JPH1026683A - Metallic fuel element for reactor and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a metallic fuel element in which radioactive wastes are reduced in its manufacture. SOLUTION: A metallic fuel element is provided with a covering pipe 2, with many granular metallic fuel pellets 1 which are filled into the covering pipe 2 and with rodium 5 which is filled into the gap between the metallic fuel pellets 1 and that between the metallic fuel pellets 1 and the covering pipe 2 and which transmits the heat of the metallic fuel pellets 1 to the covering pipe 2. The metallic fuel element is manufactured in such a way that an alloy for a fuel is melted so as to be granulated, that the metal fuel pellets 1 are formed and that the metallic fuel pellets 1 and the rodium 5 are filled into the covering pipe 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metal fuel element for a nuclear reactor using a metal fuel as a nuclear fuel and a method for manufacturing the same. More specifically, the present invention relates to a metal fuel element for a nuclear reactor, which can eliminate the need for a mold in the manufacture of a metal fuel, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, when producing a fuel body using a metal fuel such as a uranium alloy, it has been general to form the fuel body into a cylindrical shape by casting using a mold.
That is, FIG. 4 shows a general structure of a conventional metal fuel element for a fast reactor. The metal fuel element has a cylindrical fuel body 10.
1 is housed in a cladding tube 102 together with sodium 105, and sealed with an upper end plug 103 and a lower end plug 104. In this metal fuel element, the cylindrical fuel body 10
1 and a cladding tube 102 are provided with gaps of a predetermined distance in consideration of the swelling amount of the fuel body 101, and these gaps provide good heat transfer from the fuel body 101 to the cladding tube 102. Is filled with sodium 105.
The fission gas is stored in the gas plenum 106 between the sodium 105 and the upper end plug 103. The fuel body 101 is a fuel alloy such as a uranium alloy or a plutonium alloy, and is formed into a cylindrical shape by casting using a mold. That is, the melted fuel alloy is injected into a quartz pipe-shaped mold and cooled, and after the solidified fuel alloy is removed from the mold, both ends are trimmed to a predetermined length.
A cylindrical fuel body with a certain length has been obtained (BR Se
idel, LC Walters and YI Chang, "Advances in
Metallic Nuclear Fuel, "Journal of Metals, No.4, v
ol.39, (1987).).

On the other hand, as a fuel element using an oxide fuel which is not a metal fuel, a so-called vibration-filled fuel which is filled with a granular MOX fuel (mixed oxide fuel) while giving vibration to a cladding tube to obtain a high fuel density. The elements are known. That is, the low decontamination plutonium / uranium mixed nitric acid solution supplied from the reprocessing is granulated in the gelling step, and the granular MOX fuel obtained by sintering is filled into the cladding tube under vibration, whereby the fuel pin (fuel Element) can be manufactured. The development of this vibrating filled fuel element was halted due to the worldwide adoption of a pellet-type MOX fuel element with a proven track record. However, the manufacturing method based on vibration filling has been attracting attention in recent years because the MOX fuel pulverization step, mixing step, granulation step, molding step, and pellet grinding step can be dispensed with.

[0004]

However, as for a fuel element using a metal fuel, since a cylindrical metal fuel is formed by casting using a mold, the mold becomes radioactive waste and the radioactive waste is produced. Contrary to the demand to reduce things.

[0005] In view of the fact that a fuel body can be manufactured without using a mold, application of a vibration-filled fuel element for an oxide fuel element to a fuel element using a metal fuel has been studied.

However, in order to effectively transmit the heat generated in each granular fuel to the cladding tube, it is necessary to increase the filling rate of each granular fuel and fill the gap as much as possible, as in the case of pellet molding. The fuel has a larger swelling amount than the oxide fuel, and it is necessary to leave a wide gap between the granular fuel bodies in order to prevent breakage of the cladding tube. That is, in order to apply the vibration filling method to the metal fuel, both of the contradictory conditions regarding the filling of the granular fuel body must be highly satisfied, and the application is difficult.

An object of the present invention is to provide a metal fuel element for a nuclear reactor capable of reducing radioactive waste and a method for manufacturing the same.

[0008]

In order to achieve the above object, a metal fuel element for a nuclear reactor according to claim 1 comprises a cladding tube, a plurality of granular metal fuel bodies filled in the cladding tube, and A heat transfer medium is provided between the metal fuel bodies and in the gap between the metal fuel body and the cladding tube to transfer heat of the metal fuel body to the cladding tube well.

