JPH11242096A - Method and apparatus for decontamination and recovery of tray for sintering nuclear fuel pellets - Google Patents

Abstract

(57) Abstract: A molybdenum sintering tray on which nuclear fuel pellets are placed and sintered, radioactive substances are removed from the tray and molybdenum is recovered. SOLUTION: The sintering tray has an oxygen concentration of 20 wt%.
Molybdenum is sublimated by heating at a temperature of 400 to 800 ° C. under the above atmosphere, and the sublimated molybdenum is cooled and deposited to decontaminate and recover the molybdenum of the sintering tray. I have.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a sintering Mo (molybdenum) tray used for sintering nuclear fuel pellets, and more particularly to a method for decontaminating and recovering Mo from the tray. .

[0002]

2. Description of the Related Art Since a plate made of Mo has a high melting point and a high strength, it is used as a tray for placing pressed pellets (green pellets) such as UO ₂ and MOX when sintering them. I have. However, since these Mo trays are used at a high temperature of 1600 ° C. or more in a sintering furnace, they may be damaged or distorted due to a decrease in strength. Will be replaced.

[0003] Such used Mo trays are constantly generated as discarded objects, but radioactive oxides and compounds such as uranium adhere to the surfaces of the discarded Mo tray plates. Therefore, the surface contamination density does not decrease even if the surface is usually wiped.

[0004]

For this reason, when decontaminating Mo plates at present, chemical decontamination, electrodischarge dyeing and mechanical polishing have been proposed. For these reasons, Mo trays with the above defects are actually stored and discarded as they are because secondary waste such as chemicals is generated in large quantities, and it is impossible to decontaminate every detail by mechanical polishing. That is the current situation.

According to the present invention, the above-mentioned Mo tray, which has to be stored and discarded, is sublimated in a high-temperature oxidation state to separate contaminants such as the uranium compound from Mo. The purpose is to collect and reuse Mo.

[0006]

That is, a method for decontaminating and recovering a tray for sintering nuclear fuel pellets according to the present invention, which meets the above-mentioned object, comprises a tray for spent sintering made of molybdenum on which nuclear fuel pellets are placed and sintered. In this method, the sintering tray is heated at a temperature of 400 to 800 ° C. in an atmosphere having an oxygen concentration of 20 wt% or more to sublime molybdenum, and the sublimated molybdenum is cooled and deposited to form the sintering tray. Decontaminating and recovering molybdenum. In this case, the heating temperature is most preferably around 600 ° C.

Further, as an apparatus used in the method for decontaminating and recovering a tray for sintering nuclear fuel pellets of the present invention, a furnace for heating and sublimating a molybdenum tray and an atmosphere gas containing oxygen are supplied into the furnace. It is preferable to provide a supply means for supplying the gas and a cooling and recovery chamber to which vaporized molybdenum is sent from the furnace, and to form a cooling member for cooling and depositing the vaporized molybdenum in the cooling and recovery chamber.

[0008]

[Function] Molybdenum (Mo) has the property of sublimating when heated to a high temperature in the presence of oxygen.
O Uranium and plutonium which adhere or permeate to the surface of the plate to cause radioactive contamination form a stable oxide such as U ₃ O _{8 in} the presence of the oxygen. Therefore, by raising the temperature of the tray to a high temperature under appropriate oxygen conditions, it is possible to sublime only Mo and separate it from the radioactive material containing uranium, plutonium, and the like.
Then, the separated Mo precipitates by cooling the sublimated one and lowering the temperature, so that it can be easily recovered and reused.

[0009]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
An embodiment of the present invention will be described.

FIG. 1 is a sectional view showing an apparatus for decontaminating and recovering a sintering tray according to an embodiment of the present invention. In FIG. 1, 1 is a tubular furnace, 2 is a heater for heating this furnace,
Reference numeral 4 denotes an opening / closing lid provided on the upstream side and the downstream side of the furnace, respectively, and reference numeral 5 denotes a cooling and recovery device.

A pump 6 for sending atmospheric gas into the furnace is provided upstream of the tubular furnace 1. The pump 6 can adjust the amount of oxygen to be sent to the furnace, and can arbitrarily mix an inert gas such as N ₂ or Ar with the atmosphere or oxygen and send it to the inside of the tubular furnace 1. In addition, the furnace 1 of this pump 6
A pressure gauge 7 for measuring the pressure inside the furnace is provided near the connection to the furnace. Reference numeral 8 denotes a control panel, which includes a furnace temperature controller 9 and an oxygen concentration meter 10, and a chart 11 for recording the temperature and the oxygen concentration. In this example, the furnace 1 is a tubular furnace, but a muffle furnace or another furnace may be used.

