 is a graph showing the sintered density of the porous sintered UO ₂ pellets prepared from a porous sintered UO ₂ pellets as in Comparative Example 1 prepared in Example 1 of the present invention the sintering temperature;
4 is a graph showing the sintered density of the porous sintered UO ₂ pellets prepared from a porous sintered UO ₂ pellets as in Comparative Example 2 prepared in Example 2 of the present invention the sintering temperature;
5 is a graph showing the sintered density of the porous sintered UO ₂ pellets prepared from a porous sintered UO ₂ pellets as in Comparative Example 3 prepared in Example 3 of the present invention the sintering temperature;
6 is a graph showing the sintered density of the porous sintered UO ₂ pellets prepared from a porous sintered UO ₂ pellets as in Comparative Example 4 prepared in Example 4 of the present invention the sintering temperature;
FIGS. 7 to 10 are photographs of the fracture surfaces of the porous UO ₂ sintered pellets prepared in Examples 1 to 4 of the present invention by scanning electron microscopy.

The present invention

Porous UO ₂ for input to the electrolytic reduction process for metallic fuel recovery A sintered pellet characterized by comprising at least one hollow space formed from the surface of the sintered pellet so as to face the inside of the sintered pellet. The porous UO ₂ having improved electrolytic reduction efficiency The sintered pellets are provided.

The porous UO ₂ according to the present invention The sintered pellets are provided with cavities formed so as to face the inside of the sintered pellets from the surface of the sintered pellets in order to increase the contact area with the electrolytic material (molten salt) during the electrolytic reduction process and reduce the penetration depth of the electrolytic material.

As shown in Fig. 1, the conventional general cylindrical porous UO ₂ Since the sintered pellet has a limited area in contact with the electrolytic material in the electrolytic reduction process and the electrolytic material must penetrate into the sintered pellet, a long time is required for the electrolytic reduction process. In some cases, The electrolytic reduction efficiency could be lowered.

However, the porous UO ₂ of the present invention The sintered pellets have voids formed from the surface toward the inside thereof, and the porous UO ₂ The surface area of the sintered pellet is increased. As the surface area is increased, the contact area with the electrolytic material increases, so that the electrolytic reduction process can be performed more efficiently, and the electrolytic material penetration depth is short, thereby improving the electrolytic reduction rate. Furthermore, because of the short depth of penetration of the electrolytic material, larger size sintered pellets than conventional porous UO ₂ sintered pellets can be utilized, which can offset the weight reduction of the sintered pellets due to the cavities.

At this time, the porous UO ₂ of the present invention Although the sintered pellet is not limited in its shape, it can be manufactured in any form considering the ease of handling and simplification of the manufacturing process while being suitable for the subsequent electrolytic reduction process. For example, the porous UO ₂ The sintered pellets may be in the form of cylinders or polygonal columns.

Further, in the one or more cavities, the number of cavities is not particularly limited, but one or more cavities can be formed in consideration of the strength of the sintered pellets, ease of handling, and simplicity of the manufacturing process.

Further, the cavities are connected to each other to form porous UO ₂ For example, through the sintered pellets. In one example, the cavity may comprise a porous UO ₂ The sintered pellets can penetrate in the longitudinal direction. However, the shape and the structure of the cavity portion are not limited thereto, and various types and structures of cavities may be formed of porous UO ₂ Sintered pellets.

On the other hand, the porous UO ₂ In the sintered pellet, the volume ratio of the cavity to the sintered pellet is preferably 2 to 30%.

In forming the one or more cavities, the efficiency of the electrolytic reduction process is taken into consideration. At this time, the higher the volume ratio occupied by the hollow portion, the greater the electrolytic reduction rate of each sintered pellet, but the total amount of the spent fuel sintered body that can be treated at one time may decrease. Thus, the cavity may have a porous UO ₂ The sintered pellets are preferably formed at a volume ratio of 2 to 30% with respect to the sintered pellets, thereby improving the electrolytic reduction rate and the electrolytic reduction efficiency and maintaining the integrity of the sintered pellets.

Hereinafter, the porous UO ₂ according to the present invention As shown in Fig. 2, the sintered pellets were filled with a general cylindrical porous UO ₂ The sintered pellet will be described as an example.

