JP2010190718A - Method for manufacturing nuclear fuel pellet for fast breeder reactor by tumbling granulation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved method for manufacturing nuclear fuel pellets for a fast breeder reactor which is suitable for mass production. <P>SOLUTION: The method comprises the step of mixing a plutonium nitrate solution and a uranyl nitrate solution supplied from a reprocessing system for spent fuel from a fast breeder reactor with them left in solution states and adjusting the ratio of plutonium (Pu) to uranium (U) so that it can be a predetermined one, the step of transferring the adjusted mixed solution of Pu-U nitrate to a container and denitrating the mixed solution by irradiating it with microwaves to make denitrated powder before preparing MOX granulated powder by roasting and reducing the denitrated granulated powder in the container and adding a binder to the roasted and reduced powder to conduct the tumbling granulation in the container, the step of molding a certain quantity of the MOX granulated powder required for granulation into a predetermined shape of nuclear fuel pellets, and the final step of sintering the molded nuclear fuel pellets as they are for a fixed time period and adjusting the oxygen-to-metal ratio. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing nuclear fuel pellets in a fast breeder reactor cycle using a reprocessing solution provided from a fast breeder reactor spent fuel reprocessing system.

  In the fast breeder reactor cycle, FP (fission product) can be mixed into the recycled fuel, and as a next-generation fuel cycle technology, a reprocessing method improved from the conventional PUREX method (hereinafter referred to as the advanced wet method). And a plant concept that combines a pellet manufacturing method (hereinafter referred to as a simplified pellet method) in which the manufacturing process of nuclear fuel pellets is simplified as compared with the prior art. The advanced wet method collects uranium (U), neptunium (Np), and plutonium (Pu) in the co-decontamination process, thereby enabling the “uranium refining process” and “plutonium” required in the conventional Purex process. "Purification process" has been deleted. The simplified pellet method combined with the advanced wet method eliminates the powder mixing process, which occupies most of the conventional pellet manufacturing process, by adjusting the enrichment of plutonium by mixing uranium and plutonium at the nitric acid solution stage. The entire plant is greatly simplified (see Non-Patent Document 1).

  The simplified pellet method, which is a process for producing nuclear fuel pellets, is a Pu-U mixed solution adjusted for plutonium enrichment, denitrated, converted, granulated, converted into MOX powder, molded into MOX powder, sintered, This is a method for producing product pellets by adjusting O / M (the ratio of the number of atoms of oxygen and heavy metal elements). Such a simplified pellet method is described in, for example, Japanese Patent Application Laid-Open No. 2003-4883 (Patent Document 1).

JP 2003-4883 A

Japan Atomic Energy Agency, Japan Atomic Energy Co., Ltd., Strategic Research for Practical Use of Fast Breeder Reactor Cycle Phase II Final Report, 2006, JAEA-Evaluation 2006-002, 191P

  The simplified pellet method described above is still on a laboratory scale, and various verifications are necessary to realize this method as a method for producing nuclear fuel pellets suitable for mass production. For example, the problem of how to reduce radioactive waste, the problem of reducing the exposure of workers during production, and the problem of economic efficiency are also issues to be solved.

  Accordingly, an object of the present invention is to solve the above-described problems and to provide an improved method for producing nuclear fuel pellets for fast breeder reactors suitable for mass production.

  According to one aspect of the present invention, a method for producing a nuclear fuel pellet for a fast breeder reactor is obtained by mixing a plutonium nitrate solution and a uranyl nitrate solution supplied from a fast-reactor spent fuel reprocessing system as a solution. ) Adjust the ratio of uranium (U) to a predetermined ratio, transfer the adjusted Pu-U nitrate mixed solution to a container, denitrate by microwave irradiation to make denitrated powder, Denitration powder is roasted and reduced in the container, a binder is added to the roasted and reduced powder, and rolling granulation is performed in the container to produce MOX granulated powder, a certain amount necessary for pelletization The MOX granulated powder is formed into a predetermined nuclear fuel pellet shape, and finally the formed nuclear fuel pellet is sintered for a certain period of time and O / M adjusted.

  According to this method, all the processes of denitration, roasting reduction and granulation are unified, that is, the denitration powder is roasted and reduced as it is without being transferred to another container, and then the roasted and reduced powder is transferred to another container. And can be granulated as it is.

