JPS6117320B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION In the currently used reprocessing of radioactive waste from nuclear reactors, high level waste is obtained in a strong nitric acid solution. Strontium-90 was the dominant radioactive substance in the waste for the first several hundred years.
and Zesium-137. In addition, waste is further
It contains small amounts of uranium, plutonium, and transuranic elements, which have significantly longer half-lives than strontium-90 and cesium-137. Those skilled in the art generally believe that it is advantageous to solidify liquid high level waste after cooling for a suitable period of time. Solid products with good chemical resistance are targeted. It must be stable against leaching of the radioactive material contained therein by water. It must withstand the heating caused by the fission products as well as the stresses of handling and transportation. Among the materials that have been proposed for confinement are glasses such as borosilicate and phosphate glasses, quartz, titanium dioxide, certain zeolites, and other naturally occurring minerals particularly capable of retaining gas.

In the known method of enclosing high-level waste in glass, the waste is evaporated and burnt and additives are added to it at temperatures of 1000-1200°C.
When heated to , it produces a glass melt. The melt is placed in a leak-proof steel container, which is transported to a storage plant where it is cooled and monitored. High-level waste is burned at 300-500℃
The process is carried out over a temperature range of about 100 to 100 m, resulting in waste products that are converted to oxides.

In order to increase the corrosion resistance of the container after the final disposal of the high-level waste into the rock, it is specifically proposed to coat the container with a thick layer of gold, such as a layer several mm thick. There is. Electrolytic application of gold is a prerequisite. Silver and copper are also mentioned as materials for the covering layer. It is clear that practical problems exist in electrolytically applying thick anti-corrosion coatings onto objects of the size used in such applications. Furthermore, coatings of the intended thickness of said types of metals can be mechanically damaged during shipping and other handling.

If a layer of anti-corrosive material, such as a suitable ceramic material, is applied around the radioactive object and pressed and sintered by conventional means at high temperatures, there is a risk that gaseous components will separate from the radioactive object and diffuse into the surrounding environment. There is. Furthermore, it is not possible to obtain a finished layer of the required thickness with uniform high density and resistance to leaching by water.

According to the invention, a corrosion protection layer can be applied under complete control of the radioactive waste, thus eliminating the risk of gaseous components dispersing from the radioactive object into the environment. At the same time, the production of gaseous components is prevented.
Furthermore, according to the present invention, anti-corrosion layers of any desired thickness with uniform high density and resistance to water leaching can be produced under practical and easy-to-implement conditions. Furthermore, according to the present invention, a layer through which radiation is difficult to pass can be applied around a radioactive object.

The present invention is a method of enclosing an object containing radioactive waste from nuclear fuel with a casing made of a corrosion-resistant and/or radiation-resistant material, the method comprising: The method is characterized in that the object encapsulated with the material is subjected to an isostatic press at a pressure and temperature necessary to form a close-fitting, dense casing of the material. Preferably, the capsule is evacuated prior to sealing.

The object containing radioactive waste consists of the aforementioned solidified glass melt containing, inter alia, high-level waste. Furthermore, the object may consist of a sintered body of high-level waste, possibly fixed in an ion exchanger and mixed with a material that is resistant to leaching by water. It should be noted that the material resistant to leaching by water is one or more oxides of the type commonly found in various types of glasses and rocks, such as SiO ₂ , B ₂ O ₃ , Al ₂ O ₃ , MgO, alkali metal oxides, alkaline earth oxides,
Naturally occurring rocks consisting of TiO ₂ , ZrO ₂ , Fe ₂ O ₃ , or silicates, aluminates, chromates, or titanates. Also, the object is placed in a space for which radioactive waste has been blown.
The container may be made of tightly pressed ceramic material. This object does not necessarily have to consist of a single, cohesive unit. It may consist of several separate small bodies arranged adjacent to each other as well as a granular mass.

According to an embodiment of the invention, the object enclosing radioactive waste, which is embedded in a mass of particles of corrosion-resistant material and encapsulated with said mass, comprises: An isostatic press is used at the pressure and temperature necessary to form a dense casing with tightly packed particles.
Pressing) In this book, balanced pressure press refers to a technology that applies gas or hydraulic pressure evenly from all directions to the object to be pressed to form pressed bodies of various shapes, and so-called isostatic press is also included in this technology. It can be done.

