JPH0644076B2 - Method and apparatus for thermal oxidation treatment of spent nuclear fuel cladding tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a short length of a nuclear fuel cladding tube after removing the spent nuclear fuel inside, in the spent nuclear fuel reprocessing step, more specifically, it is made of zircaloy metal. The present invention relates to a method of stabilizing a coated tube having a short length (hereinafter abbreviated as "hull") by oxidizing it in an oxidizing atmosphere such as air to convert it into an oxide mainly containing zirconium oxide, and an apparatus used therefor.

[Conventional technology]

While peaceful use of nuclear fuels as an alternative to traditional fuels such as coal, oil and natural gas continues to grow steadily, amid the fear of nuclear power and the global environmental movement, the disposal of radioactive waste is now. This is a serious concern.

During the reprocessing process of spent nuclear fuel, the nuclear fuel cladding tube is separated by about 30 to separate the nuclear fuel in the fuel assembly from the cladding tube.
It is cut into short pieces with a length of up to 50 mm, and acid dissolution with nitric acid or the like is performed.

As a result, a short cladding tube or hull from which the internal nuclear fuel has been removed by acid dissolution is generated as solid waste. Hulls as solid radioactive waste are (1) the hull itself is highly activated, and (2) transuranium elements such as plutonium dioxide and fission products such as cesium adhere to the inner surface of the hull as undissolved substances. And (3) its material, that is, Zircaloy has a risk of spontaneous combustion, and special handling and storage technology are strongly required, and it has been difficult to handle in the past. Therefore, it was stored in a drum and submerged in a stainless steel-lined water tank for storage.

However, this method requires a huge storage space due to the low density of hulls of about 1.1 g / cm ³ , and it is not a stable combination in case of disturbance. However, various research efforts have been made in the past regarding volume reduction and stabilization.

In recent years, hot isostatic pressing (hereinafter abbreviated as HIP) has attracted attention as a technique capable of effectively achieving such volume reduction. HIP is an element, inert gas such as argon or heat-resistant grease, heat-resistant incompressible fluid such as molten glass, BN powder, pyroferrite, talc, zirconium oxide, magnesium oxide, etc. This is a method in which a three-dimensional hydrostatic pressure is applied to an object to be processed at a high temperature to perform isotropic compression, and sintering of metal or ceramics, diffusion bonding, or defect removal of a cast product is performed.

The applicant of the present invention also paid attention to this technology from an early stage, studied the application to the volume reduction and solidification of radioactive solid waste, and has already proposed some of the results.

For example, when subjecting a capsule containing a hull or the like to a HIP treatment, in particular, in order to prevent spontaneous ignition of zircaloy constituting the hull or the like, a method of taking measures such as deaeration prior to sealing the capsule is used. Announced. When the temperature is raised to about 60% of the melting point of Zircaloy-hal, tritium is released from Zircaloy-hal. HI about this
When the P treatment is performed, the metal capsule is degassed and sealed and subjected to a temperature rise treatment, and tritium gas released from the hull is removed by the action of the oxidant placed outside the capsule to remove the oxidation catalyst if necessary. A method was proposed in Japanese Patent Application No. 58-182598, in which tritium water was converted, and then the capsule was degassed and hermetically sealed with water contained therein.

[Problems to be solved by the invention]

In the situation described above, we have found that solid radioactive waste,
In particular, in order to reduce the volume of hulls and improve storage stability, zircaloy metal can be reformed into an oxide, that is, zirconium oxide, by contacting it with oxygen under heating conditions, and crushing such oxide. However, it has been clarified that the HIP treatment can be safely and surely converted into a ceramic dense and stable solidified body.

By the way, the amount of hulls generated in the nuclear fuel reprocessing step can reach from several thousand to several tens of thousands at a time, depending on the amount of processed hulls. To bring such a large amount of hulls into contact with an oxidizing medium such as air under heating conditions on a commercial scale, a large number of hulls are put together in a container and heated in an electric furnace or the like. The bottom hull has a problem that it is not sufficiently oxidized due to poor contact with air. Furthermore, zircaloy hull has a zirconium metal content of 95% or more in terms of chemical composition weight ratio, and especially the presence of oxygen at high temperature. In the oxidation below, there is a risk of ignition as described above.

