JP2001281377A - Method for manufacturing actinoids-containing hydride fuel pellet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a satisfactory actinoids- containing hydride fuel pellet free from crack in which an effective actinoids annihilation processing can be realized in an existing reactor. SOLUTION: This method for manufacturing an actinoids-containing hydride pellet for annihilation processing in reactor comprises manufacturing an alloy ingot composed of 20 wt% or less uranium, 50 wt% or less of actionid, and the remainder of zirconium in an arc welding furnace, cutting the ingot into a metal pellet shape having a predetermined dimension, hydrogenating the metal pellet to form a metal hydroxide. In the hydrogenation process, comprises reducing the pressure of a treatment tank containing metal pellet is reduced and hydrogen gas is introduced into the treatment tank up to 1 atmospheric pressure under a temperature condition of 800-1,000 deg.C stepwise.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing actinide element-containing hydride fuel pellets which can be annihilated in a fast reactor.

[0002]

2. Description of the Related Art In a nuclear reactor, higher-order actinoid elements are produced by a neutron capture reaction of uranium and plutonium. Among them, there are short-lived ones such as curium, but long-lived radioactive species such as americium (Am) and neptunium (Np) are included.
It is a problem in waste disposal.

At present, the method of treating and disposing of these radioactive wastes employs a stratification method in which glass is solidified and buried in a geologically stable stratum suitable for environmental safety and isolated from the human environment. Have been.

[0004] On the other hand, from the viewpoint of reducing the burden of providing a treatment plant in an appropriate stratum for such a stratum treatment method, research on annihilation treatment of radioactive waste has been advanced.
The annihilation treatment is a method for shortening or stabilizing a long-lived radioactive waste by transmutation.

[0005]

In the research on the extinction treatment of the actinoid element, development of a new dedicated reactor for efficiently extinction of the actinoid has been mainly studied.
For example, a sintering fast reactor and an accelerator driven sintering furnace having a hard neutron spectrum aiming at direct fission of actinoids have been proposed. As the fuel in this case, a nitride fuel having good thermal characteristics and mutual solubility of minor actinoids and excellent in stable americium resistance has been employed.

However, with this fuel, it is necessary to increase the neutron flux density in order to perform the actinide element annihilation processing efficiently. In addition, developing such a new dedicated annihilation reactor requires a lot of cost and time, and has many problems for practical use.

Therefore, if a hydride fuel of an actinoid element is used, fast neutrons are slowed down because the fuel itself contains hydrogen as a moderator, and thermal neutrons and epithermal neutrons, which are easily absorbed, efficiently convert the actinoid element. Focusing on the possibility that the annihilation treatment can be performed using a fast reactor for power generation, it is desirable to develop an annihilation treatment in an existing nuclear reactor, which is expected to economically reduce development costs and shorten the development period. It is rare.

In this case, first, it is necessary to produce actinide element-containing hydride fuel pellets suitable for practical use in a form that can be loaded into a nuclear reactor. As a method for producing actinide element-containing hydride fuel pellets,
For example, the process of preparing an alloy ingot containing an actinoid element and processing it into pellets, and then performing a hydrogenation treatment is simple, but simply hydrogenating the material significantly embrittles the material and cracks easily occur in the pellets. For example, a method for obtaining a hydride fuel pellet containing an actinoid element as a good fuel pellet suitable for practical use has not yet been developed.

In view of the above problems, an object of the present invention is to provide a method for producing a good actinide element-containing hydride fuel pellet free of cracks, which can realize efficient actinide annihilation treatment in an existing nuclear reactor. It is in.

[0010]

In order to achieve the above object, a method for producing a hydride fuel pellet containing an actinide element according to the first aspect of the present invention comprises: An ingot manufacturing step of manufacturing an alloy ingot having the composition of an ingot by an arc melting furnace, an ingot processing step of cutting the ingot into a metal pellet having a predetermined size, and hydrogenating the metal pellet to obtain a metal hydrogen. A method for producing an actinide element-containing hydride fuel pellet for nuclear reactor annihilation treatment, the method comprising the steps of: , 800-100
Under a temperature condition of 0 ° C., hydrogen gas is stepwise introduced into the processing tank up to 1 atm.

Further, according to a second aspect of the present invention, there is provided a method for producing an actinide-element-containing hydride fuel pellet according to the first aspect of the present invention. After the start of introduction, a hydride phase is generated in the metal pellets in the processing tank,
When the hydrogen equilibrium pressure reaches a plateau region where the hydrogen equilibrium pressure becomes constant, the stepwise introduction pressure of hydrogen gas is reduced and adjusted.