Suitable corrosion-resistant materials in the casing according to this embodiment include metallic materials such as tantalum, titanium, zirconium, tungsten, molybdenum and mixtures of at least two of these metals, and in particular copper and 70% by weight copper. and 30% by weight of nickel, as well as oxides of the type normally found in various glasses and rocks, such as TiO ₂ , Al ₂ O ₃ , SiO ₂ , B ₂ O ₃ , MgO , _ZrO2 and
Ceramic materials such as Cr ₂ O ₃ may also be mentioned, as well as silicates, aluminates, chromates and titanates. As will become clear from what follows, it is appropriate, especially when using several layers of corrosion-resistant material arranged one on top of one another, that one layer is made of noble metal. Preferred materials include tantalum, titanium and zirconium. The grain size of the corrosion-resistant material is preferably 1 mm or less. The thickness of the casing of corrosion-resistant material in the sintered state is suitably at least 2 mm, but it can be considerably thicker, for example a few centimeters or a few decimeters.

According to another embodiment of the invention, the object confining the radioactive waste is surrounded by an element of corrosion-resistant and/or radiation-resistant metallic material, which element is at least partially preferably is of a considerably larger extent than the particles of the powder material, and the body is then subjected to an isostatic press at the pressure and temperature necessary to form a cohesive, dense casing of said element. By using elements in this way, which have at least a substantial area considerably larger than the particles of the powder material, shrinkage or shape changes during pressing are considerably reduced, which makes the pressing equipment correspondingly smaller. It means to get. Another advantage is that this element is essentially bubble-free, facilitating evacuation of the capsule prior to sealing.

The element consists, at least in its main part, of a shaped element, such as a disk, ring, sphere, cylinder or rod. To facilitate compaction, the elements are preferably arranged in a geometrically well-aligned arrangement with intermediate spaces around the object. However, it is also possible to use irregularly shaped elements, for example blocks. Particularly in the latter case, amorphous elements are combined with elements in the form of powder particles in order to achieve the required filling. At least in major parts, the elements consist of sheet metal elements or cast, forged or sintered bodies.

Suitable materials for the casing in the latter embodiment, especially for the purpose of good corrosion resistance against the environment during long-term storage, are tantalum,
Titanium, zirconium, tungsten, molybdenum and mixtures of at least two of these metals, and especially copper, stainless steel, 70% by weight copper
Many metal materials are mentioned, such as an alloy containing 30% by weight nickel, and an alloy containing 94% by weight copper and 6% aluminum. The materials exemplified above will of course also provide radiation protection, but their efficiency will depend on the properties of the material and the thickness of the casing.

Suitable materials for the casing in the latter embodiment, especially for the purpose of good radiation protection, include steel, such as rolled or cast steel with a carbon content of 0.1 to 0.5%, and copper. These materials, of course, contribute to the protection of the enclosure against corrosion from the environment.

The thickness of the finished casing in this case is preferably at least 10 cm if this casing is to provide sufficient protection against radiation transmission.

In some cases, multiple layers of materials each having different corrosion resistance to different environments - for example layers each having a different electrolytic potential, or a layer that provides particularly good protection against alkaline water. 〓〓〓〓〓
In different embodiments of the invention, it is suitable to produce a multi-layer casing with layers above or below which provide particularly good protection against acidic water.

Particularly when using multiple layers of corrosion-resistant materials in the casing, it is appropriate to make one layer of noble metals such as gold, platinum or platinum iridium. The noble metal can suitably be applied electrolytically if it is applied to the outermost layer. If the precious metal is to be applied between two layers of other corrosion-resistant material which are simultaneously applied onto the object by means of an isostatic press, a thin foil of the precious metal is placed between said two layers prior to pressing. For example, gold foil can be placed between the inner and outer titanium layers.

The use of multiple layers of different materials is particularly appropriate when the casing is made to a large extent of materials that provide effective radiation protection but insufficient corrosion protection. In this case, it is appropriate to provide one or more layers of material in the casing that have good corrosion resistance rather than radiation protection. Corrosion-resistant materials suitable for such layers include:
The materials mentioned above are suitable for producing casings which require particularly good corrosion resistance. The corrosion-resistant material can be applied in the form of a powder compact. This can be applied at the same time as the radioprotective material is applied by an isostatic press, or in a separate pressing operation after the radioprotective material has been applied. In this case, the thickness of the layer of corrosion-resistant material in the sintered state is suitably at least 1 mm. It can also be applied electrolytically, for example.