Therefore, a gap is generated between the granular metal fuel body and between the metal fuel body and the cladding tube, and the metal fuel body expands (swells) due to irradiation or generation of fission gas. Also, the swelling is absorbed by the narrow gap between the metal fuel bodies or the movement of the metal fuel bodies, thereby preventing the cladding tube from being damaged. Also,
The gap between the metal fuel body and the gap between the metal fuel body and the cladding tube is filled with a heat transfer medium, so that the heat of the metal fuel body is well transmitted to the cladding tube. Moreover, the metal fuel body can be filled by pouring the granular fuel alloy into the cladding tube. In particular, by making the particle diameter of the metal fuel body sufficiently smaller than the inner diameter of the cladding tube, the fuel alloy can be more easily filled into the cladding tube. In addition, a mold or the like is not required for granulating the fuel alloy.

Here, as the heat transfer medium, a metal which becomes liquid during use, for example, sodium, mercury, lead, potassium, or an alloy thereof can be used. Preferably, sodium is employed. In this case, it is suitable for use in a fast breeder reactor using liquid sodium as a coolant.

Further, according to a third aspect of the present invention, there is provided a method of manufacturing a metal fuel element, wherein a metal alloy is formed by melting a fuel alloy and then granulating the metal alloy, and the metal fuel element and a heat transfer medium are filled in a cladding tube. Things. In this case, in order to granulate the fuel alloy, the molten alloy can be formed without using a mold by various methods such as dripping, scattering, or colliding with a jet. In addition, if the particle size is adjusted so that the size of each metal fuel body is sufficiently smaller than the inner diameter of the cladding tube,
It can be easily loaded into the cladding tube.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of the present invention will be described below in detail based on an embodiment shown in the drawings.

FIG. 1 shows an embodiment of a metal fuel element for a nuclear reactor to which the present invention is applied. This metal fuel element is for a fast breeder reactor using, for example, liquid sodium as a coolant. The metal fuel element is formed by filling a large number of clad tubes 2 with a metal fuel body 1 obtained by molding a fuel alloy into granules. The cladding tube 2 is made of steel, for example, and its upper and lower ends are closed by welding the upper end plug 3 and the lower end plug 4. Further, a gas plenum 6 for storing fission gas is provided between the sodium 5 and the upper end plug 3. When the metal fuel body 1 is formed into a granular shape having a diameter sufficiently smaller than the inner diameter of the cladding tube 2 and a large number of the particles are filled, a space between each metal fuel body 1 and between each metal fuel body 1 and the cladding tube 2 is required. There is a gap, and there is a concern that heat transfer may deteriorate. Therefore, the gap is filled with sodium 5 as a heat transfer medium, so that the heat generated in each metal fuel body 1 is transmitted well to the cladding tube 2.

As fuel alloys, for example, uranium alloys obtained by adding some elements (eg, zirconium, molybdenum, etc.) to uranium as needed, and several elements (eg, zirconium, etc.) added to plutonium as needed. , Molybdenum, etc.), and an alloy obtained by adding plutonium to a uranium alloy.

Next, a method for producing a metal fuel assembly will be described.

The metal fuel element is manufactured by first obtaining a metal fuel body 1 granulated to a predetermined size, and then filling the clad tube 2 with the granular metal fuel body 1 and sodium 5.

More specifically, first, the fuel alloy is melted in an atmosphere that does not cause a chemical reaction with the fuel alloy, and then is formed into granules. As a method of forming the fuel alloy into granules, for example, a method in which the fuel alloy is melted in an inert gas atmosphere, then dropped into a cooling atmosphere of the inert gas, and cooled and solidified during the dropping is considered. Can be At the time of the solidification, it is considered that the dropped fuel alloy becomes granular close to a sphere due to the large surface tension of the molten fuel alloy. Generally, when a fluid is dropped, each particle has a substantially constant weight and size, so that each particle of the granular fuel alloy produced as described above also has a substantially constant weight and size. Further, by adjusting the size of the fuel alloy solidified into particles to be sufficiently smaller than the inner diameter of the cladding tube 2, the fuel alloy can be easily loaded into the cladding tube 2.

According to this method, the particle size of the fuel body 1 can be adjusted by changing the amount of the molten fuel alloy dripped, and the generation of radioactive waste can be suppressed because no mold is used. . Further, since the metal fuel assembly 1 can be manufactured continuously, the productivity can be improved as compared with the case where the metal fuel assembly is manufactured using a mold.

However, the method for forming the fuel alloy into granules is not limited to the above-mentioned method, and a well-known atomizing method or rapid solidification method can be used as a means for obtaining granules. It is not something to be done.