Preferably, in the above-described method for producing a nuclear fuel pellet for a fast breeder reactor, the rolling granulation is performed by reducing the number of revolutions of the stirring blade for performing the rolling granulation or the load on the motor for rotating the stirring blade. Terminate based on fluctuations.
This is because the granulated powder becomes clogged in the clearance between the stirring blade and the inner wall of the container as the granulation progresses, and the load on the stirring blade increases or, in some cases, the stirring resistance increases as the granulation progresses. This is based on the phenomenon that the load on the stirring blade is reduced. In other words, in the case of rolling granulation, it is configured to automatically control the end point of operation of the powder rolling granulator based on the newly discovered event that any of these phenomena will appear remarkably. is doing.

  Preferably, the addition rate of water added as a binder during the rolling granulation described above is 18 wt%. This is because by setting the water addition rate to 18 wt%, a granulated powder having a grain size that is most uniform for each grain can be produced.

  Furthermore, the water as the binder of the denitration powder may be uniformly dispersed during granulation, but by adding water to the denitration powder before granulation, water is added during granulation. However, since a uniform granulated powder free from lumps can be easily produced without requiring complicated control, it is possible to make the system configuration more suitable for mass production.

  According to the method for producing nuclear fuel pellets for a fast breeder reactor according to the present invention, all the steps of denitration, roasting reduction and granulation are unified, that is, the denitrated powder is roasted and reduced as it is without being transferred to another container, and further roasted Since the powder that has been sinter-reduced is granulated as it is without being transferred to another container, it is possible not only to prevent scattering of the powder, but also to build a system suitable for mass production by eliminating the transfer process. It becomes possible.

  Furthermore, in the invention according to another aspect, it is possible to automatically control the end point of the granulation process by the rolling granulator, and to construct a nuclear fuel pellet manufacturing system for a fast breeder reactor that is more suitable for mass production. Is possible. As a result, effects such as reduction of worker exposure, initial investment and operation cost can be obtained.

It is a schematic process explanatory drawing of the manufacturing method of the nuclear fuel pellet by the simplified pellet method to which this invention is applied. FIG. 2 is a schematic explanatory view schematically showing respective steps of microwave denitration, roasting reduction, and rolling granulation in the simplified pellet method shown in FIG. 1. It is a schematic explanatory drawing of the die lubrication molding machine for nuclear fuel pellet shaping | molding used with the simplified pellet method shown by FIG. It is a graph for demonstrating the change of O / M ratio during heat processing. It is a figure which shows the result of the moisture addition rate test in the granulation process of the simplified pellet method shown by FIG. It is a figure which shows the result of the reproducibility test of a moisture addition rate in the granulation process of the simplified pellet method shown by FIG.

  First, schematic steps of the method for producing nuclear fuel pellets by the simplified pellet method according to the present invention will be described with reference to FIG. Throughout the drawings, the same reference numerals indicate substantially the same functionally.

  In FIG. 1, the plutonium enrichment is first adjusted. Specifically, plutonium nitrate solution and uranyl nitrate solution provided from the fast breeder reactor spent fuel reprocessing system (100) are mixed in solution using a vacuum combined air lift equipment, where Pu: U is used. The fuel is adjusted to a ratio (for example, 2: 8) determined by the ratio of Pu to U of the fuel (step 101).

  The mixed solution of Pu-U nitrate adjusted in plutonium enrichment is placed in a relatively shallow ceramic container (eg, 600 mmφ × 75 mm high) and denitrated by microwave irradiation (eg, 40 kW) as it is. After that (step 102), it is roasted and reduced (step 103). The roasted reduced powder produced as a result is conveyed to a granulator while being put in the same container, and is tumbled and granulated by high-speed stirring to produce MOX granulated powder (step 104).

  Each process of the above-mentioned denitration, roasting reduction, and granulation will be described in more detail with reference to FIG. FIG. 2 schematically shows each step of microwave denitration, roasting reduction, and rolling granulation. The apparatus on the left side of FIG. 2 is a microwave denitration apparatus, 21 is a container, 25 is a table for placing the container, 200 is a microwave shown in a simulated manner, and 201 is in the container 21. Denitrated powder. Further, the apparatus on the right side of the paper is a rolling granulator, 22 is a stirring blade for stirring powder, and 23 is a motor for driving the stirring blade. Reference numeral 24 denotes a cover for preventing the powder from scattering during stirring. Here, the roasting reduction apparatus is not particularly illustrated.