In view of these problems, the first object of the present invention is to
A second object of the present invention is to provide a method and a device for uniformly and easily oxidising a large amount of hulls discharged from a nuclear fuel reprocessing process on a commercial scale, and the second object of the present invention involves the risk of ignition of such a method. It is to provide a method and an apparatus for safely performing the operation.

Further, an ultimate object of the present invention is to stabilize and reduce the volume of the hull, and to secure and safely dispose of the hull.

[Means for solving problems]

According to the method of the present invention for achieving the above-mentioned first and second objects, a lidless container accommodating a deposited layer of hull mainly composed of zircaloy metal is loaded into a heating furnace, and the inside of the furnace is exposed to an oxidizing gas atmosphere. At the same time, heat treatment was performed while feeding oxidizing gas to the lower layer of the above-mentioned deposited layer, and at the same time, inert gas could be supplied into the furnace to suppress the radical oxidation reaction of the hull. It is characterized by that. Furthermore, the features of the apparatus which apply to these methods are as set out in the preambles of the claims and will be clearly understood by the following description.

Therefore, the configuration of the present invention will be described below with reference to the accompanying drawings.

First, when oxidizing a large amount of hull, as shown in the outline of Fig. 1, the accumulated layer of the hull (2) is housed in the open container (1) and loaded into an electric furnace (3) and heated. It is heated to a predetermined temperature by the heater (4).

At this time, the electric furnace (3) is supplied with an oxidizing gas (5), for example, air into the furnace by using a feeder (6), or a negative pressure state is created in the furnace by an exhaust fan (7). The hull (2) is oxidized by making the inside of the furnace an oxidizing gas atmosphere by a suction method.

However, when the oxidization is performed in this way, as shown in FIG. 2, the portion from the surface layer of the hull supplied and deposited in the open container (1) to the upper layer A is oxidized, but the lower layer is The part existing in the part B is left in an unoxidized state, and safe oxidation is difficult even if it is heated for a long time.

Therefore, the method of the present invention has an essential point of performing a heat treatment while directly feeding an oxidizing gas to the lower layer portion of the hull deposition layer to oxidize the entire deposition layer uniformly and uniformly. FIG. 3 shows the basic structure of the lidless container applied to the method of the present invention.

FIG. 3-A is a schematic side view thereof, FIG. 3-B is a plan view, and FIG. 3-C is a plan view showing an example of a modified mode thereof.

In these figures, a porous pipe (9) capable of efficiently supplying an oxidizing gas such as air is arranged at the bottom of the container (1), and a large number of blow holes (10) are provided in this porous pipe. It is perforated, and one end (11) thereof projects to the outside of the container (1).

It is more preferable that the perforated pipes (9) are arranged, as shown in FIG. 3-C, by arranging a plurality of pipes in parallel because the oxidation efficiency can be increased.

FIG. 4 is a schematic vertical sectional view showing an apparatus of the present invention in which the above-mentioned lidless container is loaded in an electric furnace.

In the figure, the electric furnace (3) has a door (12) that can be closed in an airtight manner, and an open container (1) equipped with a accumulated layer of hull is loaded into the furnace by opening and closing the door (12). . Further, the electric furnace (3) has a plurality of air supply conduits (18) (18) ... Each of which is opened in the furnace and communicates with the feeders (6) (6), and an exhaust hole in the furnace ( The exhaust pipe (20) is opened to the outside air from the exhaust fan (7) and the high performance filter (8) from the exhaust pipe (7).

A high-performance filter removes fine particles such as fumes that have sublimated at high temperatures, and the tritium gas contained in the hull is converted into tritiated water using an exhaust gas oxidation catalyst after filtering with a high-performance filter and collected.