As shown in Figure 2, the porous UO ₂ The sintered pellet 10 may be in the form of a cylinder and is provided with a hollow portion 20 facing from the surface of the sintered pellet toward the inside thereof. The hollow portion 20 may be formed to have a structure penetrating the center portion of the sintered pellet have. The shape of the hollow portion 20 is not particularly limited, but may be formed in a cylindrical shape. The size of the hollow portion can be appropriately adjusted according to the porosity of the surface of the sintered pellet 10 and the reduction treatment speed.

At this time, the width of the sintered pellet 10 shown in FIG. 2 can be determined according to the size of the cavity 20, and the narrower the width d of the sintered pellet, the more smoothly the volatilization of the fission products during sintering And further, the penetration path of the electrolytic material is short, so that electrolytic reduction can proceed at a higher speed. However, if the width d of the sintered pellet is excessively narrow, the strength of the sintered pellet may be lowered, and the weight of the sintered pellet applied to the electrolytic reduction process may be reduced. However, the width d of the sintered pellet is not limited to the specific numerical range but can be appropriately adjusted in consideration of the economical efficiency in the electrolytic reduction and the strength of the sintered pellet.

In addition,

Oxidizing spent nuclear fuel containing uranium dioxide (UO ₂ ) to form a powder containing U ₃ O ₈ (step 1);

Molding the powder formed in step 1 into a shaped body having a hollow space (step 2); And

(3) a step of sintering the formed body produced in the step (2) under a reducing atmosphere and then reducing the reduced UO ₂ sintered pellet by a reducing atmosphere gas, thereby introducing the metal fuel into the electrolytic reduction step Porous UO ₂ with improved electrolytic reduction efficiency A method for producing a sintered pellet is provided.

In the method for producing a porous UO ₂ sintered pellet according to the present invention, step 1 is a step of oxidizing a spent nuclear fuel containing uranium dioxide (UO ₂ ) to form a powder containing U ₃ O ₈ .

In the present invention, the U ₃ O ₈ powder, which is a raw material used for producing the porous UO ₂ sintered pellet, is generally formed from a spent fuel containing uranium dioxide (UO ₂ ), and generally, uranium dioxide (UO ₂ Is preferably oxidized at a temperature of 400 to 500 ° C in consideration of the particle size of the powder to be oxidized and various factors. When the spent nuclear fuel containing uranium dioxide (UO ₂ ) is oxidized at a certain temperature under the oxidizing atmosphere, it is oxidized to U ₃ O ₈ and the powderization occurs due to the volume expansion due to the decrease in density. If the oxidation of step 1 is carried out at a temperature of less than 400 ° C., it takes a long time to oxidize to U ₃ O ₈ , and there is a problem that a large amount of time is consumed for withdrawing the spent nuclear fuel from the cladding tube. It is difficult to control the size of the particles as the U ₃ O ₈ is rapidly formed, and thus the coarse U ₃ O ₈ There is a problem that particles are generated.

In the method of producing the porous UO ₂ sintered pellet according to the present invention, step 2 is a step of molding the powder formed in step 1 into a molded body having a hollow space.

Molding the molded body using the powder formed in the step 1 of the step 2, and molding the molded body to have a cavity with the molded body, and the molding of the step 2 may be performed by a conventional method such as compression molding. At this time, the molding in the step 2 may be performed by compressing the powder using a mold designed to provide a cavity with a molded body, but not limited thereto, and using all the methods capable of forming a cavity with a molded body .

On the other hand, the molded body manufactured through the molding in the step 2 is preferably in the form of a cylindrical or polygonal column suitable for a subsequent process, but is not limited thereto.

Further, the molding of step 2 is preferably carried out at a molding pressure of 100 to 500 MPa. When the molding pressure is less than 100 MPa, the powder is not sufficiently compressed, so that the integrity is poor, which makes it difficult to handle in the process of moving and processing to the next step. When the molding pressure is more than 500 MPa, the molded body is densified as it is compressed under excessive pressure, and thus the fission products in the molded body are not easily volatilized during the sintering process. The porosity of the shaped body can be controlled by appropriately controlling the molding pressure in the production of the molded body at the molding pressure within the above range. When performing the subsequent sintering process by appropriately controlling the porosity, the volatilization of the fission product Can be performed smoothly.

The method for producing a porous UO ₂ sintered pellet according to the present invention may further include homogenizing the powder formed in the step 1 before performing the step 2 molding, wherein the " homogenization "≪ / RTI > means homogenization of various materials contained within the matrix.