  In the above-described denitration, roasting reduction, and granulation processes, first, a Pu-U nitrate mixed solution (not shown) in which the plutonium enrichment is adjusted so that Pu: U is in a ratio of, for example, 2: 8. Then, it is injected into the container 21 for microwave denitration. Thereafter, the container 21 is put into a microwave denitration apparatus, and the microwave 200 is irradiated to the solution in the container 21 while the table 25 is slowly rotated by a motor (not shown) on the lower side of the figure, thereby removing the denitration. Is done. The code | symbol 201 shown on the left side toward the paper surface of FIG. 2 has shown the denitrated powder. After irradiation with the microwave 200, the powder 201 is taken out together with the container 21, and roasted and reduced in a batch-type heating furnace (not shown) while being put in the container.

The uranium oxide solution was tested for roasting reduction.
This test was performed for the batch heating furnace and the rotary kiln heating furnace, respectively. As a result, both of them can be roasted in 1 hour at roasting conditions of 650 ° C or higher, and stable reduction can be achieved with reducing conditions of 650 ° C or higher and hydrogen equivalents of 3 times or higher. A roasted reduced powder having a weight of 2.0 to 2.2 was obtained. However, in the present invention, a batch-type heating furnace that has been used so far is employed from the viewpoint of handling denitration powder.

  The obtained roasted reduced powder (powder 201) is transferred to the right rolling granulator toward the paper surface of FIG. 2 while being put in the same container, and after the binder is added (for example, water is added). After spraying), the mixture is stirred at a constant speed by the stirring blade 22 driven by the motor 23, which is given from above the container 21, and granulated powder is produced (step 104). At this time, the water may be sprayed uniformly over the entire powder, and may be performed before or during granulation. As a result of granulation, MOX granulated powder with good fluidity, such as Carr's fluidity index or the like, is obtained. Although granulation is performed by a rolling granulator here, the present invention is not limited to this granulation method, and crushing rolling granulation which is a similar granulation method can also be used. . Details of the above granulation process will be described in more detail based on test data in paragraph [0027] and thereafter.

  The MOX granulated powder obtained in step 104 is conveyed to the next molding step for pelletization. The transported MOX granulated powder is supplied to the molding machine only in a certain amount necessary for pelletization, where it is molded into a preset nuclear fuel pellet shape to be inserted into the cladding tube of the nuclear fuel rod for the fast breeder reactor. (Step 105).

  The above molding process will be described in more detail with reference to FIG. FIG. 3 is a schematic explanatory diagram of a die lubrication molding machine. In FIG. 3, 30 is a die, 31 is a die wall surface, 32 is a solid cylindrical portion for making a pellet hollow, and 33 and 34 are an upper punch and a lower punch, respectively. 35 is a tank for storing MOX granulated powder, and 36 is a device for spraying a powder lubricant onto the die wall surface 31.

  In pellet molding, first, the powder lubricant is applied almost uniformly to the die wall surface 31 by the spraying device 36. Thereafter, a certain amount of powder is supplied from the tank 35 to the die 30 and formed into hollow pellets by the upper and lower punches 33 and 34.

  When compacting powder, if the powder lubricant is non-uniform and the frictional resistance between the powder and the molding die wall surface is large, the compressive load transmitted to the powder will be non-uniform, resulting in chipping, cracking, non-uniform density, and mechanical strength of the molded product. Causes a drop. In order to obtain a molded product of stable quality by die lubrication molding, the powder lubricant is applied from below the die 30 and the excess powder lubricant in the die 30 is sucked from above the die 30. It is preferable. In addition, due to the difference in fluidity of the powder lubricant, the amount of the powder lubricant filled in the die 30 varies. Therefore, in order to reduce the influence on the quality of the molded body due to the variation in the filling amount of the powder lubricant, the spraying device 36 is preferably provided with a quantitative filling mechanism for filling a certain amount of the powder lubricant. Furthermore, in order to improve maintainability, it is good also as a structure where only die set parts, such as the die | dye 30 and the upper and lower punches 33 and 34, are installed in a glove box (not shown) and a power part is installed outside a glove box.

