JPH0219920B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an arrangement for containing waste materials for long-term storage. The invention is particularly applicable to the immobilization of high dose radioactive waste materials such as those produced in nuclear reactors. The safe storage of nuclear waste for very long periods of time is a major problem for the nuclear industry, and various proposals have been made to solve this problem. One proposal is to immobilize the waste in a suitable borosilicate glass;
This then involves embedding it into a suitable geological structure. However, regarding the safety of this technique, vitreous devitrification may occur;
As a result of this, the question of leaching of radioactive elements was discovered. Other recent proposals include the formation of synthetic rocks to immobilize nuclear reactor waste. For details on this method
AE Ringwood et al., Nature,
Described in the March 1979 issue. According to this disclosure, the selected synthetic rock is formed with radioactive elements in solid solution. The component minerals of the rock or related structural congeners are believed to withstand a wide range of geochemical environments over millions of years and are highly resistant to leaching by water. The nuclear reactor waste is introduced in the form of a dilute solid solution into the crystal lattice of the synthetic rock and should therefore be safely immobilized. A dense, consolidated, mechanically strong mass of synthetic rock containing nuclear waste is produced under pressure and heat by densification methods. This mass can then be safely disposed of in a suitable geological structure. The following patent application was filed at the Australian National University (Australian National University) based on research by A.E.
National University) filed: U.S. Patent Application No. 54957, entitled “Safe
Immobilization of High Level Nuclear
Reactor Wastes” and U.S. Patent Application No. 124953, entitled “A Process
for the Treatment of High Level Nuclear
The present invention relates in some embodiments to the use of the synthetic rock arrangement of AE Ringwood et al. and to a method of producing a disposable mass of material that can contain radioactive waste in immobilized form. However, the invention is not necessarily limited to the particular type of synthetic rock of AE Ringwood, and the apparatus and methods described herein are
It can also be applied to synthetic rocks other than those specifically described by AE Ringuud et al. Other examples of synthetic rock systems that may be used in the context of the present invention include: 1 Supercalcine [GJ McCarthy, Nuclear
Technology, Volume 32, January 1977]. 2 Products of zeolite solidification method (IAEA
Technical Report No. 176, p. 51). 3 Product of titanate solidification method (IAEA
Technical Report No. 176, p. 53). 4 Products of the Sandia process [RW Lynch and RG Dosch, US
Report SAND-75-0255 (1975)]. For purposes of this specification, synthetic rock is chemically defined as one or more metal oxides (or metals) formed into a rock-like structure by subjecting raw solid particles to heat and pressure. It is defined as a substance consisting of a compound derived from an oxide. According to a first aspect of the invention, (a) a quantity of feedstock is placed in a metal can, provided that during the method a means for preventing the metal can from deforming outwardly as a whole is provided with a groove; the metal can is sufficiently heat resistant and corrosive to contain the feedstock during and after carrying out the method;
and the feedstock comprises materials for forming a synthetic rock and a small proportion of nuclear reactor waste which, when densified into a mass, is immobilized in the synthetic rock; (b) a metal can shaft; applying pressure to compress the feedstock along and applying heat to induce densification and production of a synthetic rock mass comprising nuclear reactor waste; and (c) of the densification step. Either before or after
sealing the metal can with a metal cap, thereby making the sealed metal can suitable for removal and placement in a suitable long-term storage location (where nuclear reactor waste is A method is provided for directly forming a solid mass (including a synthetic rock to be immobilized) in a metal can for disposal. In one embodiment, the metal can is disposed within a cavity within the refractory support element that prevents it from deforming generally outwardly. In other embodiments, the metal can is made to collapse like a bellows under axial pressure, but the wall structure itself does not deform outwardly in its entirety. At least the preferred embodiments of the present invention provide a simple and effective method that can be easily performed in a "hot cell." This results in a product that is relatively safe and easy to handle. Radiation damage within the synthetic rock during very long storage is thought to cause only a small expansion, on the order of 2-3%, if any. In at least preferred embodiments, such long-term expansion can be adapted to the environment without increasing the risk of environmental contamination, for example by leaching in ground water. Another important factor from an economic point of view is that the method is relatively simple and can therefore be easily carried out in a thermal cell. Since contamination of the equipment is unavoidable during the process and decontamination and disposal of worn equipment must therefore be an expensive and inconvenient operation, equipment with a long service life is required. Further advantages may be achieved using various embodiments of the invention, including preferred or optional features described below. Preferably, the metal can has a sealed bottom end wall and only the final step of welding or otherwise permanently securing the metal cap to the top of the metal can is required in the heat cell. For the formation of at least some types of synthetic locks, the method includes the inclusion of suitable metals in suitable amounts in or in contact with the feedstock to provide a selected oxygen potential; This appears to be best done by facilitating the effective formation of synthetic rocks containing immobilized radioactive waste. Suitable metals considered to provide the desired oxygen potential are nickel,
Titanium and iron. The metal can be provided in the form of a lining for a metal can or as an inner can for the feedstock, or alternatively the metal can be provided in particulate form mixed with the feedstock. Most advantageously, the invention includes the additional step of first bringing the feedstock into a granular form that is easily injectable. This is a spillage in the heat cell.