The capsule containing the object with radioactive waste and casing-forming material is of course made of a material that yields under pressing conditions. The capsule consists, inter alia, of a plate of steel, tantalum, titanium, zirconium or an alloy of these metals, such as zircaloy. In some cases it is also possible to use quartz capsules.

The pressure at which the casing surrounding the object containing radioactive waste is isostatically pressed is suitably at least 10 MPa, preferably at least 50 MPa. A range of 50 MPa to 300 MPa is particularly suitable. The temperature will, of course, depend on what material is used to form the casing. However, the temperature is always at least 500°C. A suitable temperature for applying a steel radiation protection casing without simultaneous application of a corrosion protection layer is 1000°C. A suitable temperature when using tantalum is about 1400℃, when using titanium it is about 950℃, when using zirconium it is about 1000℃, and when using tungsten it is about 1500℃. If molybdenum is used, the temperature is approximately 1350℃.

The present invention will be described in more detail by way of example with reference to the accompanying drawings.

Cylinder 1 in FIG. 1 consists of a solidified melt of borosilicate glass containing high-level waste from a plant that reprocesses fuel used in nuclear reactors. The cylinder is surrounded by a leak-proof acid-resistant steel container 2. The radioactive waste was contained using the well-known glass entrapment method described in the introduction to this specification. The cylinder has a diameter of 30 cm and a length of 2 m.

Container 2 that houses cylinder 1 is then 2mm
It is placed in a capsule in the form of a cylindrical container 3 made of thick tantalum plate. The container 3 contains at its bottom a layer 4 of tantalum powder with a particle size of less than 200 microns. container 2
The space 5 between the envelope surfaces of and 3 is also filled with the same tantalum powder. A layer 6 of tantalum powder is also placed on top of the container 2. The thickness of the layer of tantalum powder is 10 mm. Once the container 3 has been filled as described above, a tartar plate lid 7 is welded to the container. The lid is provided with a tube 8, which is connected to a vacuum pump for evacuation of the capsule. After evacuation, the capsule is sealed by heating and melting the tube just above the top of the lid.

In FIG. 4, 32 indicates a movable press stand. This stand is supported by wheels 33 running on rails 34 on the floor. The press stand consists of an upper yoke 36 and a lower yoke 3 held together by a prestressed strip sheath 39.
7 and a pair of spacers 38. The press stand can be moved between the position shown in FIG. 4 and a position in which it surrounds the hyperbaric chamber 42. The high pressure chamber 42 is supported by a column 49, and has an inner pipe 50,
Prestressed struts surrounding the inner tube
It houses a high-pressure cylinder consisting of a strip sheath 51 and an end ring 52 that holds the strip sheath axially and constitutes a suspension system that attaches the high-pressure chamber 42 to the column 49. The high pressure chamber 42 has a lower end cap 53 that projects into the inner tube 50 of the high pressure cylinder. In the lower end lid there is a slot in which a sealing ring 54 is arranged and a passage 55 for supplying a pressurized medium, suitably argon or helium.
and a cable passageway 56 for supplying power to a heating element 57 for heating the furnace. The heating element 57 is supported by a cylinder 58 that rests on an insulating bottom 59 that projects into an insulating sheath 60. The upper end lid includes an annular portion 61 with a sealing ring 62 sealing against the inner tube 50 . Sheath 60 is suspended from and hermetically connected to annular portion 61. The upper end lid is
It also includes a lid 63 for closing the opening in the annular portion 61 which is permanently attached to the high pressure cylinder. This lid includes a sealing ring 64 that seals against the inner surface of the annular portion 61 and a heat insulating shell that protrudes into the cylinder 60 when the high pressure chamber is closed and forms part of an insulating shell that surrounds the true furnace space 66. A lid 65 is provided. The lid 63 is
It is attached to a bracket 67 carried by an operating rod 68 that can move up and down and rotate. Yokes 36 and 37 absorb compressive forces acting on end caps 63 and 53 when pressure is applied to furnace space 66.

