JPS6239960B2 - - Google Patents

Description

Detailed Description of the Invention The present invention relates to spent nuclear fuel cladding tubes called hulls, upper and lower ends of nuclear reactor fuel assemblies, spacers,
The present invention relates to a volume reduction and stabilization treatment method suitable for safely storing radioactive metal waste such as springs (hereinafter abbreviated as "hull etc.") over a long period of time.

Uranium fuel burned in a nuclear reactor is usually sheared and melted at a reprocessing facility, and separated into plutonium, uranium, and radioactive waste.The plutonium and uranium are recovered, refined, and processed again as fuel for reuse. ing. However, after the radioactive waste is separated, the gas and liquid are treated separately, but the remaining melted fuel cladding tube, so-called hull, and the upper and lower ends of the fuel assembly,
Solid metal waste such as spacers and springs is generated, and its disposal has become an important technical issue.

Solid waste such as hulls is stored for long periods of time as high-level radioactive waste, but they are not only highly radioactive, but also have special characteristics due to the danger of spontaneous combustion due to the material, Zircaloy. Handling and storage techniques are required. Due to this difficulty in handling, hulls are currently stored in drums and submerged in stainless steel-lined water tanks. One reason for this is that water can be used as a shield, and the other reason is that water prevents spontaneous combustion.

However, with this method, Hull et al.
Due to this low density, the storage space becomes enormous, which significantly reduces storage efficiency, and there are problems such as the fact that it does not form a strong bond against unexpected disturbances, so volume reduction treatment is strongly desired. There is.

By the way, many proposals have been made regarding volume reduction treatment methods for such hulls, etc., including those at the idea stage, but most of them are still at the development stage, and only a few are at the plant-scale demonstration stage. There is only one method, the Zirflex method.

However, this method has not been particularly put into practical use, and the actual state of the method does not lead to a large volume reduction ratio.

On the other hand, research into more compact and stable storage forms is progressing in various fields, and some have proposed, for example, a method of melting and solidifying the hull in a heating furnace to form a dense block body. However, this method also has problems with the disposal of secondary waste, such as radioactive gas during melting and radioactivity of refractories in the furnace, and has not yet been put into practical use.

Despite this situation, on the other hand, the demand for reprocessing of spent fuel is increasing due to the effective use of resources and the increasing proportion of nuclear energy, and along with this, the amount of hulls generated is gradually increasing. The problem of storage facilities is becoming increasingly urgent, and the early development of volume reduction processing technology is strongly desired.

In view of the above-mentioned situation, the present invention aims to safely and reliably process metal waste such as hulls generated during the reprocessing of spent fuel, which is a type of radioactive waste, and convert it into a form suitable for long-term storage. The purpose of this is to provide a method to ensure the safety of processing operations during the pressing process after housing hulls, etc. in containers, taking into account spontaneous combustion and tritium countermeasures. It is characterized by its ability to be secured and processed into a form suitable for storage.

Hereinafter, specific characteristics and aspects of the present invention will be described in detail.

The attached drawings are schematic diagrams of the steps of the method of the present invention, and sequentially show each step of putting the hull etc. into a container 1 and reducing the volume by press-compressing the container together.

In the figure, the important points of the method of the present invention, especially for processing hulls, etc., are: (1) Avoiding spontaneous combustion during the process; (2) What to do with tritium absorbed by hulls, etc. , (3) not to generate dust, and (4) to suppress the spread of contamination to each processing equipment, and each step of the method of the present invention is configured by achieving these.

That is, for (1) above, a getter such as Ti or Zr is placed in the container to fix oxygen, and the oxygen in the container is removed by vacuum suction to prevent reaction with Zircaloyha. Therefore, Zircaloy powder, which is thought to be generated during press compression, is normally likely to catch fire, but it can be sufficiently prevented.

In addition, for (2), Cu or
This is achieved by using Al. Tritium usually has high permeability and can pass through any material, but among these, Cu, Al, etc. have relatively low permeability. Therefore, even if tritium is generated in gaseous form, the use of a container prevents the spread of contamination.

Next, regarding points (3) and (4), since containers are used in both cases as described above, diffusion is sufficiently prevented. In particular, it is conceivable that the container may be damaged during press compression, but since both Cu and Al have excellent malleability, if the thickness is appropriately selected, it is almost impossible for the container to be damaged. In addition, since air is removed by vacuum suction, pressure is not maintained within the container.
Therefore, it cannot be destroyed. In addition to the above, the method of the present invention is intended to: (5) Simplicity of the process (6) Remote operability (7) Ease of maintenance (8) Ease of handling (9) Volume reduction ratio (10) ) Stability of the treated product, ease of storage, etc., and the method of the present invention provides sufficient and satisfactory results in all of these respects. In other words, it is clear that (5) to (7) are satisfied because a simple press compression method is used, and (8) can be transported by magnet by welding magnetic stainless steel plates. The simplest handling is possible.