Uranium dioxide (UO ₂ ), a spent fuel in PWR, is known to have a constant distribution of burning over the length of the same fuel assembly and fuel rod. Due to this difference in the degree of combustion distribution, the distribution of the contents according to the composition of U, TRU and fission product, which are fissile materials generated in the nuclear fuel, becomes uneven, and also affects the microstructure of the fuel. You can give.

The microstructure of the nuclear fuel varies in size depending on the degree of combustion or linear power. Depending on the degree of burning, the UO ₂ sintered pellet has an undistributed area before the irradiation from the edge to the central axis, an equiaxed area, and a columnar area. Thus, the particle size change depending on the degree of combustion affects the particle size of the oxidized U ₃ O ₈ powder, and the particle size of the oxidized powder tends to increase with increasing the degree of combustion.

This heterogeneity of distribution and non-uniform particle size of the fissile material, especially the plutonium-containing TRU material present in the U ₃ O ₈ powder after use according to the difference in the degree of combustion, ie the position of the fuel rod, Can affect the accurate weighing measurement of the < RTI ID = 0.0 > In addition, the inhomogeneity distribution of the nuclear fuel material may be difficult to achieve stabilization and reproducibility of the process parameters in producing the porous UO ₂ sintered pellets.

In order to prevent these problems, the manufacturing method according to the present invention may further include homogenizing the powder containing U ₃ O ₈ formed in the step 1, which can be controlled through the refinement of the powder. That is, the homogeneity of the powder can be improved by making the powder finer, and the homogeneity of the powder increases as the powder becomes finer. In addition, through homogenization, the concentration distribution of the different fissile materials in the spent nuclear fuel can be made uniform, and the accuracy of nuclear material quantification can be improved. Furthermore, the concentration distribution of the fission products present in the spent nuclear fuel can be homogenized to improve the trapping ability of the fission product volatilized during the sintering and reduction treatment, to uniformize the degree of volatilization of the fission product, There is an effect that reproducibility of the manufacturing process can be exhibited when the sintered pellet is manufactured.

On the other hand, when the homogenization is performed, the molding of the step 2 can be performed at a relatively low pressure as compared with the powder in which homogenization has not been performed. This is because the powder containing U ₃ O ₈ is made finer by homogenization and the size distribution of the particles is homogenized. Accordingly, sintered pellets having a desired sintering density can be produced even at relatively low molding pressure or sintering temperature than in the prior art, thereby providing excellent productivity.

On the other hand, the homogenization can be performed through a process such as blending or milling. However, if the particle size of the U ₃ O ₈ -containing powder formed in step 1 is not uniform, homogenization may be performed by mixing to cause problems such as segregation, which may result in poor homogenization . Therefore, it is more preferable that the homogenization of step 2 is performed through a milling process, and the homogeneity can be further improved by making the particle size of the powder similar. However, the homogenization of step 2 is not limited to this, and it may be carried out by appropriately selecting means for homogenizing the particle size of powders containing U ₃ O ₈ formed in step 1 and homogenizing the materials in the powder have.

At this time, the milling may be performed using a ball mill, a basket mill, an attrition mill, a bead mill, a hammer mill, etc., but the homogenization The present invention is not limited thereto.

In the method for producing porous UO ₂ sintered pellets according to the present invention, step 3 is a step of sintering the formed body produced in step 2 under an atmospheric gas and then reducing the reduced UO ₂ sintered pellets under a reducing atmosphere gas to form a porous UO ₂ sintered pellet to be.

Powder in the containing U ₃ O ₈ formed from the spent nuclear fuel is present in a wide variety of semi-volatile and volatile fission products, such fission products can result in poor in the electrolytic reduction process for reducing the ceramic fuel of a metallic nuclear fuel have. Therefore, it is preferable to volatilize by heating at a proper temperature in the pre-treatment process of pyro-processing, and the volatilized fission product should be collected by a filter and treated.

In order to remove this fission product, in step 3, the formed U ₃ O ₈ The sintered body was sintered under an atmospheric gas, and U ₃ O ₈ The fission product in the formed body is volatilized and removed. The sintered pellet of step 3 is present in the form of UO _{2 + x} (0.01? _X ? 0.67). To reduce it to UO ₂ , the sintered pellet is reduced in a reducing atmosphere, and the sintering and reduction step To form the shaped body produced in step 2 into UO ₂ pellets.

On the other hand, the sintering and reduction process in the step 3 is generally performed using porous UO ₂ Can be carried out through the methods used for producing sintered pellets.