  The pellets molded in step 105 are sintered at a predetermined temperature for a certain period of time, and the O / M ratio, which is the ratio of the number of oxygen and heavy metal elements, is adjusted to be as low as possible (step 106). This is because, when this O / M ratio is large, for example, a nuclear fuel rod cladding tube made of nuclear fuel pellets and oxide dispersion strengthened (ODS) steel exerts a chemical interaction (PCCI). Here, since the MOX granulated powder does not contain additives such as a binder, pre-sintering and degassing are not necessary.

  Although the O / M ratio is closely related to the sintering time, the O / M ratio tends to increase when the temperature is lowered at the end of sintering. FIG. 4 is a graph for explaining the change in the O / M ratio when 30% Pu-MOX fuel pellets are heat-treated with an atmospheric gas containing 40 ppm of H2O. Reference numbers 41a and 41b denote time-dependent changes in the heat treatment temperature. , 42a and 42b show changes with time in the O / M ratio. Both (a) and (b) show the relationship between time (h) and heat treatment temperature (° C.) and the relationship between time (h) and O / M ratio. (A) is the case where the temperature decrease rate is 600 ° C./h, and (b) is the case where the temperature decrease rate is 1000 ° C./h. As is apparent from these figures, in any case, the O / M ratio increases almost simultaneously with the end of the heat treatment time. However, when the cooling rate is 600 ° C / h, the O / M ratio finally rises to 1.959, whereas when the cooling rate is 1000 ° C / h, the O / M ratio rises to 1.951. Therefore, the O / M ratio can be further reduced by rapidly lowering the temperature.

  The sintered pellets are ground on the outer periphery, inspected for density and appearance (dimensions), and further subjected to government inspection to become product pellets (step 106). It is preferable that the size / density inspection, outer periphery grinding and appearance inspection of the sintered pellets can be processed at once by a multi-function composite facility.

[Granulation test]
Here, the granulation process performed in step 102 of FIG. 1 will be described in more detail with reference to FIGS. 5 and 6. FIG. 5 shows the results of a water addition rate test during granulation. In FIG. 5, (a) shows the granulation test result at a moisture addition rate of 16 wt%, (b) shows the granulation test result at a moisture addition rate of 17 wt%, and (c) shows the granulation test result at a moisture addition rate of 18 wt%. (D) shows the granulation test result at a moisture addition rate of 19 wt%, and (e) shows the granulation test result at a moisture addition rate of 20 wt%. Further, FIG. 6 shows the result of the reproducibility test of granulation at a moisture addition rate of 18 wt% where the result of the moisture addition rate test was good.

  The test results in FIGS. 5 and 6 were obtained by a granulation test performed under the conditions shown in Table 1 below.

  As shown in Table 1, a 2 liter high-speed mixer equipped with a stirring blade and a crushing blade was used as a rolling granulator. The mixing blade shape of this mixer is 3 blades and the rotation speed is 300 to 2000 rpm. Moreover, the shape of the crushing blade is a C-shaped blade, and the rotation speed is 500 to 4500 rpm. The operation of the rolling granulator was intermittent operation every 30 seconds for each water addition rate in order to confirm the granulation state at regular intervals. The operation of the rolling granulator is intermittently operated in order to confirm the state of granulation every predetermined time, but in an actual machine, it should be controlled so as to be continuously operated in order to increase the processing efficiency.

  In this test, MOX powder having the specifications shown in Table 2 was used.

  Please refer to FIG. Rolling granulation was attempted for a maximum of 8.5 minutes in increments of 1 wt% from 16 wt% to 20 wt%. When the moisture addition rate was 16 wt% and 17 wt%, rolling granulation was performed for the maximum time of 8.5 minutes, but almost as shown in the micrographs of (a) and (b). I could not. On the contrary, when the moisture addition rate is 19 wt% and 20 wt%, as shown in the micrographs of (d) and (e) by rolling granulation for a very short time (0.5 minutes), Grams with a unit of mm exceeding the specifications shown in Table 3 to be described later were granulated, and no grains having a substantially uniform particle size to meet the specifications were obtained.