On the other hand, regarding (9), the volume reduction ratio that can be obtained by the method of the present invention is about 1/4 to 1/5, and the true density of Zircaloy is 6.5 g/cm ³ and Hull et al. (1.1 g/cm ³ ) is 1/6 even if the volume is reduced to the maximum, which confirms that this is a sufficient value.

Furthermore, regarding point (10), as is clear from the above, the obtained treated body and its storage method have no fear of spontaneous combustion and do not release tritium. Furthermore, a storage container with a suitable shielding thickness allows the application of a conventional and simple storage system, and does not require large-scale underwater storage facilities.
Note that the amount of heat generated by the hull, etc. is small, so forced cooling is not necessary, and cooling to the level of ventilation is sufficient.

As described above, the method of the present invention satisfactorily fulfills each important point intended by the present invention. The contents will be explained in order according to each step shown in FIG.

First, as shown in FIG. 1A of the accompanying drawings, the method of the present invention begins with the step of appropriately charging a hull or the like (H) into a container 1 using a conveying means 2. At the time of charging, it is advantageous to connect the container 1 as closely as possible to the supply line of the hull or the like in order to prevent crud or damaged fuel adhering to the surface from scattering in the form of dust.

The material of the container used is generally Cu or Al.
use. Normally, tritium is diffused in the hull, etc., and there is a possibility that it will come out during processing. Of course, the amount released at room temperature is considered to be relatively small, but if the temperature rises even locally during processing, more will be released, so it is necessary to take measures against this. Due to its nature, tritium has high permeability and can pass through almost all materials, but materials that are relatively difficult to penetrate include Au,
It is said to be W, Al, Cu, Mo, etc. Therefore, any of these materials can be used and each can achieve the intended purpose of the present invention, but among them, Al or Cu is economically the most advantageous and practical. Furthermore, both of these materials are highly malleable and have the advantage of being able to easily cope with local deformation of the container during the press compression process.

In addition, when feeding the hull, etc., another supply means 3 installed close to the conveyance means is used.
Add and mix getters such as Ti and Zr. This is to remove as much oxygen as possible within the container 1 and to prevent oxidation and ignition of the surface of the hull and the like during pressurization, which will be described later.

As is well known, most of hulls are made of Zircaloy and have the property of being highly flammable, and can ignite not only in the air but also in CO ₂ and N ₂ depending on the temperature. Furthermore, it has been found from a study of the relationship between the particle size of zirconium powder, the ignition temperature, and the amount that when Zircaloy is turned into powder, its ignition temperature gradually decreases. In addition, the above has been confirmed from the results of considering the ignition temperature and surface area of zirconium foil relative to its volume, as well as the consideration of each shape and ignitability of zirconium. For example, the hull is equivalent to a sheet of 0.762 mm. Although its area per unit weight is 4 cm ² /g, the ignition limit of sheet-like zirconium is usually 12.5 cm ² /g, and it has been found that the hull itself does not spontaneously ignite. Also, from this value, the ignition temperature of Hull itself is 900℃
It is estimated that the temperature is around ~1000℃, which is a little high, but small pieces are generated when the fuel tube is shredded, and since the Zircaloy tube is brittle during press compression, it is easily broken into pieces, and powder is generated. This is a considerable amount, so we must not forget to consider this point.

In the above situation, Hull etc. (H) is put into the container and filled, but the opening of the container that has been filled in this way is then sealed by welding the lid with an exhaust pipe. . B in the figure shows this container lid welding process, in which a lid 4 having an exhaust pipe is welded to the opening of the container 1 which has been filled with the hull etc. (H).
When welding the container lid 4, the welding heat is particularly transmitted to the hull etc. and tritium is easily released. Therefore, the lid structure shown in FIG. 2 is used and the welding means 5 is placed as far away from the hull etc. as possible. welded by
Perform (a) to prevent heat transfer as much as possible.

Thus, when the welding of the container lid is completed, air is evacuated from the container through the exhaust pipe. The most common means for degassing is a rotary vacuum pump, and the dust discharged at this time is collected through a filter. Furthermore, since the gas to be exhausted may contain tritium, etc., gas treatment corresponding to this is performed as appropriate, as shown in (c) of the figure. However, in any case, these are considered to be in trace amounts, so they cannot be considered particularly important.

Degassing is particularly effective in suppressing spontaneous combustion of the hull powder as described above, and furthermore, it can prevent the container from breaking due to internal pressure during subsequent compression.