The capsule 3 is placed in the furnace space 66 and the first raised lid 63 is lowered to close the furnace space so that the pressure and temperature are approximately 100 MPa and approximately 100 MPa, respectively.
When raised continuously to 1400°C and maintained at this value for about 2 hours, a layer of tantalum powder 4,5
and 6 the desired density and sintering is achieved.
The capsule with the encapsulated material is cooled, then the pressure is reduced to atmospheric pressure and the capsule is removed from the furnace. Typically, capsules may remain as pressed products when stored for long periods of time.

According to another embodiment, the container 2 with the glass cylinder 1 can be placed in a titanium container, in which case said container has two layers of titanium powder placed one outside the other and It is embedded in a layer of gold in the form of a film placed between these layers.

Figure 2 shows object 1 that confines radioactive waste.
Another example of 1 is shown below. This can be manufactured in the manner shown schematically in FIG. A prestressed cylinder 12 of corrosion-resistant material (for example aluminum oxide) is provided with holes 13 and has polished end faces. This cylinder has a base plate 1 which is prestressed and has a polished surface.
It is provided with a lid 15 which rests on 4 and has a similarly polished surface to close the hole. The bottom plate and the lid are made of the same material as the cylinder, and the pressure is 100 MPa in the apparatus shown in Fig. 4, for example.
and by isostatic pressing at 1350° C. for 1 hour. Spent fuel rods 16 removed from the reactor are placed in holes 13. The cylinder with a lid and a bottom is housed in a container 17 with a welded lid 18 made of sheet steel approximately 3 mm thick. The lid has a pipe 19 connected to a vacuum pump for evacuating the container.
is provided. The space between the inner wall of the container and the cylinder, and between the lid and bottom of the container, is filled with a powder 20 of the same corrosion-resistant material used for the cylinder, bottom plate and lid. After evacuating the container, it is sealed by melting the tube 19. The container, together with its contents, is then placed in the apparatus shown in Figure 4.
It is subjected to an isostatic press at 100MPa and 1350℃. This results in a bond between the end face of the cylinder 12, the base plate 14 and the lid 15, and a densification of the powder 20, which is transferred to the prestressed object 1.
2, 14 and 15 together form a single monolithic object 11 (FIG. 3). hole 1 in object 11
3 has a dimension larger than the outer diameter of the fuel rod 16 even after isostatic pressing. In this way, stress problems caused by different coefficients of thermal expansion of the various materials contained can be avoided.

As can be seen in FIG. 3, the object 11, preferably after removing the container 17, is placed in a capsule 21 consisting of a cylindrical container 22 made of sheet steel with a thickness of 5 mm. The container 22 contains a plurality of layers 23 or steel discs. A large number of turns of steel plates 24 are wound in the space between the inner covering surface of the container 22 and the outer covering surface of the object 11. The container 22 includes a number of steel plate disk layers 25 on the object 11.
Ru. The steel of the parts 23, 24 and 25 constituting the element made of a material that is difficult to transmit radiation has a low carbon content and the thickness of the steel plate is 10 mm. Element 2
3, 24 and 25 and inside the lid 26, the container 22 is made of a material that has better corrosion resistance to the environment than steel during long-term storage, for example stainless steel in the form of a powder with a grain size of up to 0.7 mm. It includes a layer 27 with a thickness of 10 mm. A lid 26 made of sheet steel is attached to the top of the container 22. The lid is provided with a tube 28 connected to a vacuum pump for evacuating the capsule. After evacuation, the capsule is sealed by melting the tube 28.

The capsule 21 is placed in the furnace space 66 in the high-pressure furnace according to FIG. The desired density and sintering of layers 23, 24, 25 and 27 is achieved when the temperature is increased continuously to and held at this value for about 2 hours. The capsule with encapsulated material is then cooled, after which the pressure is reduced to atmospheric pressure and the capsule is removed from the furnace. layer 2
3, 24 and 25 together form a casing around the object 11, which casing provides effective protection against radiation. This casing has a thickness of approximately 25 cm in this example. or,
This casing also has a layer 27 which becomes firmly fixed to the other layers and which provides good protection against corrosion if the pressed object is stored for a long period of time.

If the application of the radiation protective casing is carried out in the same press operation in which the object containing the radioactive waste is sealed, a suitable temperature is 1350°C.