For example, the sintering of the step 3 may be performed under an atmosphere gas at a temperature of 1000 to 1600 ° C, and the atmosphere may be air, carbon dioxide (CO ₂ ), nitrogen (N ₂ ) Argon (Ar) gas or the like can be used. When the sintering is carried out in an oxidizing gas atmosphere such as air, carbon dioxide, nitrogen gas atmosphere, or inert gas atmosphere such as argon, it is possible to adjust the O / U ratio (ratio of oxygen atoms to uranium atoms) It is easy to remove the fissile product which is a single component of the metal.

In sintering the molded body in the step 3, the sintering time is preferably 1 to 10 hours. When the sintering time is less than 1 hour, the mechanical strength of the sintered body is so weak that it may be broken even in a small impact, which makes it difficult to handle in a subsequent step. When the sintering time exceeds 10 hours, pores in the sintered body are formed coarsely, There is a problem of uneven distribution.

When the sintering is completed, UO _{2 + x} (0.01? _X ? 0.67) is formed, and the UO _{2 + x} sintered pellet There is a problem in that productivity is deteriorated in the process speed and the throughput in the electrolytic reduction process which is a subsequent process because there is more surplus oxygen than UO ₂ . That is, the sintered pellets having UO _{2 + x} (0.01? _X ? 0.67) are completely reduced to UO ₂ after the sintering is completed. For this purpose, the sintered body that has been sintered is reduced in the reducing atmosphere gas. The UO ₂ sintered pellet can be produced through the reduction, and through this, a high-quality UO ₂ sintered pellet which is porous and free from defects such as cracks can be produced. In addition, the O / U ratio of the produced UO ₂ sintered pellet is 2.00, which facilitates the subsequent electrolytic reduction process, and can perform the reduction and further volatilize the non-volatilized fission products during sintering.

At this time, the sintering and reduction of the step 3 may be carried out continuously or discontinuously.

In the case where the sintering and reduction in step 3 are continuously performed, the sintering atmosphere is reduced by converting the sintering atmosphere gas into a reducing atmosphere gas during the cooling process after sintering, Time-keeping, or immediately cooling.

That is, the above-mentioned sintering and reduction are continuously carried out means that the hydrogen gas is injected into the reducing atmosphere in order to form a reducing atmosphere immediately after the sintering is performed, and thereby the sintering and reduction can be continuously performed Can be performed.

In this case, when the sintering is performed in an air atmosphere, an inert gas such as argon is first injected to remove the oxidizing atmospheric gas, and then the hydrogen gas is injected for forming the reducing atmosphere.

When the sintering is performed in carbon dioxide, nitrogen, argon and atmospheric gas, hydrogen gas may be directly injected to form a reducing atmosphere, but the present invention is not limited thereto.

When the sintering and reduction process of step 3 is discontinuously performed, the UO _{2 + x} sintered pellet produced by sintering is cooled to room temperature and then reduced to a temperature of 1000 to 1400 ° C in a reducing atmosphere To form a porous UO ₂ sintered pellet.

The above-mentioned sintering and reduction processes are performed discontinuously means that the sintered pellets after completion of sintering are cooled to room temperature and then reduced in a reducing atmosphere at a temperature of 1000 to 1400 ° C. When the sintering and reduction are continuously performed, a wide working space is not required and the process can be performed in a relatively short time. However, since analysis of the sintered pellet after completion of sintering can not be performed, It may be difficult to improve the characteristics. However, when the sintering and reduction processes are discontinuously performed, the characteristics of the sintered pellets can be analyzed, and the temperature at which the reduction is performed can be appropriately controlled through the characteristic analysis. This makes it easier to control the O / U ratio of the porous UO ₂ sintered pellets. In addition, when the sintering and reduction processes are discontinuously performed, the trapping filters used in the sintering and reduction processes can be used individually, and when the same trapping filters are used in the sintering and reduction processes, the fission products captured in the filter during the sintering process The problem of separation from the filter due to the reaction during the reduction process can be prevented.

When the sintering and reduction processes are performed discontinuously, the reduction is preferably performed for 1 to 10 hours. If the reduction in this case is performed in less than one hour because the reduction to UO ₂ is performed, and a problem of UO _{2 + x} are produced, the reduction is carried out for more than 10 hours, the cost problems according As an unnecessary time-consuming Lt; / RTI >

The reason that the sintering and reduction processes are carried out continuously or discontinuously is that the porous UO ₂ Sintered pellets can be produced by one embodiment, and the production method according to the present invention is not limited thereto.