After the above-mentioned deaeration, the exhaust pipe is sealed, and this is done by heat-pressing using appropriate pressure bonding means 6, as already seen in other cases, and unnecessary portions of the exhaust pipe are cut and removed. Here,
In order to protect the exhaust pipe afterwards and to facilitate handling, a magnetic stainless steel plate 7 is further attached to the top of the lid 4 by welding means 8 as shown in FIGS. 1D and 2. and weld. In this case, since the container itself is already sealed, activation of tritium by welding heat will not be a problem, but it is natural that it is better to leave it absorbed in the hull etc. as much as possible.
It is desirable that the weld (a)' be located far away, similar to the lid weld. This welding of the magnetic stainless steel plate 7 enables handling with a magnet.
Very useful.

After filling the container with hulls, etc. as described above, and completing the degassing and sealing, this filled container A is then placed in the first container.
Insert it into the press mold 9 and apply compression as shown in Figure E. It is preferable to compress to a surface pressure of 40 kg/ ^mm2 , and by doing so, a true density ratio of about 70 to 80% can be obtained, which is sufficient to achieve the purpose of volume reduction. .

In this case, it is preferable that the press mold 9 used be a split mold that is divided in the vertical direction, so that the processed body can be easily taken out after press compression. That is, as a result of compression, the processing body has a large frictional force against the inner wall of the press, and therefore a large press load is required even when knocked out.

Press compression treated body produced in this way
Finally, a plurality of A's are inserted into the storage container 10 as shown in FIG. In this case, it is effective to use a magnetic stainless steel plate as the lid 11 for ease of handling, and easy handling using a magnet is possible.

By appropriately selecting the shielding material and wall thickness of the storage container, it is possible to achieve the same ease of handling as ordinary low-level waste drums, and there is no need for a large-scale underwater storage pit. gone,
A relatively small space can be used effectively. Moreover, since the amount of heat generated by the hull, etc. is small, there is no need for forced cooling.

As described above, the method of the present invention removes radioactive metal waste.
Fill a container made of Cu or Al with a getter, close the lid, deaerate, seal, compress and reduce the solubility, and then insert multiple pieces into a storage container with a magnetic stainless steel lid and store with the lid closed. (1) By using a container made of Al or Cu, it is possible to prevent the spread of radioactive contamination and to prevent the contamination of equipment such as presses, and to keep the hull etc. away from the outside air. Since there is no contact with tritium, it prevents spontaneous combustion, and also eliminates the risk of container damage during containment and compression of tritium.

(2) The addition of getter fixes the oxygen in the container and prevents spontaneous combustion of hulls, etc.

(3) By capping, degassing, and sealing the container, heat during welding is prevented from being transferred to the hull, etc. and generating tritium, and by removing oxygen, spontaneous ignition is prevented, and at the same time, the container during press compression is Eliminates internal pressure and prevents damage to the container.

(4) The use of magnetic stainless steel material enables handling using a magnet, which facilitates remote control and protects the degassing tube.

(5) A volume reduction ratio of 1/4 to 1/5 can be obtained, which satisfactorily achieves the intended volume reduction purpose, and also allows storage in air since there is no risk of ignition.

This not only makes it possible to reduce the storage space and make efficient use of the storage space, but also eliminates the need for forced cooling and enables normal storage handling, making it extremely useful. This is an extremely effective method from a safety standpoint as a method of disposing of radioactive waste, which will be needed in the future, when the use of energy is expected to increase.

[Brief explanation of the drawing]

FIG. 1 is a process outline diagram sequentially showing the steps of the method of the present invention, and FIG. 2 is a partially enlarged view showing the sealing structure of the container lid. DESCRIPTION OF SYMBOLS 1... Container, 2... Transport means, 3... Getter supply means, 4... Lid, 5, 8, 12... Welding means, 6... Crimping means, 7... Magnetic stainless steel plate,
9...Press mold, 10...Storage container, 11...
Lid, A... Filled container, A'... Press compression treated body.

Claims (1)

[Claims] 1 Radioactive metal waste is filled into a container made of Cu or Al, a getter such as Ti or Zr is added and mixed, the container is capped, deaerated, and sealed, and then compressed. A method for volume reduction and stabilization of radioactive metal waste, which comprises reducing the volume, then inserting a plurality of compressed bodies having reduced volume into a storage container having a magnetic stainless steel lid, and welding the lid. . 2. The method for volume reduction and stabilization of radioactive metal waste according to claim 1, wherein a magnetic stainless steel plate is welded and attached to the top surface of the lid of a container made of Cu or Al. 3. The method for volume reduction and stabilization of radioactive metal waste according to claim 1 or 2, wherein the compression is performed using a split press mold.