and must minimize contamination. The granules are processed by spray drying/disc granulation in a cold compression process.
It can be produced by calcination or by a fluidized bed/calcination process. In a preferred and important embodiment of the invention, the feedstock is charged into a thin-walled metal can that remains solid at the initial sintering temperature used, typically on the order of 1200°C. The metal can may have a tight-fitting lid, and the feedstock can be poured into the can and cold-pressed before being capped. Preferably, the lid is tight-fitting to hold tight any components of the nuclear waste that are somewhat volatile at high sintering temperatures. This step is very important for the economics of operation since contamination of the heat cell by such volatile components can be minimized. Thin-walled cans may have a tight-fitting lid, rather like paint cans, and may be made of nickel or iron. Certainly such metal selection can provide a suitable oxygen potential. A useful material for the metal can is stainless steel, which is sufficiently corrosion resistant and has sufficient high temperature resistance to be easily used in the present method. One such steel is known as an austenitic steel. Typically heated to about 1260℃ and about 7MPA
pressure is a suitable sintering condition. This pressure can be increased to, for example, 14 MPa. However, in order to effectively sinter and densify the feedstock, there are practical limits on the maximum height of the feedstock column. Therefore, in a preferred embodiment of the invention, the method comprises a series of arcs arranged such that the refractory support element can selectively supply heat to each region extending along the axis of the metal can. including using a device that includes a vertical induction heating coil. As a result, the densification process occurs starting at one end of the metal can, and the induction coil is continuously applied after pre-densification and sintering of the feedstock. Most conveniently, it is equipped with water-cooled induction coils in partially overlapping relation. In this process, constant pressure is applied to the feedstock by a refractory-faced plunger inserted into the open end of a metal can, causing gradual densification. It is most economical to finalize the metal can, at least prior to the final step of sintering, to accommodate the densification that has occurred up to that step. Additional amounts of feedstock or additional small amounts of feedstock can be inserted before the final step. A tight refractory spacer is then inserted between the top of the feed and the refractory surfaced plunger to prevent it from entering the final heating zone. Pressure is most conveniently applied from a lower support, a hydraulic ram, and from a metal ram with a refractory surface in contact with the feedstock. Fireproof surface prevents overheating of the metal ram. Water cooling of the metal ram is also desirable. The invention is best practiced in a manner that carefully minimizes outward deformation of the metal can and yet provides a long service life to the device. In one advantageous embodiment, a refractory support element having slightly inclined holes into which the metal can fits closely spaced is injected and filled into the space between the metal can and the inclined holes. It is used in conjunction with refractory particles to provide a relatively dense cushioning agent that inhibits substantial deformation of the metal can during the densification process. The removal process consists only of driving the ram at the bottom to push up a metal can that can slide over the particles. The particles then fall through the cavities in the support element, are collected and recycled. The method includes shaking the refractory particles to impart good density and resistance to deformation of the metal can. According to a second aspect of the invention, there is provided an apparatus for use in a method as described above with respect to any of the embodiments according to the first aspect of the invention. The apparatus comprises a refractory support element containing a hole suitable for disposing a metal can containing a feedstock with a space between the metal can wall and the cavity wall; Means for introducing a refractory material into the space between the metal can and the wall of the cavity, filling it with particulate material and thereby substantially preventing outward deformation of the metal can under heating and pressure. Means for applying heat to the feedstock in the metal can in a sintering process; Means for applying densification pressure in the axial direction of the metal can; densification It comprises means for removing the metal can after the process and means for collecting and reusing particulate matter after removal of the metal can. Most preferably, the device includes a water-cooled induction heating coil. In commercially advantageous embodiments, the apparatus is suitable for handling relatively long metal cans up to about 3.6 m in length and up to about 375 mm in diameter. The apparatus in this embodiment includes a series of separate induction coils to densify and sinter the feedstock in sequentially separate stages starting at one end of the metal can, thereby densifying and sintering the feedstock in the metal can. Effective densification and sintering throughout the entire body should be ensured. Preferably, the heating coils overlap to produce a suitably densified continuous body of raw material within the metal can at the end of the process. According to a third aspect of the invention, there is provided a disposable element comprising a sealed metal can containing a densified synthetic rock body containing a small proportion of nuclear reactor waste in its crystal structure. provided. This element can be applied by the method of the first aspect of the invention or by the method of the second aspect of the invention.