Before the sintering of the step 3, the molded body formed in the step 2 may be sintered by gradually raising the sintering temperature to the sintering temperature in consideration of the volatilization temperature of the fission product.

When the shaped body is heated stepwise to the sintering temperature for sintering, the fission product can be separated and collected at each temperature interval in which the volatile fission products are volatilized. Various types of quasi-volatile and volatile fission products are present in the U ₃ O ₈ powder formed from spent nuclear fuel. The volatilization temperature for removing such fission products varies depending on the type of fission product. The volatilization temperature for removing the fission product is, for example, volatilized at a temperature of about 150 ° C in the case of iodine (I) and bromine (Br), and is lower than that of technetium (Tc), ruthenium (Ru), molybdenum (Rh), tellurium (Te), carbon (C), etc. are volatilized at about 800 ° C and cesium (Cs), rubidium (Rb) and cadmium (Cd) are volatilized at about 1000 ° C. Thus, the fission products having different volatilization temperatures can be captured by heating and volatilizing stepwise to the sintering temperature and selecting a suitable filter for the volatilized fission products. That is, by heating stepwise to the sintering temperature, the fission product volatilized in each step can be collected more efficiently by using a suitable filter, and the treatment of the waste filter in which the fission product is captured can be easily performed.

On the other hand, a method of preparing a porous UO ₂ sintered pellets according to the invention for the next low density by using a raw material powder containing the after use of the raw material powder fuel (UO ₂₎ instead of plutonium (Pu), gadolinium (Gd), such as UO ₂ - PuO ₂ , UO ₂ -Gd ₂ O ₃ The present invention is not limited thereto.

Further,

And a method of performing an electrolytic reduction process using the porous UO ₂ sintered pellet.

The pyrolytic process for reusing spent fuel is performed through electrolytic reduction, electrolytic refining, and electrolytic smelting, through which metal-type fuel can be recovered. The porous UO ₂ sintered pellets according to the present invention can be used to recover metallic fuel through the pyrolysis process and can be used for the electrolytic reduction process.

Accordingly, the present invention provides a method for performing an electrolytic reduction process using the porous UO ₂ sintered pellet according to the present invention. To the porous UO ₂ sintered pellets according to the invention as a porous sintered pellets, whereby those having at least one cavity (hollow space) is formed toward the inside from the surface of the sintered pellet, electrolytic during the reduction process substance from penetrating into the Electrolytic reduction can be carried out at a higher speed. Further, as the contact area where the electrolytic material and the sintered pellet can contact with each other increases, a larger amount of electrolytic material can penetrate into the sintered pellet, thereby improving the electrolytic reduction efficiency.

Meanwhile, as a method for performing the electrolytic reduction process using the porous UO ₂ sintered pellet, for example, there is a method of immersing the porous UO ₂ sintered pellet in a hot molten salt, preferably a LiCl-Li ₂ O solution; (U), a transuranic element (TRU), and a fission product (FP) through the electrolytic reduction process to produce a metal-type metal conversion material . However, the method of performing the electrolytic reduction process using the porous UO ₂ sintered pellet according to the present invention is not limited thereto, and the porous UO ₂ sintered pellet can be carried out by an appropriate method and apparatus capable of electrolytically reducing the porous sintered pellet.

Hereinafter, the present invention will be described in more detail by way of examples. It should be noted, however, that the following examples are illustrative of the invention and are not intended to limit the scope of the invention.

Example 1 Preparation of porous UO ₂ sintered pellets having improved electrolytic reduction efficiency 1

Step 1: U ₃ O ₈ powder was prepared using unirradiated UO ₂ sintered body instead of uranium dioxide (UO ₂ ) sintered body after use irradiated from the reactor. At this time, the unirradiated UO ₂ sintered body showed a sintered density of about 96% theoretical density (TD). The non-stylized four hours oxide the irradiated UO ₂ sintered at 450 ℃, an air atmosphere, a sintered UO ₂ was prepared the U ₃ O ₈ powder, the volume expansion due to the density decrease according As oxidized to U ₃ O _8. The average particle size of the U ₃ O ₈ powder was 10 μm and the specific surface area was 0.53 m ² / g.