According to a third aspect of the present invention, there is provided a method for producing an actinide element-containing hydride fuel pellet according to the second aspect of the present invention. Is introduced by 2.67 kPa to 6.67 kPa or less, and in the non-plateau region, hydrogen gas is introduced by 6.67 kPa to 20.0 kPa.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, as shown in the examples described later, metal pellets obtained by cutting an uranium-zirconium (UAZr, A: actinoid element) alloy ingot containing an actinoid element. By introducing hydrogen gas stepwise in the treatment tank, hydride fuel pellets that are less likely to crack can be manufactured.

In particular, when the hydride phase is formed after the start of the introduction of hydrogen gas and the plateau reaches a plateau region where the hydrogen equilibrium pressure is constant, the introduction pressure of the stepwise hydrogen gas is reduced and adjusted so that cracks do not occur. Fuel pellets can be produced.

This is because the hydrogen absorption of the metal pellet is the difference between the hydrogen introduction pressure and the hydrogen equilibrium pressure, so that the hydrogen equilibrium pressure reaches a plateau region where the hydrogen equilibrium pressure becomes constant without increasing the hydrogen concentration. When the hydrogen introduction pressure is higher than the hydrogen equilibrium pressure in the hydride generation state, the amount of hydrogen stored in the metal pellets increases and the expansion becomes large and sharp, so cracks occur in the fuel. Thought,
By gradually introducing the introduction pressure of the hydrogen gas little by little, it is possible to prevent the generation of cracks by avoiding rapid hydrogen occlusion of the metal pellets.

As described above, since the plateau region where the hydrogen equilibrium pressure is constant is a partial period in the hydrogen gas introduction step, the other period, that is, the hydrogen equilibrium pressure also increases with the increase in the hydrogen concentration. Since the amount of hydrogen occluded in the metal pellets is relatively small during the region period, it is not necessary to reduce the hydrogen gas introduction pressure as much as the plateau region level. If the pressure is increased and reaches the plateau region, the efficiency of the hydrogenation process can be improved by reducing and adjusting the hydrogen gas introduction pressure.

For example, in a processing tank under reduced pressure vacuum, 80
1 atmosphere (101.3 kP) under the temperature condition of 0 to 1000 ° C.
When hydrogenation of the metal pellets is performed by introducing hydrogen gas until a), the stepwise introduction pressure of hydrogen gas is set to 6.67 to 20.0 kPa (50 to 15 kPa) at a time in the non-plateau region period.
0 Torr), and if the introduction pressure is reduced and adjusted so as to be 2.67 to 6.67 kPa (20 to 50 Torr) or less in the plateau region period, good actinoid element-containing hydride fuel pellets free from cracks Is obtained.

The composition of the UA-Zr alloy ingot in the present invention is as follows: U: 20 wt% or less, A: 50 wt%
% Or less, Zr: balance. The reason that the actinide element content is 50 wt% or less and the uranium content is 20 wt% or less is because uranium has a small hydrogen absorption under the temperature conditions of the present invention, and therefore, increases the hydrogen storage efficiency. Also,
The actinoid element in the present invention is thorium, neptunium, and americium. The metal pellet processing size is set appropriately small in consideration of the volume expansion of about 5% or less due to hydrogen absorption later.

The above-mentioned plateau region may be determined in advance from a relationship between the hydrogen concentration in the sample and the hydrogen equilibrium pressure using a preliminary sample. The amount of hydrogen absorbed into the sample was measured at a predetermined pressure initially introduced into the processing tank containing the sample, and the absorption of hydrogen into the sample progressed, and the absorption of hydrogen was completed to reach an equilibrium pressure. A calculated value is determined based on the pressure decrease at the time.

As a result, for example, in FIG.
A plateau region corresponding to the hydrogen concentration in the sample, that is, a region where the hydrogen equilibrium pressure does not increase with an increase in the hydrogen concentration can be determined from the curve. Therefore, during the metal pellet hydrogenation step, while measuring the metal pellet hydrogen absorption during the process,
The stepwise introduction of hydrogen gas is advanced, and based on the above determination result, the introduction of hydrogen gas after the point at which the hydrogen concentration reaches the plateau region can be achieved by reducing and adjusting the introduction pressure.

The hydrogenation of the metal pellets while measuring and adjusting the pressure for introducing the hydrogen gas can be easily performed by using a hydrogen absorption test apparatus. As illustrated in the schematic configuration diagram of FIG. 2, a general hydrogen absorption test apparatus mainly includes a quartz tube 11 containing a sample S, a decompression system including a turbo molecular pump 18, a hydrogen gas cylinder 13, And a hydrogen gas supply system having a Baratron pressure gauge 16 for measuring hydrogen pressure.