Manufactured using equipment from the viewpoint of For purposes of illustration only, embodiments of the invention will now be described with reference to the accompanying drawings, in which: FIG. Referring first to FIG. 1, the apparatus includes a steel framework 1 supporting top and bottom hydraulic rams 2 and 3 and a feedstock 6 for densification and sintering in a metal can 5. an electric induction furnace arranged to receive a metal can 5 in which the
As shown in FIG. 1, the bottom ram 3 has a head suitable for supporting the bottom of a metal can and has a suitable refractory block 12 at the top of the ram, and the top ram has a head suitable for supporting the bottom of a metal can. In order to apply pressure in the axial direction of the metal can, it is suitable to provide a plunger with a refractory surface 7a extending into the metal can. The guide 4 consists of a block 8 of refractory material having sufficient tensile strength to withstand substantially the applied pressure and to absorb the forces which would tend the metal can to expand radially outward. The refractory block 8 has an angled central hole 9 for receiving a refractory particle fill to support a metal can.
Additionally, block 8 includes a series of internally water-cooled electrical induction coils 10. The particulate refractory material flows out of the hopper 20 and fills the space between the metal can 5 and the hole 9 when the valve 21 is opened. When the metal can is removed and the process is completed, the particulate refractory material descends into a collection box 22 from where it is pumped back through conduit 24 to hopper 20 by pump 23. Referring now to FIG. 2, it can be seen that the filled particulate refractory packing 11 is located in the inclined annular space between the outside of the circular cross-section metal can 5 and the hole 9 of the block 8. Recognize. The induction coil 10 also comprises a series of separate induction coil taps overlapping each other. Each terminal tap is labeled AA, BB, etc. FIG. 2 also shows that the bottom ram 3 can be moved upwards through the cavity 9 to remove the final product. A typical method of operation includes the following steps: (i) Retracting the upper ram and placing the metal can 5 with a bottom sealed end on top of the refractory block 12 in the space 9 and in the position shown in the drawings; Deploy. (ii) Nuclear waste material is mixed in small proportions with constituents that form synthetic rocks into granules that can be easily injected.
A quantity of the granulated feedstock is then injected into the metal can 5 to substantially fill it and the upper ram 2 is lowered. (iii) The refractory particulate material 11 is then injected into place and filled, for example by vibration, so that the metal can is sufficiently supported against radially outwardly expanding deformations. (iv) Activate the hydraulic rams 2 and 3 to apply pressure and charge the feedstock 6 in the metal can. Typically a pressure of about 7 MPa is applied. (v) Heating only the bottom region of the feedstock is effected by connecting the ends A--A of the induction coil 10 to a power supply. A typical power supply operates at 3KHz. Typically over a period of 45 minutes, the temperature of the feedstock in zone AA is heated to the sintering temperature of about 1260°C. The power source is then turned on while maintaining the pressure for about 3 hours. (vi) Next, turn off the power to the induction coil A-A section,
Connect the induction coil B-B section to the power source. It will be appreciated that the degree of overlap is such that a continuous densified solid phase is produced in the metal can. Each induction coil section is activated sequentially for about 3 hours until only a small amount of feedstock is present between the densified zone and the ram surface 7A. Then, the ram 2 is withdrawn, the feedstock is injected to the top of the metal can, and the above steps are continued until just before the induction coil GG section is activated. Prior to this, a refractory surface 7a is inserted into the space to isolate the ram from the heated material. (vii) After the densification of the upper portion of the feedstock is completed, maintain the pressure and allow the element to cool to 300°C. The pressure is then released and the irregularity with the upper refractory surface is pulled out. (viii) actuating the bottom ram 3 to eject the metal can 5 from the induction furnace and simultaneously lowering the refractory particulate material 11 to be collected in the circulation device; (ix) Remove the excess upper wall portion of the metal can 5, and weld a metal cap that tightly adheres to the metal can. This metal can can then be disposed of in a suitable geological structure. Referring now to FIG. Figure 3 illustrates a preferred embodiment of a metal can, but is not to scale. In a preferred embodiment, the metal can 5 has an entire bottom wall 6 and typically has a wall thickness of 6 to 8 mm and a diameter of 100 mm or more. Third
The figure illustrates the final unit after welding the lid 13 in place. For convenience, the metal is austenitic steel. In this embodiment the feedstock is at the bottom 15 of the entire
and a thin-walled can 14 with a lid 16 suitable for pressurization and introduced into a metal can. This can is similar to a conventional paint tin can and is preferably made of metal which provides a suitable oxygen potential to facilitate incorporation of the waste into the synthetic rock. That is, the can may be made of nickel or iron. To manufacture the units of FIG. 3, the feedstock is first cold-pressed or otherwise granulated;
It is desirable to pour this into the can. Then, the lid is crimped. Subsequently, when the arrangement is as shown in FIG. 2, the can is inserted into the metal can 5 prior to the densification process. During the densification process, for convenience, a can whose height corresponds to each induction coil portion AA, BB, etc.