Step 2: The powder produced in the step 1 was charged into a die of a press provided with a mold so as to form a cavity, and a molding pressure of 300 MPa was applied to form a cylindrical molded article (Diameter: 10 mm, length: 12 mm, width (d): 2.5 mm, weight: about 4.1 g) was manufactured and the variation of molding pressure was controlled within 10 MPa

Step 3: The formed body prepared in step 2 was charged into a sintering vessel of zirconia (ZrO ₂ ) and sintered in a batch-type sintering furnace, and air was shed and sintered. At this time, the sintering temperature T was 1000 to 1400 占 폚 (1000, 1100, 1200, 1300 and 1400 占 폚), and the sintering temperature was maintained for 2 hours.

After the sintering was performed, the air was purged with Ar gas and replaced with hydrogen gas for the reduction treatment. After completion of the substitution with hydrogen gas, the sintered body was subjected to reduction treatment by cooling the sintered body.

In this case, when the sintering is carried out at a temperature of 1000 ° C, the substrate is replaced with hydrogen gas so that the O / U ratio is 2.00, and then the sintering temperature is maintained for 2 hours and then cooled. On the other hand, in the case of sintered pellets and sintered at 1000 ℃ higher temperatures (T> 1000 ℃), was the reduction treatment by directly cooling was replaced with hydrogen gas O / U ratio to be 2.00, thereby producing a porous UO ₂ sintered pellets through Respectively.

The heating and cooling rates of the sintering and reduction treatment were all set at 4 ° C per minute.

Example 2 Preparation of porous UO ₂ sintered pellets with improved electrolytic reduction efficiency 2

Porous UO ₂ sintered pellets were prepared in the same manner as in Example 1, except that Step 3 of Example 1 was carried out up to Step 2 and then Step 3 was performed as follows.

Step 3: The formed body was placed in a sintering vessel made of zirconia (ZrO ₂ ), charged into a batch-type sintering furnace, and sintered with flowing carbon dioxide (CO ₂ ). At this time, the sintering temperature T was 1000 to 1400 占 폚 (1000, 1100, 1200, 1300 and 1400 占 폚), and the sintering temperature was maintained for 2 hours.

After the sintering was performed, carbon dioxide was immediately replaced with hydrogen gas for the reduction treatment, and after the completion of the substitution with hydrogen gas, the sintered body subjected to sintering was cooled to be reduced.

In this case, when the sintering is carried out at a temperature of 1000 ° C, the substrate is replaced with hydrogen gas so that the O / U ratio is 2.00, and then the sintering temperature is maintained for 2 hours and then cooled. On the other hand, in the case of sintered pellets and sintered at high temperature (T> 1000 ℃) than 1000 ℃, O / U reduction treatment was it preparing porous UO ₂ sintered pellets through by rain immediately cooled and then substituted with hydrogen gas in order to be 2.00 Respectively.

The heating and cooling rates of the sintering and reduction treatment were all set at 4 ° C per minute.

Example 3 Preparation of Porous UO ₂ Sintered Pellets with Improved Electrolytic Reduction Efficiency

Porous UO ₂ sintered pellets were prepared in the same manner as in Example 2 except that nitrogen gas was supplied instead of carbon dioxide in Step 3 of Example 2 and sintering was carried out.

Example 4 Preparation of Porous UO ₂ Sintered Pellets with Improved Electrolytic Reduction Efficiency

Porous UO ₂ sintered pellets were prepared in the same manner as in Example 2, except that sintering was carried out by flowing argon gas instead of carbon dioxide in Step 3 of Example 2 above.

≪ Comparative Example 1 &

Porous UO ₂ sintered pellets were prepared in the same manner as in Example 1 except that a cylindrical molded body was formed without forming a hollow portion in the molded body in Step 2 of Example 1 above.

≪ Comparative Example 2 &

Porous UO ₂ sintered pellets were prepared in the same manner as in Example 2, except that in Step 2 of Example 2, a cylindrical molded body was formed without forming a cavity in the formed body.

≪ Comparative Example 3 &

Porous UO ₂ sintered pellets were prepared in the same manner as in Example 3, except that in step 2 of Example 3, a cylindrical molded body was formed without forming a cavity in the formed body.

≪ Comparative Example 4 &

Porous UO ₂ sintered pellets were prepared in the same manner as in Example 4, except that in step 2 of Example 4, a cylindrical shaped body was formed without forming a cavity in the formed body.

<Experimental Example 1> Sintered density analysis

(1) Sintering density analysis of UO ₂ sintered pellets under sintering in air atmosphere

The sintered density of the porous UO ₂ sintered pellets prepared by sintering in the air atmosphere in Example 1 and Comparative Example 1 was measured using an immersion method, and the results are shown in FIG.