In the hydrogen absorption test using such an apparatus, the inside of the quartz tube 11 containing the sample S is depressurized by a turbo molecular pump 18 driven by a rotary pump 19, and the degree of vacuum is confirmed by an ionization vacuum gauge 17. After heating in an electric furnace 12 installed on the outer periphery to set a predetermined temperature environment, a reservoir tank 15 is supplied from a hydrogen gas cylinder 13 through a hydrogen purifier 14.
Hydrogen gas is introduced into the inside only at a predetermined pressure, the hydrogen gas cylinder valve VH is closed, and the quartz tube introduction valve V1 is opened to make the pressure in the system of the reservoir tank 15 and the quartz tube 11 uniform, and the hydrogen pressure (P1) is measured with a Baratron manometer. Measured at 16,
When hydrogen is absorbed by the sample S and the sample S has absorbed hydrogen and is in an equilibrium state, a hydrogen equilibrium pressure (P2) is measured,
The amount of hydrogen absorbed by the sample can be obtained by calculating based on the decrease in hydrogen pressure (P1-P2) in consideration of the volume of the pipe and the quartz tube 11 and the like.

The determination of the plateau region using the preliminary sample indicated by ● in FIG. 3 was made using this hydrogen absorption test apparatus. That is, the hydrogen gas is introduced into the reservoir tank 15 according to the hydrogen absorption operation of the sample from 6.67 kPa to 1 kPa.
Hydrogen is introduced into the range of 3.33 kPa (50 to 100 Torr) to absorb the hydrogen of the preliminary sample, and the amount of hydrogen absorption of the preliminary sample is determined from the difference between the initial pressure of hydrogen gas introduction and the equilibrium pressure after the absorption. The operation of introducing and absorbing hydrogen gas, which is to be determined, is repeated stepwise, and the measurement results are summarized in FIG. 3 as the hydrogen concentration (wt%) of the preliminary sample on the horizontal axis.

The preliminary sample used here had the same composition (containing thorium as an actinoid element) and the same process as the metal pellets used in Examples described later.
Area B in FIG. 3 based on the curve obtained from the
Was determined to be a plateau region. The hydride fuel pellets obtained in this preliminary test had cracks.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As one embodiment of the present invention, the production of hydride fuel pellets using thorium (Th) as an alternative element to the actinoid element is described in the hydrogenation step based on the plateau region shown in FIG. The case of using the hydrogen absorption test apparatus shown in FIG. FIG. 1 is a flow chart showing a process for producing actinide element-containing hydride fuel pellets according to the present embodiment.

First, in the ingot manufacturing step (1), uranium having a composition of 11 wt%
A U-Th-Zr alloy ingot made of 45 wt% thorium and 44 wt% zirconium was produced.

Next, in the ingot processing step (2), the alloy ingot is cut into metal pellets of a predetermined size using a cutter and a lathe, and the metal pellets are cut in a hydrogenation step (3). Hydrogen addition / absorption operation was performed using the hydrogen absorption test apparatus No. 2 to obtain hydride fuel pellets.

In the hydrogenation step (3), hydrogen gas was introduced stepwise from the vacuum to 1 atm (101.3 kPa) in the quartz tube 11. However, during the plateau region period, the hydrogen introduction pressure was 2.67 to 5.33 kPa (20 to 5.33 kPa).
(40 Torr).

Specifically, the metal pellets are first accommodated in a quartz tube 11, and the inside of the quartz tube 11 is evacuated by a turbo molecular pump 18 driven by a rotary pump 19 while confirming the degree of vacuum with an ionization vacuum gauge 17. After that, the inside of the quartz tube 11 was heated to about 900 ° C. for about 2 hours in the electric furnace 12, also serving as degassing of the metal pellet and annealing for removing the processing distortion.

Thereafter, hydrogen gas is supplied from the hydrogen gas cylinder 13 into the reservoir tank 15 through the hydrogen purifier 14.
The hydrogen gas is introduced within a range of 67 to 13.33 kPa (50 to 100 Torr), the hydrogen gas cylinder valve VH is closed, and the quartz tube introduction valve V1 is opened to make the system of the reservoir tank 15 and the quartz tube 11 uniform, and the hydrogen pressure ( P1) is measured by the Baratron pressure gauge 16, and the metal pellet S absorbs hydrogen. When the metal pellets have absorbed hydrogen and are in an equilibrium state, the quartz tube introduction valve V1 is closed and the hydrogen equilibrium pressure (P2) is measured, and the hydrogen pressure is reduced (P1-P2).
Based on the above, the hydrogen absorption amount of the metal pellet was calculated in consideration of the volume of the pipe and the quartz tube 11 and the like.