It is compressed with the feedstock contained therein, thereby assisting in retaining any volatile components in the feedstock. Furthermore, contamination of the apparatus of FIG. 2 can be minimized by this thin can technique. It has been found that this can is compressed inside the metal can 5 and is tightly fixed therein, without any noticeable distortion in its wall sections. FIG. 3 illustrates the final product having a block of synthetic rock 18 within a thin-walled metal can 14. Fireproof spacer 1
9a remains inside the metal can and fills the space. The second specific example shown in FIG. 5 is characterized by the use of a metal can 20 made of stainless steel and having a bellows-like structure that prevents the entire metal can from deforming outward during the pressurizing process. . FIG. 5 schematically shows the entire process and the equipment that can be used. Outside the heat cell, the “SYNROC”
A non-radioactive synthetic rock precursor is produced as indicated by the process labeled "Precursor". The synthetic rock has a composition as shown in the table below, containing tetraisopropyl titanate and tetrabutyl zirconate. It is prepared by using it as the ultimate TiO ₂ and ZrO ₂ source. This component is mixed with a nitrate solution of the other components, co-precipitated by addition of sodium hydroxide, and then washed.

Table: The precursor feedstock is a very high surface area product and acts as an effective ion exchange medium. Additives and high-dose nuclear waste (HLW)
in the form of a nitrate solution and a thick homogeneous slurry is produced in a mixing stage located in a thermal cell. Typically up to about 20% of the slurry may contain high dose waste. The slurry is then fed through conduit 22 to a rotary kiln 23 operating at about 850 DEG C. where it is heated, evaporated and calcined.
The obtained fired product is mixed in a mixer 24 with 2% by weight of metallic titanium powder supplied from a hopper 25. The mixer 24 then feeds the powder into a stainless steel and bellows shaped main metal can as shown. From the drawing, the metal can has a coefficient of about 3.
It will be appreciated that it can only be compressed and not deform outwardly overall. As shown in the drawings, before the mixer feeds the powder into the metal can, a thin perforated metal liner 26 is placed inside the metal can and the space between the liner and the metal can is filled with zinc oxide. Fill with powder 27. Alternatively, other powders with low thermal conductivity may be used instead. The metal can is then filled with powder 28 from mixer 24. A stainless steel plug or lid 29 is then used to seal the metal can, and the metal can is
A piston 30 made of an alloy based on molybdenum and capable of operating at temperatures up to 1200° C. is placed between the pistons 30. A radio frequency induction coil 31 is then used to raise the temperature of the end of the piston 30 and the metal can and its contents to approximately 1150°C. After sufficient time has elapsed for the interior of the synthetic rock powder to reach a uniform temperature, a compressive force is applied through the piston 30, causing the walls of the metal can to collapse axially, similar to a bellows. The resulting sealed compacted container containing the synthetic rock structure is then removed and placed in a disposable cylinder 31a made of a highly corrosion resistant alloy such as based on Ni ₃ Fe.
stack inside. The space between the main metal can 20 and the inner wall of the cylinder 31a is filled with molten lead 32,
Seal the cylinder for eventual disposal. The examples shown in FIGS. 6 and 7 are variations on the example shown in FIG. In this case, the steps up to the mixer 24 in FIG. 5 are the same. However, in this specific example, the outside of the bellows-like metal can 41 and the cylinder 40 are
The space inside the metal can is substantially retained until after the compression process, thus satisfying the need to handle the metal can for insertion into the cylinder after compression and the space around the metal can in the embodiment of FIG. The outer cylinder 40 and the bellows-like metal can 41 are dimensioned so that lead injection for cleaning is omitted. As shown in FIG. 6, the cylinder 40 is supported by a base 43, and a metal can 41 is inserted into the cylinder 40.
