JP4863313B2 - Method for producing nuclear fuel pellet for fast breeder reactor in fast breeder reactor cycle - Google Patents

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, and denitrate by irradiating with microwave to make denitrated powder After that, a binder is added to the denitrated powder, and granulated in the container to obtain a granulated powder. 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, denitration and granulation are unified, that is, the denitration powder is granulated as it is without being transferred to another container, so that not only the powder can be prevented from scattering, but also the transfer process is eliminated and mass production is performed. It becomes possible to construct a system suitable for the system.

  Furthermore, when water is used as the binder of the denitration powder, for example, it may be sprayed uniformly during granulation, but by adding to the denitration powder before granulation, The system configuration can be made more suitable for mass production without requiring complicated control than adding water.

  Furthermore, it is preferable to rapidly lower the sintering temperature at a rate higher than the rate of temperature increase at the end of sintering. The faster the temperature drop rate, the more the O / M ratio can be suppressed, but it is also necessary to consider economic issues such as the furnace structure.

  In the present invention, since the machines, instruments, etc. used in the production of nuclear fuel pellets can be minimized, the automation of the production line is facilitated accordingly, so that radioactive waste is reduced, worker exposure is reduced, Effects such as initial investment and reduction of operating costs can be obtained.

  In addition, since the container used for powder processing is commonly used at least for each process of denitration and granulation, there is no need to transfer between containers, and the processing work for powder adhering to the container is reduced. Can do.

It is a schematic process explanatory drawing of the manufacturing method of the nuclear fuel pellet by the simplified pellet method which concerns on one Embodiment of this invention. It is a schematic explanatory drawing of the top access type microwave denitration granulation apparatus used by one Embodiment of this invention. It is a schematic explanatory drawing of the die lubrication molding machine for nuclear fuel pellet shaping | molding used by one Embodiment of this invention. It is a graph for demonstrating the change of O / M ratio during heat processing.

  First, schematic steps of a method for producing nuclear fuel pellets by a simplified pellet method according to an embodiment of 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, the plutonium nitrate solution and the uranyl nitrate solution given by the fast breeder reactor spent fuel reprocessing system (100) are mixed as they are using a vacuum combined air lift system, where Pu: U is the fuel specification. To a ratio determined by the ratio of Pu to U based on (for example, 2: 8) (step 101).

  The Pu-U nitrate mixed solution whose plutonium enrichment is adjusted is denitrated by microwave irradiation and then granulated (step 102). The granulated powder is roasted and reduced by batch processing while being put in the same container, and MOX granulated powder is produced (step 103).

  The above-described denitration granulation step will be described in more detail with reference to FIG. FIG. 2 shows an upper access type microwave denitration granulator that unifies denitration granulation. In FIG. 2, reference numeral 200 denotes a simulated microwave. 21 is a container for denitration granulation, 201 is the powder denitrated in the container 21, 22 is a stirring blade for stirring the 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, and reference numeral 25 denotes a table on which the container is placed. In this figure, it is shown that the microwave 200 is irradiated in a state where the stirring blade driving motor 23 is lowered, but FIG. 2 is only an explanatory diagram of denitration granulation integration, As will be described later, the stirring blade 22 is arranged at a position not affected by the microwave irradiation, and is configured to be lowered to the position of FIG. 2 after the microwave irradiation. Note that the microwave denitration granulator is divided into a microwave denitration apparatus (not shown) and a granulation apparatus (not shown), and the Pu-U nitrate solution (not shown) in the container 21 is made into a micro. After denitration by the wave denitration apparatus, the container 21 may be moved to the granulation apparatus as it is, and granulation may be performed there.

  In the denitration granulation process using the above-described apparatus, 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 2: 8, for example, It is injected into the container 21 in the microwave denitration granulator. Thereafter, while the table 25 is slowly rotated by a motor (not shown) on the lower side of the figure, the solution in the container 21 is irradiated with the microwave 200 to perform denitration. After microwave irradiation, for example, water is sprayed as a binder on the obtained powder 201 (not shown), the stirring blade 22 driven by the motor 23 from above the container 21 is lowered to the center of the container 21, and the stirring blade 22 Is rotated at a constant speed to stir the powder 201 to produce a granulated powder (step 102). At this time, the water may be sprayed uniformly over the entire powder, and may be performed before or during granulation.

  The granulated powder is carried to the furnace while being placed in the same container, and roasted and reduced by batch processing (step 103). As a result, MOX granulated powder having good fluidity such as Carr's fluidity index or the like is obtained. In the above description, the roasting reduction process is to be roasted and reduced by batch processing using the same container as that for granulation, but the granulated powder is automatically taken out from the container and the rotary kiln furnace is installed. It may be used and continuously roasted and reduced.

  Note that FIG. 2 and the above description based on it are explanatory text on the premise of rolling granulation, but the present invention is granulated as is apparent from the description of [Summary of Invention] described above. This method is not limited to the rolling granulation described here. That is, you may make it granulate using granulation methods, such as kneading granulation and crushing rolling granulation.

  The MOX granulated powder obtained in step 103 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 104).

  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 (for example, powdered Zn-St) 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 article 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, only the die set portions such as the die 30 and the upper and lower punches 33 and 34 may be installed in the glove box (not shown), and the power unit may be installed outside the glove box.

  The pellets molded in step 104 are sintered at a predetermined temperature for a predetermined 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 105). 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 changes in the O / M ratio when 30% Pu-MOX fuel pellets are heat-treated with an atmospheric gas containing 40 ppm of H ₂ O. Reference numerals 41a and 41b denote heat treatment temperatures. Time changes 42a and 42b indicate time changes 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.

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 / Granulation Process 103 Roasting Reduction Process 104 Pellet Molding Process 105 Sintering (O / M Adjustment) Process 106 Pellet Producting Process 30 Die

Claims (1)

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. And
Transfer the adjusted Pu-U nitrate mixed solution to a container, denitrate it by irradiating it with microwaves, and make a denitrated powder. Then add a binder to the denitrated powder, granulate in the container and granulate Powder and
The granulated powder is roasted and reduced while placed in the container to produce MOX granulated powder,
A certain amount of MOX granulated powder required for pelletization is molded into a predetermined nuclear fuel pellet shape,
Finally molded nuclear fuel pellets directly fixed time sintering, in the manufacturing method of the nuclear fuel pellets for fast breeder reactor, characterized by performing the heat treatment for adjusting the O / M ratio, at the end sintering, the heat treatment A method for producing nuclear fuel pellets for fast breeder reactors, wherein the temperature is rapidly lowered at a rate of 1000 ° C / h .

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