The layer 24 may be replaced, for example, by rings made of sheet steel, which have holes slightly larger than the diameter of the object 11 and are stacked one on top of the other. Also, layer 24
may be replaced by elongate sector-shaped rods arranged in particular parallel to the axis of the object 11 and with an intermediate gap facilitating effective fixation to said object during pressing. . Layer 24 may also be replaced with a cast cylinder made of radiation protection material. Similarly, the mass of layers 23 and 25 may be replaced by one or several cast relatively thick discs. The layers 23, 24 and 25 may also be replaced by geometrically regularly shaped objects (eg cylinders or spheres) packed around the object 11. Particularly when using layers in the form of irregularly shaped masses, it is suitable to use them simultaneously with powdered filler elements, as mentioned above.

If the steel in the example described above is, for example, 70% Cu
If replaced by a 30% Ni copper alloy or a 94% Cu and 6% Al copper alloy, a casing is obtained that is specifically aimed at good corrosion resistance. In this case, it is not necessary to apply a layer corresponding to layer 27.

As is clear from the subject matter described above, the object 11 confining radioactive waste in the case exemplified above is, inter alia, an object consisting of a glass melt containing radioactive waste or an incineration of radioactive waste. It may be replaced by an object made of solid and preferably corrosion-resistant material.

The method according to the invention is not only useful for the confinement of fuel used in nuclear reactors as illustrated. It can also be used for the containment of high-level waste for fuel reprocessing in the production of plutonium for atomic weapons and for the containment of other radioactive waste.

[Brief explanation of the drawing]

Figure 1 schematically shows a capsule enclosing a radioactive waste object and a mass of corrosion-resistant material;
Figure 3 schematically depicts an object confining radioactive waste during manufacturing; Figure 3 schematically depicts said object in its finished state, placed in a capsule and surrounded by material forming a radioprotective casing; , FIG. 4 shows a high-pressure furnace in which the corrosion-resistant material according to FIG. 1 and the radiation-protective material according to FIG. 3 are respectively compressed and sintered to form a casing over a radioactive waste object. 1...Cylinder, 2...Container, 3...Capsule, 11...Object, 12...Cylinder, 13...
... hole, 16 ... spent fuel rod, 21 ... capsule, 22 ... container, 32 ... press stand, 42 ... high pressure chamber, 66 ... reactor space.

Claims (1)

[Scope of Claims] 1. A method of enclosing an object containing radioactive waste from nuclear fuel with a casing made of a material that is corrosion-resistant and/or difficult for radiation to pass through, the method comprising: said material being corrosion-resistant and/or difficult for radiation to pass through; said object surrounded by and further encapsulated with said material is subjected to an isostatic press at a pressure of at least 10 MPa and a temperature of at least 500° C. to form a cohesive dense casing of said material; The method characterized in above. 2. In the method described in claim 1,
The object, which is embedded in a compact containing particles of corrosion-resistant material and is further encapsulated with the compact, is at least 10 MPa in order to form a close-fitting dense casing of the particles of corrosion-resistant material. A method characterized in that it is subjected to an isostatic press at a pressure of at least 500°C and a temperature of at least 500°C. 3. In the method described in claim 1,
A method characterized in that the material forming the casing includes elements of metallic material having a significantly larger area than the particles of powder material. 4. In the method described in claim 3,
A method characterized in that the element is a shaped element, at least in its main part, such as a disk, ring, sphere, cylinder or rod. 5. A method according to claim 3 or 4, characterized in that the element comprises, at least in its main part, a sheet element, a cast body, a forged body or a sintered body. 6. The method of any one of claims 1 to 5, wherein the body containing radioactive waste comprises a dense body of corrosion-resistant material sintered under pressure; A method characterized in that radioactive waste is placed inside the object. 7. A method according to any one of claims 1 to 6, characterized in that the material forming the casing consists of tantalum, titanium or zirconium. 8. A method according to any one of claims 1 to 6, characterized in that the material forming the casing consists of tungsten or molybdenum. 9. A method according to any one of claims 3 to 6, characterized in that the material forming the casing consists of steel. 10. A method according to any one of claims 1 to 9, characterized in that the capsule is made of tantalum, titanium, zirconium or steel. 〓〓〓〓〓
11. A method according to any one of claims 1 to 10, wherein the object is coated with a plurality of layers of different materials forming the casing, one of which is disposed outside the other. A method characterized by being present.