A cylinder 41a with an open end is arranged inside. The mixture from the mixer 24 is then poured into the metal can to fill the area within the cylinder 41a and the top lid 44 is placed in place. The whole object is then placed in a radio frequency induction coil 45 surrounding the outer cylinder.
After sufficient time has elapsed to achieve a uniform temperature, the metal can 41 is compressed using a ram 46 having a piston presser 47. As shown in FIG. 7, the metal can expands slightly outward and collapses, forming a cylinder 40.
The walls are arranged so as not to significantly restrict outward expansion of the bellows-like metal can 41.
While the cylinder 41a actually bends somewhat during the collapse of the can, it substantially prevents the synthetic rock material from entering the bellows area, thereby preventing insufficient compaction within the bellows area. The risk of improperly formed composite rock between the corrugations of the bellows is avoided. In reality, adjacent corrugations of the bellows will come together during the compression stage. FIG. 7 also shows that the induction coil 45 can be easily moved upwards to the next position so that the next metal can that is to be inserted into the top of the metal can 41 can be processed. ing.

[Brief explanation of drawings]

FIG. 1 is a schematic elevational view of equipment arranged for carrying out an embodiment of the invention; FIG.
3 is an enlarged axial elevational view of the central portion of the illustrated apparatus; FIG. 3 is an enlarged schematic illustration of a disposable element manufactured by using an embodiment of the invention; FIG. Figure 4 is a graph showing a typical applied pressure and temperature cycle; Figure 5 is a graph showing a typical applied pressure and temperature cycle;
FIG. 6 is a schematic diagram representing a third embodiment prior to compression; and FIG. 7 is a view of the metal can of FIG. 6 after compression.

Claims (1)

Claims: 1. A method of forming a solid mass comprising a synthetic rock in which nuclear reactor waste is immobilized, comprising: (a) placing an amount of feedstock in a metal can; provided that the metal can is generally cylindrical and includes a bellows-like wall structure, and the metal can has sufficient heat and corrosion resistance to contain the feedstock during and after the process; and (b) the feedstock comprises materials for forming the synthetic rock and a small proportion of nuclear reactor waste which, when densified and agglomerated, is immobilized in the synthetic rock; applying pressure along the axis of the canner to compress and applying heat to induce densification and production of a synthetic rock mass containing nuclear reactor waste; and (c) converting the metal canner into a feedstock. At some stage after filling, the metal can is sealed with a metal cap, thereby making the sealed metal can suitable for storage for a reasonably long period of time. A method of forming a solid mass. 2. Positioning the tubular screen inside the metal can, leaving a space between the screen and the inner wall of the metal can,
2. The method of claim 1, further comprising injecting an insulating powder into said space and then injecting said feedstock into an area within said tubular screen to fill a metal can. 3. positioning the metal can within an outer cylinder and applying the pressure and heat when the metal can is filled with feedstock and sealed with a lid such that the dimensions of the outer cylinder and metal can are 2. The method of claim 1, wherein the metal cans are provided with a loose, tight fit rather than mutual interference when compressed within the cylinder of the metal can. 4. The metal is placed in contact with the feedstock prior to the application of heat and pressure, so that the metal provides an oxygen potential that aids incorporation of nuclear reactor waste into the synthesis lock. the method of. 5. A method according to claim 1, comprising as a preliminary step forming the feedstock into granules. 6 Filling the feedstock into a thin-walled metal can that is solid at the densification temperature but deforms as it forms a composite rock during densification of the feedstock, and then converting this metal can into a metal can. 6. The method according to claim 4 or 5, wherein the method is loaded into the container. 7. The method according to claim 1, wherein the metal can has a long cylindrical shape, and the densification is performed using an electric induction heating coil in a series of areas starting from one end of the metal can. . 8. The method of claim 7, wherein the heating zones overlap each other. 9. The electric induction heating coil is placed within a refractory support element having an angled aperture for receiving a metal can, and the particulate refractory material is placed within the angled annular space between the metal can and the angled aperture. 8. A method as claimed in claim 7, in which a densification step is carried out. 10. The method of claim 9 in which vibration is used to fill the particulate refractory material and the particulate material is collected and circulated after removal of the metal can following the densification step. 11. The method according to claim 1, wherein the metal can used in the method is made of heat-resistant austenitic steel. 12 High density at a temperature around 1260℃ and 7MPA
2. The method of claim 1, wherein the method is carried out for about 3 hours at a pressure of about 100 ml.