The lidless container (1) loaded in such an electric furnace (3) is
As shown in Fig. A, one end (11) of the perforated tube (9) is a furnace body (13).
It is fitted and connected to the oxidizing gas supply hole (14) formed at the end of the air supply pipe (18) protruding from the inside to the furnace. On this occasion,
The supply hole (14) is provided with a stopper (15) so that the end (11) of the perforated pipe (9) and the supply hole (14) are coupled and tightly fitted.

In this way, as shown in FIG. 4-B, it becomes possible to supply an oxidizing gas such as air to the lower layer of the hull deposition layer through the perforated pipe (9).

FIG. 5 is a schematic longitudinal sectional view showing an apparatus equipped with an oxidation reaction control means for suppressing the ignition of the hull itself by abruptly reacting with oxygen in the oxidizing gas and for immediately extinguishing the ignition. is there.

In the figure, a cylinder of an inert gas such as nitrogen gas (17)
(17) ... Can be communicated with the inside of the furnace by inert gas supply pipes (22) (22) ..

The inert gas supply pipes (22) (22) may be directly connected to the furnace body (13) or may be connected to the air supply pipe lines (18) (18).

On the other hand, a detector (16) for detecting the temperature inside the furnace and the ignition phenomenon, such as a thermocouple or an optical thermometer, is provided in the furnace, and the opening and closing is performed by the abnormal behavior signal of the temperature inside the furnace accompanying the ignition phenomenon. Operate the valves (21) (21) ...
The supply rate of the inert gas from 7) is controlled to adjust the oxidation rate, or the inert gas is rapidly ejected to extinguish the fire.

At this time, it is preferable to control or stop the oxidizing gas feeders (6) and (6) at the same time, and the same applies when the oxidizing gas is taken in from the outside by the suction method. Control the suction volume. In order to perform those operations by the detection signal of the detector (16), well-known and commonly used means can be appropriately applied.

The oxidation temperature applied to the above-mentioned method of the present invention is 1000 ° C or higher, preferably 1200 ° C or higher.

[Action]

The present invention is as described above, and then the operation of the present invention will be described.

The hull (2) discharged from the reprocessing step is loaded on the lidless container (1) and loaded into the electric furnace (3) to seal the door (12).

At that time, the attitude of the container (1) is regulated by rails in the furnace, and the end (11) of the perforated pipe (9) protruding from the bottom of the container
As shown in FIG. 4-A, the holes are fitted into the oxidizing gas supply holes (14) and tightly connected by the action of the stopper (15). In this state, the heater (4) of the electric furnace is energized to raise the temperature inside the furnace and the feeders (6) (6) ...
Injects oxidizing gas such as air, and at the same time, exhaust fan (7)
To discharge the gas in the furnace through the exhaust pipe.

The exhaust gas is filtered by the high-performance filter (8) to remove activation products such as minute dust and radionuclides contained therein.

In this way, a high-temperature oxidizing gas atmosphere of about 1000 ° C. or higher, preferably 12000 ° C. or higher is formed in the furnace, and the oxidizing gas sent to the lower part of the hull deposit layer through the perforated pipe (9). As a result, the oxidizing gas is evenly distributed from the lower part to the upper part of the deposited layer to uniformly oxidize the zircaloy forming the hull in a short time and no unoxidized part remains.

It is preferable to preheat the oxidizing gas supplied via the air supply line (18). Further, in the above description, the case where the oxidizing gas is forcibly fed from the air supply line by the feeder is shown, but the exhaust fan (7) exhausts it to make the inside of the furnace a negative pressure, and to oxidize air etc. Of course, it is also possible to adopt a method of inhaling the characteristic gas.

Further, since the zircaloy constituting the hull has a risk of ignition due to rapid oxidation as described above, if the reaction rate is adjusted by the device of the present invention shown in FIG. You can extinguish the fire and eliminate the danger.