On the other hand, the cooling / recovering device 5 includes a chamber 12 provided adjacent to the opening / closing lid 4 on the downstream side, and a cooling member as a cooling member installed in the chamber 12 in a spiral or serpentine shape, for example. And a plate 13. The cooling plate 13 is detachable from the cooling and collecting device 5 so that the deposited Mo can be easily collected.

Cooling water flows so as to circulate inside the cooling plate 13, and the vaporized Mo sent from the tubular furnace 1 is cooled by the cooling plate 13,
3 and is collected. The atmosphere in the furnace in which the Mo has been recovered is discharged from the discharge port 14 of the chamber 12 through a filter (not shown) while removing radioactive substances.

Using the decontamination / recovery apparatus of this embodiment, a laboratory tray T made of Mo (dimensions 20 × 60 × 1.5 mm, weighing about 19 g) contaminated by radioactivity was used in a tubular furnace 1 as shown in the figure. , The opening / closing lid 3 is opened, and atmospheric gas (air of about 23 wt% of oxygen) is directly supplied to the furnace 1.
After holding for 6 hours under the condition of 00 ° C. (preferably 600 ° C.), the tray T in the furnace showed a weight loss of 99% or more. When the residue remaining in the furnace 1 was analyzed, U, M
o and Fe were detected respectively. Then, in the cooling recovery device 5 downstream of the furnace 1, the crystalline M
Deposition of oO ₃ was confirmed.

[0015] In the presence of oxygen Thus, uranium and plutonium adhering to the tray T from forming a stable oxide such as U ₃ O _8, oxygen 20 wt% or more manufactured Mo under appropriate atmospheric conditions By raising the temperature of the tray to a high temperature, only Mo can be sublimated and separated from radioactive substances containing uranium, plutonium, and the like. It is considered that a reaction represented by the following formula 1 occurs in the Mo tray treated in the high temperature oxygen atmosphere.

[0016]

## EQU1 ## 2Mo + 3O ₂ = 2MoO ₃ (→ sublimation)

[0017]

As described above, according to the present invention, the above-mentioned sintering tray made of molybdenum is heated at a temperature of 400 to 800 ° C. in an atmosphere having an oxygen concentration of 20 wt% or more to sublime molybdenum. The molybdenum in the tray for sintering is decontaminated and recovered by cooling and precipitating the sublimated molybdenum.The sublimation of molybdenum by heating the tray in the specific oxygen atmosphere while the radioactivity in the tray is performed. The substance is converted into a stable oxide and left as a residue, whereby it is possible to separate and recover only the molybdenum in a decontaminated state.
In addition to enabling the decontamination of molybdenum trays, which were difficult to separate from radioactive materials, the amount of sintering trays that had to be stored and discarded has been greatly reduced, and the recovered expensive molybdenum has been reduced. It has a remarkable effect that it can be reused.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an example of an apparatus for performing a decontamination and recovery method according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Tubular furnace 2 Heater 3 Opening / closing lid 4 〃 5 Cooling / recovering device 6 Pump 7 Pressure gauge 8 Control panel 9 Controller 10 Oxygen concentration meter 11 Chart 12 Chamber 13 Cooling plate 14 Discharge port

Claims (3)

[Claims] 1. A molybdenum used sintering tray on which nuclear fuel pellets are placed and sintered, the sintering tray is placed in an atmosphere having an oxygen concentration of 20 wt% or more in an atmosphere of 400 to 80 wt.
Molybdenum is sublimated by heating at a temperature of 0 ° C., and the sublimated molybdenum is cooled and deposited to decontaminate and recover the molybdenum of the sintering tray. Decontamination recovery method. 2. The method according to claim 1, wherein the heating temperature is around 600 ° C. 3. An apparatus used in the method for decontaminating and recovering a tray for sintering nuclear fuel pellets according to claim 1 or 2, wherein a furnace for heating and sublimating a molybdenum tray is provided with oxygen in the furnace. Supply means for supplying an atmosphere gas containing
Decontamination of a tray for sintering nuclear fuel pellets, comprising: a cooling and recovery chamber for sending vaporized molybdenum from the furnace; and a cooling member for cooling and depositing the vaporized molybdenum in the cooling and recovery chamber. Collection device.