As shown in FIG. 3, the theoretical density of the porous UO ₂ sintered pellets prepared in Example 1 and Comparative Example 1 was analyzed according to the sintering temperature. As a result, the porous UO ₂ sintered pellet having the cavity prepared in Example 1 Compared with the cylindrical sintered pellets of Comparative Example 1, the sintered density was about 2 to 5% TD higher than that of the sintered pellet of Comparative Example 1. It can be seen from this that the sintered density of the sintered pellets provided with the cavities is higher under the same sintering temperature condition, and it can be understood that the weight reduction of the sintered pellets due to the cavities can be partially offset.

(2) Sintering density analysis of porous UO ₂ sintered pellets under sintering under carbon dioxide atmosphere

The sintered density of the porous UO ₂ sintered pellets prepared by sintering under the carbon dioxide atmosphere in Example 2 and Comparative Example 2 was measured using an immersion method, and the results are shown in FIG.

As shown in FIG. 4, the theoretical density of the porous UO ₂ sintered pellets prepared in Example 2 and Comparative Example 2 was analyzed according to the sintering temperature. As a result, the porous UO ₂ sintered pellet having the cavity prepared in Example 2 Compared with the cylindrical sintered pellets of Comparative Example 2, the sintered density was about 1 to 5% TD higher than that of the sintered pellet of Comparative Example 2 at the entire sintering temperature range. It can be seen from this that the sintered density of the sintered pellets provided with the cavities is higher under the same sintering temperature condition, and it can be understood that the weight reduction of the sintered pellets due to the cavities can be partially offset.

(3) Sintered density analysis of porous UO ₂ sintered pellets under sintering under nitrogen atmosphere

In Example 3 and Comparative Example 3, the sintered density of the porous UO ₂ sintered pellets produced by sintering under a nitrogen atmosphere was measured using an immersion method, and the results are shown in FIG.

As shown in FIG. 5, the theoretical density of the porous UO ₂ sintered pellets prepared in Example 3 and Comparative Example 3 was analyzed according to the sintering temperature. As a result, the porous UO ₂ sintered pellet having the cavity prepared in Example 3 Compared to the cylindrical sintered pellets of Comparative Example 3, the sintered density was as high as about 1 to 5% TD at the entire sintering temperature range. It can be seen from this that the sintered density of the sintered pellets provided with the cavities is higher under the same sintering temperature condition, and it can be understood that the weight reduction of the sintered pellets due to the cavities can be partially offset.

(4) Sintered density analysis of porous UO ₂ sintered pellets under sintering in argon atmosphere

The sintered density of the porous UO ₂ sintered pellets prepared by sintering under argon atmosphere in Example 4 and Comparative Example 4 was measured using an immersion method, and the results are shown in FIG.

As shown in FIG. 6, the theoretical density of the porous UO ₂ sintered pellets prepared in Example 4 and Comparative Example 4 was analyzed according to the sintering temperature. As a result, the porous UO ₂ sintered pellet having the cavity prepared in Example 4 Compared to the cylindrical sintered pellets of Comparative Example 4, the sintered density was about 1 to 5% TD higher than that of the cylindrical sintered pellets of Comparative Example 4. It can be seen from this that the sintered density of the sintered pellets provided with the cavities is higher under the same sintering temperature condition, and it can be understood that the weight reduction of the sintered pellets due to the cavities can be partially offset.

From the above analysis results, it can be seen that the porous UO ₂ sintered pellets according to the present invention have a higher sintered density than the sintered pellets not provided with cavities under the same sintering temperature condition, It can be seen that these trends are the same.

<Experimental Example 2> Analysis of Porous UO ₂ Sintered Pellet by Scanning Electron Microscope

The fracture surfaces of the porous UO ₂ sintered pellets prepared in Examples 1 to 4 were analyzed by a scanning electron microscope. The results are shown in FIGS. 7 to 10.

As shown in FIGS. 7 to 10, it can be seen that there are pores in the inner circumferential surface and the outer circumferential surface of the porous UO ₂ sintered pellets prepared in Examples 1 to 4, and these pores are connected to the inside of the sintered pellet, The electrolytic material can easily penetrate into the inside. That is, the porous UO ₂ sintered pellet having the cavity according to the present invention has a large contact area with which the electrolytic reducing material can contact, and it is easy to penetrate the electrolytic reducing material due to the pores provided in the contact area. have. In addition, it can be predicted that the electrolytic reduction process can be performed at a higher rate because the distance that the electrolytic reducing material penetrating through the pores reaches the inside of the sintered pellet is short.