Next, the cladding tube 2 whose lower end opening is hermetically closed by the lower end plug 4 is filled with a required amount of the metal fuel body 1 and sodium 5 and the metal fuel body 1 is settled. For example, after a required amount of the metal fuel body 1 is put in the cladding tube 2 into which the solid sodium 5 is inserted without vibration, the sodium 5 is dissolved by heating. However, the present invention is not limited to this method, and a mixture of the required amount of the metal fuel body 1 and the melted sodium 5 is prepared before insertion into the cladding tube 2, and the mixture is injected into the cladding tube 2, The body 1 may be settled, or the metal fuel body 1 and sodium 5 may be filled by other methods and procedures to settle the metal fuel body 1.

Granulation of the metal fuel body 1 is sufficient if it is extremely small compared to the inner diameter of the cladding tube 2 and can be formed without using a mold or the like. Radioactive waste such as molds can be reduced. Further, since the metal fuel assemblies 1 are merely settled, a sufficient gap can be left between the metal fuel assemblies 1. That is, although the swelling amount of the metal fuel is larger than that of the oxide fuel, a gap can be formed between the respective metal fuel bodies 1 to avoid the influence of the swelling, and the metal fuel is merely charged and settled. It can contribute to improvement of the performance.

Here, the component ratio and the weight of the metal fuel body 1 filled in one cladding tube 2 may be managed so as to have predetermined values as follows. That is, the component ratio of the metal fuel body 1 can be controlled to a predetermined value by adjusting the component ratio when, for example, melting the raw material of the fuel alloy. The weight of the metal fuel assembly 1 can be managed by measuring the height of the metal fuel assembly 1 in a state where each metal fuel assembly 1 is settled in, for example, sodium 5. However, it goes without saying that the component ratio and the weight of the metal fuel body 1 may be controlled by a method other than these methods.

After the metal fuel body 1 and the sodium 5 are filled, the upper end plug 3 is welded to the upper opening of the cladding tube 2 to hermetically close the opening. At this time, for example, argon is sealed in the gas plenum 6.

When the metal fuel element is loaded in the fast breeder reactor, the heat generated in the metal fuel body 1 is transmitted to the cladding tube 2 by the sodium 5 filling between the metal fuel bodies 1. Therefore, it can be used similarly to a fuel body formed by casting using a mold. Further, even if swelling occurs in each metal fuel body 1 due to irradiation or fission gas, a sufficient gap is provided between each metal fuel body 1 and each metal fuel body 1 can move freely. Breakage of the cladding tube 2 is prevented.

Further, the metal fuel bodies 1 having different particle diameters are filled and the mixing ratio thereof is changed variously so that the metal fuel bodies 1 are mixed.
Can be adjusted.

Although the above-described embodiment is an example of a preferred embodiment of the present invention, the present invention is not limited to this embodiment, and various modifications can be made without departing from the gist of the present invention. For example, in the above description, the metal fuel element used in the fast breeder reactor is taken as an example, but the present invention is not limited to the fast breeder reactor, but may be used in other nuclear reactors such as light water reactors, heavy water reactors, and gas reactors. Of course, it may be applied to the element.

The above description is about a metal fuel element for a fast breeder reactor using liquid sodium as a coolant. Although sodium is used as the heat transfer medium 5, sodium is not necessarily used for heat transfer. It is not necessary to use the medium 5, and for example, mercury, lead, potassium or the like may be used as the heat transfer medium 5. However, it is necessary to appropriately select the coolant in consideration of the reaction between the heat transfer medium 5 and the like.

Further, in the above description, the cladding tube 2 is made of steel. However, the material of the cladding tube 2 is not limited to steel. For example, a zirconium alloy, a magnesium alloy, an aluminum alloy or the like can be used. .

[0029]

Next, an embodiment of a metal fuel element will be described, but it is needless to say that the present invention is not limited to this embodiment.

FIG. 2 shows an example of an apparatus for producing the granular metal fuel body 1. In the figure, reference numeral 7 denotes a molten fuel alloy, reference numeral 8 denotes a container having a hole at the bottom, reference numeral 9 denotes a fuel alloy which is dripped and solidifying, reference numeral 10 denotes a fuel alloy which has been solidified, and reference numeral 11 denotes a bottom of the container. The accumulated granular fuel alloy, reference numeral 12 denotes argon gas, reference numeral 13 denotes cooling argon gas, reference numeral 14 denotes a sealed container holding the cooling argon gas, reference numeral 15 denotes a heater for melting the fuel alloy, and reference numeral Reference numeral 16 denotes a pipe through which a refrigerant for cooling the cooling argon gas 13 flows, and reference numeral 17 denotes a pipe through which a refrigerant for cooling the bottom of the container 14 flows.

