JP3315510B2 - Molybdenum container decontamination method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decontaminating a molybdenum container for removing nuclear fuel material such as uranium from a molybdenum container used for firing nuclear fuel pellets such as uranium dioxide pellets. More specifically, the present invention relates to a method of decontaminating a molybdenum container by a dry batch processing method.

[0002]

2. Description of the Related Art Uranium dioxide pellets for nuclear fuel are fired in a metal molybdenum container from the viewpoints of compatibility with uranium compounds at high temperatures, heat resistance, stability against hydrogen in a firing atmosphere, and price. Since this container is heated at a high temperature for a long time during sintering, uranium diffuses not only on the surface of the container but also into the inside of the container and is contaminated. Conventionally,
Methods for removing uranium from this molybdenum container have been proposed in Japanese Patent Publication No. 33877/1984 and Japanese Patent Application Laid-Open No. 62-182699. The decontamination method disclosed in JP-B-59-33877 is a method in which a molybdenum container is used as an anode, the surface of the container is oxidized by electrolytic oxidation treatment in a specific electrolyte solution, and a generated muddy oxide layer is separated. . See also
The decontamination method disclosed in Japanese Patent Application Laid-Open No. 2-182699 is a method in which a molybdenum container to which a contaminant is adhered is subjected to electrolytic cleaning and then ultrasonically cleaned in a poor solvent for the contaminant. However, since the above two conventional decontamination methods are chemical wet methods by electrolysis, not only an electrolyte solution is required, but also various waste liquids are generated by electrolysis, and the waste liquid treatment is required. This kind of waste liquid treatment has a problem that it is complicated and requires a large treatment cost. In addition, the above-mentioned Japanese Patent Publication No.
The decontamination coefficient of the decontamination method shown in JP-A-77 is 40 to 30.
It was about 0, and the decontamination coefficient was relatively low.

In order to solve this problem, the present applicant has continuously and reliably separated nuclear fuel material from a molybdenum container by a dry method without generating various waste liquids by electrolysis, and regenerated the molybdenum container. In addition, an apparatus for treating nuclear fuel material containing impurities for recovering nuclear fuel material has been disclosed (JP-A-63-45594). The processing apparatus includes an open-ended heating furnace configured to allow a container (for example, a molybdenum container) containing an impurity-containing nuclear fuel material or a nuclear fuel material-containing material to pass from an inlet at one end to an outlet at the other end. Means for simultaneously introducing an oxidizing atmosphere gas from inlets and outlets at both ends of the furnace, and an oxidizing atmosphere gas provided at an intermediate portion of the heating furnace, which is selectively heated from the impurity-containing nuclear fuel material or the nuclear fuel material-containing material by heating in the heating furnace. A discharge pipe for discharging together with the impurity oxide or the material oxide that evaporates.

[0004]

However, the above-mentioned processing apparatus has a heating furnace for completely separating impurity oxides, material oxides and nuclear fuel material from a container in a heating furnace having both ends open. Must be heated to an extremely high temperature above their melting point, for example, 900 ° C., to evaporate or vaporize them, resulting in a long processing time and a large amount of heat energy.

An object of the present invention is to provide a method for decontaminating a molybdenum container which can remove nuclear fuel materials such as uranium in a short time with a relatively small heat energy and with a relatively high decontamination coefficient by a dry batch processing method. It is in.

[0006]

In order to achieve the above-mentioned object, as shown in FIG. 1, the decontamination method of the present invention uses a molybdenum container 14 used for burning nuclear fuel pellets in an air supply port 11.
Into the electric furnace 10 having a closed end and an exhaust port 12, while supplying oxidizing air from the air supply port 11 into the electric furnace 10 and discharging air from the electric furnace 10 from the exhaust port 12. A method of heating the molybdenum container 14 at 600 to 620 ° C. for 0.75 to 2 hours, cooling the heated molybdenum container 14 to at least room temperature, and removing the nuclear fuel oxide deposited on the surface of the molybdenum container 14 by polishing or washing with water. It is.

If the heating temperature is less than 600 ° C. or the heating time is less than 0.75 hours, the transfer of contaminants to the surface inside the molybdenum container is small, and if it exceeds 620 ° C. or exceeds 2 hours. The contaminants evaporate or evaporate, making subsequent collection difficult and complicated. Cooling temperature is 50
Above ° C, nuclear fuel oxides are unlikely to appear on the container surface. In order to efficiently deposit nuclear fuel oxide from the surface of the molybdenum container, the molybdenum container is heated at 10 to 20 ° C. during heating.
It is preferable that the molybdenum container be cooled at a rate of 25 to 30 ° C./min after the temperature is increased at a rate of 25 to 30 ° C./min. The specific temperature, time, heating rate, and cooling rate are appropriately determined in the above ranges according to the capacity of the electric furnace, the capacity of the molybdenum container, the oxidizing power of the supplied air, and the like.

[0008]

The molybdenum container used for firing nuclear fuel pellets is supplied with oxidizing air for 600 to 620 minutes.
When heated at 0.75 ° C. for 0.75 to 2 hours, oxides containing nuclear fuel oxides such as U ₃ O ₈ and MoO ₃ appear on the surface of the contaminated molybdenum container 14 into which the nuclear fuel material such as uranium diffuses and enters. It is considered that the nuclear fuel oxide appears on the surface of the container because UO _{2 in the} container is changed to U ₃ O ₈ by heating and is transferred to the surface of the container by this oxidation phenomenon. Since the above-mentioned temperature is lower than the melting point of the nuclear fuel oxide, the nuclear fuel oxide that has appeared on the container surface does not evaporate or evaporate, but precipitates as a scale upon cooling. This scale is removed by polishing or washing with water.