10: Porous UO ₂ sintered pellet
20: Cavity
d: width of sintered pellet

Claims (20)

Oxidizing spent nuclear fuel containing uranium dioxide (UO ₂ ) to form a powder containing U ₃ O ₈ (step 1);
Molding the powder formed in step 1 into a shaped body having a hollow space (step 2);
Sintering the porous U ₃ O ₈ molded body produced in step 2 with an atmospheric gas at a temperature of 1200 to 1600 ° C to produce a UO _{2 + x} sintered pellet (step 3); And
(Step 4) of cooling the UO _{2 + x} sintered pellet produced in step 3 to room temperature and reheating the UO ₂ sintered pellet to a temperature of 1000 to 1400 ° C in a reducing atmosphere to form a UO ₂ sintered pellet A method for producing a porous UO ₂ sintered pellet having improved electrolytic reduction efficiency for charging into an electrolytic reduction process for nuclear fuel recovery.
delete delete Oxidizing spent nuclear fuel containing uranium dioxide (UO ₂ ) to form a powder containing U ₃ O ₈ (step 1);
Molding the powder formed in step 1 into a shaped body having a hollow space (step 2); And
(Step 3) of forming porous UO ₂ sintered pellets by converting into a reducing atmosphere gas and reducing the porous sintered pellets during sintering the green compacts prepared in step 2 above at a temperature of 1000 to 1600 ° C. and cooling them, A method for manufacturing a porous UO ₂ sintered pellet having improved electrolytic reduction efficiency for charging into an electrolytic reduction process for recovering a metal fuel contained therein.
The method of claim 1 or 4, wherein the oxidation of step 1 is performed in an oxidizing atmosphere at a temperature of 400 to 500 ° C. In order to recover the metallic fuel, the porous UO having improved electrolytic reduction efficiency ₍₂₎ Process for producing sintered pellets.
According to claim 1 or 4, wherein the electrolytic reduction efficiency is improved process for producing a porous sintered UO ₂ pellets according to claim 1, further comprising the step of homogenizing the powder formed in the above step 1.
The method of claim 1 or 4, wherein the molding of step 2 is performed under a molding pressure of 100 to 500 MPa. The method of claim 1, wherein the porous UO having improved electrolytic reduction efficiency ₍₂₎ Process for producing sintered pellets.
delete The method of claim 1 or 4, wherein the atmospheric gas is air or a carbon dioxide atmosphere. The method for producing porous UO 2 sintered pellets according to claim 1 or 4, wherein the electrolytic reduction efficiency is improved.
The method of claim 1 or 4, wherein the atmospheric gas is nitrogen or an argon atmosphere. The method for producing a porous UO 2 sintered pellet according to any one of claims 1 to 4, wherein the electrolytic reduction efficiency is improved.
The method of claim 1 or 4, further comprising the step of heating the shaped body formed in step 2 to a sintering temperature stepwise before sintering the step 3 to collect the fission product. A method for producing a porous UO ₂ sintered pellet having improved electrolytic reduction efficiency for charging into an electrolytic reduction process for recovery.
delete The method according to claim 1 or 4, wherein the sintering in step 3 is performed for 1 to 10 hours. The porous UO2 sintered pellet having improved electrolytic reduction efficiency for the electrolytic reduction process for metal fuel recovery Way.
The method for producing porous UO 2 sintered pellets according to any one of claims 1 to 4, wherein the reducing atmosphere is a hydrogen gas atmosphere, and the electrolytic reduction efficiency is increased for the electrolytic reduction process for metal fuel recovery.
delete The method of claim 4, wherein the sintering and reduction are performed continuously. The method for producing porous UO ₂ sintered pellets according to claim 4, wherein the sintering and reduction are performed continuously.
delete delete The method of claim 1, wherein the reheating is performed for 1 to 10 hours. The method for producing porous UO ₂ sintered pellets according to claim 1, wherein the reheating is performed for 1 to 10 hours.
According to claim 1, wherein said sintering and reducing the efficiency of the electrolytic reduction process for producing an improved porous sintered UO ₂ pellets for introducing the electrolytic reduction process for recovering metal fuel, characterized in that is carried out discontinuously.

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