The fuel alloy 7 melted by the heater 15 is dropped into the argon gas 13 through a hole provided in the container 8. Here, in order to perform the dripping successfully,
An appropriate differential pressure is provided between the 12 argon gas and the 13 cooling argon gas. The dropped fuel alloy 9 completes solidification while falling through the cooling argon gas 13 and accumulates at the bottom of the closed container 14. The cooling argon gas 13 in the closed vessel 14 is cooled to room temperature or lower by the refrigerant flowing in the pipe 16 so that the dropped fuel alloy solidifies during the fall. The diameter of the granular fuel alloy 11 can be controlled by adjusting the diameter of the hole provided at the bottom of the container 8 and the above-described differential pressure.

Therefore, a uranium alloy melted at about 1550 ° C. (containing about 30 atomic percent of zirconium)
Was dropped from a hole having a diameter of 0.3 mm drilled at the bottom of the container 8 and the height of the closed container 14 was set to about 1.5 m for the test. As a result, a spherical metal fuel body 1 having a particle size of about 0.8 mm could be manufactured.

The metal fuel body 1 thus manufactured is
The outer diameter is 7.5 mm and the inner diameter is 6.5 mm as shown in FIG.
The case where the cladding tube 2 was filled with molten sodium was examined. When filling the cladding tube 2 with the metal fuel body 1 having one kind of outer diameter sufficiently smaller than its inner diameter,
As a result of calculating the volume ratio of the fuel alloy in the region where the metal fuel body 1 exists in the cladding tube 2, it was found that the theoretical ratio was about 74% at the maximum. However, it is empirically known that when a single type of sphere is filled in a container, the same ratio is actually about 60%. In such a case, two or more types of spheres are used. By manufacturing the metal fuel body 1 having various kinds of outer diameters and adjusting the composition thereof, the same ratio can be increased to obtain a desired same ratio. Here, among metal fuel elements using a fuel body molded by casting using a conventional mold, those designed to have the same ratio of about 75% have the best use results. Therefore, in the metal fuel element shown in FIG. 3, in a region where the metal fuel body 1 exists in the cladding tube 2, if the volume ratio of the fuel alloy occupying the space in the region is set to 75%, the same performance is obtained. It can be determined that it has.

As is apparent from this embodiment, it is understood that there is no need to use a mold when manufacturing a granular metal fuel body, and it is not necessary to mold the granular fuel alloy itself after manufacturing. it can. Therefore, it is possible to reduce radioactive waste and improve productivity.

[0035]

As described above, in the metal fuel element for a nuclear reactor according to the first aspect, a cladding tube, a large number of granular metal fuel bodies filled in the cladding tube, a space between the metal fuel bodies and Since the heat transfer medium that is filled in the gap with the cladding tube and transfers the heat of the metal fuel body to the cladding tube is provided, the metal fuel body is shaped into a predetermined size in consideration of the swelling amount, and a mold is used in manufacturing. It is not necessary, and a molding step after casting can be eliminated. Therefore, radioactive waste can be reduced. Further, since molding after casting is unnecessary, the manufacturing process of the fuel body can be simplified, which can contribute to improvement in economic efficiency.

Further, since the metal fuel element for a nuclear reactor according to the present invention uses sodium as a heat transfer medium, a metal fuel element suitable for a fast breeder reactor using liquid sodium as a coolant is provided. be able to.

Further, in the method of manufacturing a metal fuel element according to the third aspect, the fuel alloy is melted and then granulated to form a metal fuel body, and the metal fuel body and the heat transfer medium are filled in the cladding tube. Therefore, the fuel alloy can be granulated without using a mold by various methods such as melting and colliding with a jet or a drop. In addition, by adjusting the particle size so that the size of each metal fuel body is sufficiently smaller than the inner diameter of the cladding tube, loading into the cladding tube can be easily performed.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of an embodiment of a metal fuel element according to the present invention.

FIG. 2 is a sectional view showing an example of an apparatus for producing a granular metal fuel body.

FIG. 3 is a sectional view showing a metal fuel element filled with a metal fuel body manufactured by the apparatus of FIG. 2;

FIG. 4 is a sectional view of a conventional metal fuel element.

[Explanation of symbols]

 Reference Signs List 1 metal fuel body 2 cladding tube 5 sodium (heat transfer medium)

Claims (3)

[Claims] 1. A cladding tube, a number of granular metal fuel bodies filled in the cladding tube, and a gap filled between the metal fuel bodies and between the metal fuel body and the cladding tube. A metal fuel element for a nuclear reactor, comprising a heat transfer medium that transfers heat of a metal fuel body to a cladding tube. 2. The metal fuel element for a nuclear reactor according to claim 1, wherein said heat transfer medium is sodium. 3. The production of a metal fuel element for a nuclear reactor, characterized in that a fuel alloy is melted and then granulated to form a metal fuel body, and the metal fuel body and a heat transfer medium are filled in a cladding tube. Method.