[0009]

Next, an embodiment of the present invention will be described in detail with reference to the drawings. As shown in FIG. 1, an electric furnace 10 which is a batch type muffle furnace is provided with a supply port 11 and an exhaust port 12, and an opening / closing door 1 is provided between the supply port 11 and the exhaust port 12.
3 are provided. The electric furnace 10 has an inlet 11 and an outlet 1
With the exception of 2, the structure is sealed. A table 15 on which the molybdenum container 14 used for burning nuclear fuel pellets is placed inside the electric furnace 10. An air pipe 16 for supplying air rich in oxygen is connected to the air supply port 11,
An air pipe 17 is connected to the exhaust port 12. The air pipe 17 is connected to an air inlet 19 of a trap 18. An air outlet pipe 21 is provided through the lid 18 a of the trap 18, and the outlet pipe 21 is connected to the filter 2 through an air pipe 22.
3 is connected to one end. The other end of the filter 23 is the exhaust pipe 2
4 is connected.

Next, a method for decontaminating the molybdenum container 14 using the decontamination apparatus will be described. First, two used molybdenum containers 14 were prepared. The radioactive contamination concentration was 4 Bq (Becquerel) for one molybdenum container and 25 Bq for the other molybdenum containers. The door 13 of the electric furnace 10 is opened, and these molybdenum containers 14
After being placed on 5, the opening and closing door 13 was closed, and the inside of the electric furnace 10 was heated. In this example, from room temperature to 610 ° C. at a rate of 10 to 20 ° C./min while supplying fresh oxygen-rich air at 25 ° C. from the air pipe 16 through the air supply port 11 at a rate of 10 liter / min. The temperature was raised and maintained there for 1.5 hours.
Since the temperature of 610 ° C. is lower than the evaporation or vaporization temperature (about 800 ° C.) of the oxide of the nuclear fuel material such as uranium, almost no oxide vapor of the nuclear fuel material such as uranium is generated.

The heated air in the electric furnace 10 discharged from the exhaust port 12 is introduced into a trap 18 through an inlet 19 through an air pipe 17. Here, very small amounts of oxide vapor of nuclear fuel substances such as uranium contained in the air are cooled,
Solidified and collected. The air discharged from the trap 18 passes through a filter 23 through an air pipe 22. The oxide dust that could not be collected by the filter 23 is collected,
The purified air is discharged from the exhaust pipe 24.

The two molybdenum containers 14 heated in the electric furnace 10 are cooled in the electric furnace 10 at a rate of 25 to 30 ° C./min. Removed from 10 and cooled at room temperature. Two molybdenum containers 1 at the same temperature as room temperature (25 ° C.)
Scale was formed on the surface of No. 4. When lightly polished with a wire brush, the scale peeled off, resulting in a silver-white molybdenum container. When the radioactivity concentrations of the two molybdenum containers 14 were measured, both were found to be 0.04 Bq. Thereby, the decontamination coefficient of one molybdenum container was 100, and the decontamination coefficient of another molybdenum container was 600.
When the peeled scale was analyzed, U ₃ O ₈ , MoO ₃
And the like containing nuclear fuel oxides.

[0013]

As described above, the decontamination method of the dry method of the present invention requires only heating by an electric furnace as compared with the conventional wet method, so that no secondary waste such as waste liquid is generated. And 10
There is a feature that a relatively high decontamination coefficient of about 0 to 600 can be obtained. Compared with the dry continuous processing method disclosed in JP-A-63-45594, the dry batch processing method of the present invention heats the electric furnace to a temperature equal to or higher than the melting point of molybdenum oxide or nuclear fuel oxide and heats them from the molybdenum container. It is not necessary to completely separate the nuclear fuel material such as uranium from the molybdenum container in a short time and easily with relatively little heat energy, and the processing cost is greatly reduced. In particular, the method of the present invention is suitable for decontamination of a small number of used molybdenum containers because it is a batch process.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a decontamination apparatus of the present invention.

[Explanation of symbols]

 Reference Signs List 10 electric furnace 11 air supply port 12 exhaust port 14 used molybdenum container

Continuation of the front page (72) Inventor Toshiyuki Kai 622, Funaiishikawa, Tokai-mura, Naka-gun, Ibaraki Prefecture 1 In the Tokai Works of Mitsubishi Nuclear Fuel Co., Ltd. (56) References JP-A-55-44917 (JP, A) 61-194497 (JP, A) JP-A-51-20900 (JP, A) JP-A-63-45594 (JP, A) JP-A-55-159200 (JP, A) (58) Fields investigated (Int. Cl. ^7, DB name) G21F 9/28 G21C 21/02

Claims (2)

(57) [Claims] 1. A molybdenum container (14) used for burning nuclear fuel pellets is placed in a closed electric furnace (10) having an air supply port (11) and an exhaust port (12). The electric furnace (1) is supplied from the air supply port (11) while supplying oxidizing air from the air supply port (11) and from the exhaust port (12).
0) 0.75 at 600 to 620 ° C while exhausting the air in
Heating the molybdenum container (14) for about 2 hours, removing the heated molybdenum container (14) from the electric furnace (10) and cooling it to at least room temperature, and oxidizing nuclear fuel deposited on the surface of the molybdenum container (14). Method of decontaminating molybdenum containers to remove material. 2. The method for decontaminating a molybdenum container according to claim 1, wherein the oxide is removed by polishing or washing the surface of the molybdenum container.