That is, the signal that detects the abnormally high temperature in the furnace by the detector (16) is converted into the operating force of the on-off valve (21) through the electronic processor or the pneumatic operation mechanism, and the valve (21) is operated to operate the cylinder ( Adjust the amount of inert gas supplied from 17). As a result, the oxidation reaction rate is controlled appropriately, and in the case of emergency fire extinguishing, the valve (21) is fully opened to eject a large amount of inert gas and at the same time the supply of oxidizing gas can be cut off to immediately extinguish the fire. .

Further, since the inert gas can be supplied through the air supply line (18), the rapid termination of the oxidation reaction can be surely achieved.

〔Example〕

(Comparative Example) The outer diameter is 14 mm and the thickness is 0.1, which is substantially the same as the nuclear fuel cladding tube.
3 mm of Zircaloy short tube 8 mm long and 30-50 mm long
It was filled and loaded in an open box type container made of Hastelloy X of 00 mmW × 300 mmL × 100 mmH. This container consists of a single perforated pipe (tube diameter 10 mmΦ, blow hole diameter 2 mm as shown in Fig. 3-B.
Φ, the number of holes 20) was used at the bottom.

This container was placed in an electric furnace having an inner diameter of 500 mmW × 400 mmL × 500 mmH and sealed with a door.

The electric furnace has a structure as shown in FIG. 5, and the air supply hole and the end of the perforated tube of the container were fitted and connected by a coupling mechanism.

First, the air supply pipe that supplies air to the perforated tube is closed, and air is introduced into the furnace only at the top of the electric furnace at a flow rate of 0.5 m ³ / min and discharged from the exhaust pipe. The heater was energized while forming a stream. The air supply line was heat-exchanged by passing through the exhaust line in an opposed manner, and the supply air was preheated.

The temperature in the furnace reached 1200 ° C. after 80 minutes, the temperature was maintained at that temperature and the operation was continued for 120 minutes, and then the energization and the air flow were stopped. After cooling, the oxidation state of the Zircaloy short tube deposited layer was examined. A layer with a thickness of about 70 mm from the surface of the deposited layer was well oxidized. The short length of the lower layer having a thickness of 30 mm was partially oxidized and mixed with a black metal peculiar to Zircaloy.

(Embodiment) Next, as an embodiment of the present invention, the lower air supply conduit is opened, air is also fed from the container bottom perforated pipe at a flow rate of 0.4 m ³ / min, and the upper air supply is 0.3 m ³ / min The zircaloy short tube was oxidized under the same conditions as in the comparative example except for the above.

As a result of observing the oxidation state after the treatment, it was confirmed that the entire surface of the deposited layer was uniformly transformed into an oxide up to the deep part.

Thus, the excellent effects of the method and apparatus of the present invention were confirmed by the above comparative example and the present invention example.

〔The invention's effect〕

INDUSTRIAL APPLICABILITY Since the present invention has the above-described structure and operation, it is possible to safely, quickly and reliably oxidize a large amount of hulls discharged from the spent nuclear fuel regeneration process on a commercial scale to make it radioactive. There is an excellent effect that the nuclide can be densified by sintering into a stable volume-reduced solidified body.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an outline of a method and an apparatus for heat-oxidizing a hull of a general mode, FIG. 2 is an explanatory view showing an oxidation state of the hull obtained by the method of FIG. 1, and 3-A.
FIG. 3 is a schematic vertical sectional view of a lidless container used in the present invention,
-B is a plan view thereof, FIG. 3-C is a plan view of another mode thereof, FIG. 4 is a schematic vertical sectional view of the device of the present invention, FIG. FIG. 5B is an explanatory view of the operation of the method by the same apparatus, FIG. 5 is a schematic vertical sectional view showing an improved method and apparatus of the present invention, and FIG. (1) …… Uncovered container, (2) …… Short length of nuclear fuel cladding, (3) …… Electric furnace, (4) …… Heating heater, (5) …… Oxidizing gas, (6) …… Sending Feeder, (7) …… Exhaust fan, (8) …… High performance filter, (9) …… Perforated tube, (10) …… Blowout hole, (11) …… End, (12) …… Door , (13) …… Furnace body, (14) …… Supply hole, (15) …… Stopper, (16) …… Detector, (17) …… Cylinder, (18) …… Air supply line, (19 ) …… Exhaust hole, (20) …… Exhaust pipe, (21) …… Open / close valve, (22) …… Inert gas supply pipe,