  On the other hand, when the water addition rate was 18 wt%, as shown in the micrograph of (c), grains having a substantially uniform particle size were granulated by a rolling operation for 2.5 minutes. Table 3 shows the analysis results of the granulated particles.

  Therefore, under the conditions shown in Table 1, a granulation reproducibility test was performed. The reproducibility test is performed for granulation times of 1.5 minutes, 2.5 minutes, 3.5 minutes, and 5.25 minutes. Just in case, repeatability tests are performed twice under the same conditions for 3.5 minutes. went.

  The results of these reproducibility tests are shown in FIG. From the results shown in FIG. 6, when the moisture addition rate is 18 wt%, it is possible to obtain grains satisfying the specifications shown in Table 3 above in any case of granulation time from 1.5 minutes to 5.25 minutes. all right. It can be seen that a granulated powder meeting the specifications can be obtained at least for an operation with a granulation time of 1.5 minutes or longer.

  The inventors further conducted a test in which the rolling granulator was continuously operated for a longer time under the conditions shown in Table 1. Visual inspection was performed when 180 seconds (3.0 minutes) had passed, but no significant changes were observed in the granulation state and the number of rotations. Therefore, when the operation was continued further, the rotational speed of the stirring blade decreased from 807 rpm to 797 rpm after 333 seconds (180 seconds + 153 seconds) from the start of the operation. At that time, the operation of the rolling granulator was terminated and the granulation state was inspected. As a result, it was found that particles having an average particle diameter of 250 μm, a fluidity index of 75, and an O / M of 2.13 were obtained that meet the specifications. In another test, a phenomenon was observed in which the number of revolutions increased this time after a certain period of time had elapsed from the start of operation, and even if the operation was terminated at that time, particles that still meet the specifications were obtained. From these events, it is understood that the operation of the rolling granulator can be automatically terminated by monitoring the rotational speed of the stirring blades or detecting the load fluctuations relating to the motor for rotating the stirring blades. .

Such an automatic control device can be configured by a well-known technique that has been used in various fields in the past, so a detailed description thereof will be omitted here. For example, when the detected rotation speed fluctuates from a steady rotation speed (for example, 810 rpm) beyond a predetermined rotation speed (for example, 10 rpm), the power to the motor that rotates the stirring blade is cut off. it can.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and is included in the claims of the present application without departing from the scope of the technical idea of the present invention. For example, in the above description, a reprocessing system using an advanced wet method is described, but the present invention can be applied to a reprocessing system using a conventional method as it is. In order to simplify the reprocessing, a reprocessing system in which some impurities such as minor actides (MA) are not removed in the reprocessing facility is also considered. However, the present invention is based on such a reprocessing system. Even a solution is applicable.

100 Nuclear Fuel Reprocessing System 101 Pu-U Solution Mixing Process 102 Microwave Denitration Process 103 Roasting Reduction Process 104 Granulation Process 105 Pellet Molding Process 106 Sintering (O / M Adjustment) Process 107 Pellet Producting Process 20 Granulating Apparatus 30 Die

Claims (3)

  Mix the plutonium nitrate solution and uranyl nitrate solution supplied from the fast breeder spent fuel reprocessing system in solution and adjust the plutonium (Pu) to uranium (U) ratio to a predetermined ratio. Then, the adjusted Pu-U nitrate mixed solution is transferred to a container and denitrated by microwave irradiation to obtain a denitrified powder, and then the denitrated powder is roasted and reduced in the container, and the roasted and reduced powder Add a binder to the body and roll and granulate in the container to produce MOX granulated powder, mold a certain amount of MOX granulated powder necessary for pelletization into the shape of a predetermined nuclear fuel pellet, and finally A method for producing nuclear fuel pellets for fast breeder reactors, in which the nuclear fuel pellets molded into sinter are sintered for a certain period of time and O / M adjusted.   The method for producing nuclear fuel pellets for a fast breeder reactor according to claim 1, wherein in the rolling granulation step, the rotation speed of the stirring blade for performing rolling granulation or a motor for rotating the stirring blade is rotated. A process for producing nuclear fuel pellets for a fast breeder reactor, wherein the process is terminated based on fluctuations in load.   3. The nuclear fuel pellet for a fast breeder reactor according to claim 1 or 2, wherein an addition rate of water added as a binder during the rolling granulation is 18 wt%. Pellet manufacturing method.

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