The above-described stepwise hydrogen gas introduction / metal hydrogenation operation is performed at a hydrogen introduction pressure of 6.67 to 13.33 kPa (50 to 10 3 kPa).
(0 Torr), and the hydrogen concentration (wt%) based on the amount of hydrogen absorbed by the metal pellet at each stage was determined. The results are indicated by a circle (open circle) in FIG.

When the above operation is repeated and the hydrogen concentration reaches the region B determined to be the plateau region in FIG. 3, the hydrogen introduction pressure is reduced and adjusted before the next operation (the fourth step of stepwise hydrogen introduction). 2.67 to 5.33 kPa (20 to 40 Torr)
r). After the change of the hydrogen introduction pressure, the same stepwise hydrogen gas introduction / metal hydrogenation operation is repeated.
Before the next operation (the eighth stepwise hydrogen introduction), the hydrogen introduction pressure was adjusted again to 13.33 kPa (100 Torr).
And the same stepwise hydrogen gas introduction and metal hydrogenation operations,
Repeated until 1 atm was reached.

Thereafter, the quartz tube valve V1 is closed and 100
Cool to room temperature at a cooling rate of ° C / h. By this cooling process, all the hydrogen in the quartz tube 11 can be absorbed in the metal pellets, and finally the target hydrogen concentration can be achieved.

[0034] The thorium-containing hydride fuel pellets hydrogenated by the above-described operations are subjected to the inspection and appearance in the inspection step (4).
Measure dimensions, weight, etc. The actinide element-containing hydride fuel pellets obtained in this example did not crack.

In the above embodiment, a hydrogen absorption test apparatus was used in the hydrogenation step. However, the present invention is not limited to this, and the pressure in the processing tank containing metal pellets can be reduced and evacuated. Any mechanism may be used as long as it has a mechanism and a mechanism that can introduce hydrogen gas at a desired introduction pressure in a stepwise manner and measure the hydrogen gas pressure.

[0036]

As described above, according to the present invention, a good actinoid element-containing hydride fuel pellet free from cracks can be produced.
There is an effect that the actinoids, which are long-lived radioactive wastes, can be efficiently eliminated in the existing fast reactor.

[Brief description of the drawings]

FIG. 1 is a schematic flowchart illustrating a production process of an actinoid element-containing hydride fuel pellet according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a hydrogen absorption test apparatus used in a hydrogenation step in an example of the present invention.

FIG. 3 shows the hydrogen concentration of a sample in the hydrogenation step and the pre-hydrogenation step for the hydrogenation step (horizontal axis:
FIG. 4 is a graph showing the relationship between the hydrogen equilibrium pressure (wt%) and the hydrogen equilibrium pressure (vertical axis: Torr).

[Description of Signs] 1: Ingot manufacturing process 2: Ingot processing process 3: Hydrogenation process 4: Inspection process 11: Quartz tube 12: Electric furnace 13: Hydrogen gas cylinder 14: Hydrogen purifier 15: Reservoir tank 16: Baratron pressure gauge 17: Ionization vacuum gauge 18: Turbo molecular pump 19: Rotary pump V1: Quartz tube introduction valve VH: Hydrogen gas cylinder valve

Claims (3)

[Claims] 1. An ingot manufacturing process for manufacturing an alloy ingot having a composition of 20 wt% or less of uranium and 50 wt% or less of an actinoid element and a balance of zirconium by an arc melting furnace, and cutting the ingot into metal pellets having predetermined dimensions. An ingot processing step of processing, and a hydrogenation step of hydrogenating the metal pellets to obtain a metal hydride, comprising: The hydrogenation step is characterized in that, after decompressing the treatment tank in which the metal pellets are accommodated, under a temperature condition of 800 to 1000 ° C., hydrogen gas is stepwise introduced into the treatment tank to 1 atm. For producing actinide element-containing hydride fuel pellets. 2. After the stepwise introduction of the hydrogen gas, when a hydride phase is generated in the metal pellets in the processing tank and reaches a plateau region where the hydrogen equilibrium pressure becomes constant, the stepwise introduction of the hydrogen gas is performed. The pressure is reduced and adjusted.
3. A method for producing a hydride fuel pellet containing an actinoid element according to item 1. 3. In the plateau region, hydrogen gas is supplied at 2.6.
The method for producing actinide element-containing hydride fuel pellets according to claim 2, wherein the hydrogen gas is introduced at a rate of 7 kPa to 6.67 kPa or less, and the hydrogen gas is introduced at a rate of 6.67 kPa to 20.0 kPa in the non-plateau region.