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masao Shiotsuki Inventor Masao Shiotsuki 4002 Narita-machi, Oarai-cho, Higashi-Ibaraki-gun, Ibaraki Oarai Engineering Center, Power Reactor and Nuclear Fuel Development Corporation (72) Fumiaki Komatsu Chikugaoka, Kita-ku, Kobe-shi, Hyogo 6-2 / 11 (56) Reference JP-A-59-73040 (JP, A) JP-A-60-198498 (JP, A)

Claims (10)

[Claims] 1. An open-ended container containing a deposited layer of a spent nuclear fuel cladding short piece mainly composed of zircaloy metal is placed in an electric furnace to create an oxidizing gas atmosphere in the furnace and a lower layer of the deposited layer. Spent nuclear fuel, which is characterized by performing a heat treatment while feeding an oxidizing gas to the section, and further supplying an inert gas into the reactor in order to suppress the radical oxidation reaction of the nuclear fuel cladding tube by the above treatment. Thermal oxidation treatment method for cladding. 2. The method for thermal oxidation treatment of a spent nuclear fuel cladding tube according to claim 1, wherein the oxidizing gas is air. 3. The method for heat-oxidizing a spent nuclear fuel cladding tube according to claim 1 or 2, wherein the heat treatment is performed at a temperature of at least 1000 ° C. 4. The method for heat-oxidizing a spent nuclear fuel cladding tube according to claim 3, wherein the heat treatment is performed at a temperature of at least 1200 ° C. 5. The used gas according to any one of claims 1 to 4, wherein the oxidizing gas is introduced through a perforated pipe provided in a lower layer portion inside the deposition layer. Method for thermal oxidation treatment of nuclear fuel cladding. 6. The used gas according to any one of claims 1 to 5, wherein the oxidizing gas atmosphere in the furnace is replaced with an inert gas atmosphere by supplying the inert gas. Method for thermal oxidation treatment of nuclear fuel cladding. 7. The method for thermal oxidation treatment of a spent nuclear fuel cladding tube according to any one of claims 1 to 6, wherein the inert gas is nitrogen gas. 8. The method for thermal oxidation treatment of a spent nuclear fuel cladding tube according to claim 1, wherein the inert gas is supplied to a lower layer portion of the deposited layer. 9. An open-ended container for accommodating a deposited layer of a short piece of a spent nuclear fuel cladding tube, and an electric furnace having a door for inserting and removing the open-ended container, wherein the electric furnace is oxidizable in the furnace. It comprises a plurality of air supply pipes each communicating with a feeder for introducing gas, and an exhaust pipe having an exhaust fan and a high-performance filter, respectively, and the lidless container has a perforated pipe at its bottom. Heating the spent nuclear fuel cladding tube, characterized in that one end of the perforated tube is connected to one end of the air supply line when the container is loaded in an electric furnace. Oxidation treatment equipment. 10. An open-ended container for accommodating a deposited layer of a short material of a spent nuclear fuel cladding tube, and an electric furnace having a door for inserting and removing the open-ended container, wherein the electric furnace is oxidizable in the furnace. It comprises a plurality of air supply conduits each communicating with a feeder for feeding gas, an exhaust conduit provided with an exhaust fan and a high-performance filter, and the lidless container has a perforated pipe at the bottom thereof. In addition, one end of the perforated pipe is connected to one end of the air supply pipe in a state where the container is placed in an electric furnace, and an inert gas communicating with the inside of the pipe via an on-off valve is provided. A supply pipe is additionally provided, and an electric furnace temperature detector is further provided so that the on-off valve can be operated by a signal from the detector to control the oxidation reaction rate in the furnace. Thermal oxidation treatment equipment for nuclear fuel cladding.