Not Applicable
1. Field of Invention
The present invention relates to the treatment and disposal of radioactive waste and more particularly to systems and processes for pyrolyzing and vitrifying radioactive waste materials in order to reduce the volume of waste material and to prevent leaching or leaking of radioactivity into the environment.
2. Description of the Related Art
The stabilization and disposition of radioactive waste is a complex field that includes a number of techniques and methods. In some processes, radioactive isotopes that are the by-products of nuclear reactions are combined with various admixture materials designed to isolate and capture specific radioactive isotopes or to render the immediate nuclear by-products safer and easier to manipulate. The various admixture materials, collectively referred to herein as “media,” include a number of inorganic and organic substances, including some organic resins. The mixture comprising media and radioactive isotopes is generally referred to herein as “radioactive waste,” “waste material,” or simply “waste.”
The disposal of radioactive waste material is an expensive process that is highly dependent upon the volume of waste material being disposed. Therefore, it is highly desirable to find methods and systems for compacting waste material, thereby reducing the volume of waste material to be disposed or stored.
Other stabilization technologies can offer some volume reduction to varying degrees depending on the additives and volumes required. While volume reduction of inorganic sludges is limited by the nature of the material (i.e. totally inorganic and not able to undergo pyrolysis), organic sludges or organic resins can undergo much higher volume reductions when totally pyrolyzed.
Disclosed herein are systems and processes for reducing the volume of radioactive waste materials through pyrolysis and vitrification carried out by microwave heating and, in some instances, a combination of microwave heating and inductive heating. In some embodiments, the microwave-enhanced vitrification system comprises a microwave system for treating waste material combined with a modular vitrification system that uses inductive heating to vitrify waste material; in other embodiments, the microwave system is combined with a vitrification system that uses some other process to achieve vitrification. The final product of the microwave-enhanced vitrification system is a denser, compacted radioactive waste product.
The present invention, in some of its embodiments, provides a microwave system for treating radioactive waste material. In some embodiments, the microwave system comprises a microwave waveguide positioned to direct microwaves at radioactive waste in a waste container. The microwaves excite the waste material through coupled heating in order to pyrolyze and vitrify the waste material into a more compact form. In particular, where waste coming into the microwave system (“incoming waste material”) comprises media combined with radioactive isotopes in a non-dense mixture, the microwave system acts to reduce the volume of waste material by heating the incoming waste material with microwaves, pyrolyzing the waste material, destroying the crystalline structure of the incoming waste material, producing a molten mixture of the waste material components, allowing gases within the incoming waste material to escape the molten mixture, and allowing the molten mixture to cool into a dense, vitrified composition (the “final waste product”).
One embodiment of the microwave-enhanced vitrification system includes a microwave source, a waveguide, and a canister. The microwave source generates microwaves suitable for pyrolyzing and liquefying solid radioactive waste material for the purpose of stabilizing the waste material for safe storage and disposal in accordance with knowledge common to one skilled in the art. The waveguide directs the microwaves generated by the microwave source toward the waste material within the canister. The canister is suitable for long term storage of treated radioactive waste material. In some embodiments, the canister is constructed of a suitable material for external decontamination and durability, such as stainless steel. The canister receives the unvitrified solid incoming waste. Initially, the canister receives a first layer of unvitrified incoming waste material. Each layer of incoming waste has a depth that is completely penetrable by the microwaves. The waveguide is positioned with respect to the first layer of solid waste feed such that the microwaves generated by the microwave source are directed toward and applied to the first layer. In some embodiments, the microwave-enhanced vitrification system supplements the first layer of solid waste feed with a “starter material,” such as silicon carbide, iron filings, iron powder, or similar substance, which facilitates coupling until the melt is self-sustaining.
After the first layer of solid waste feed is treated as discussed above, a second layer of incoming waste material is added to the canister such that the second layer is deposited on top of the first layer. The second layer is then treated in the same manner as the first layer. Each additional layer of solid waste feed is received by the canister and treated by the microwaves in accordance with the above discussion, which can be continuous or semi-continuous in nature. The pyrolyzed waste in the lower portions of the canister cools as additional waste material is received and treated. When the waste cools, it forms a stable vitrified final waste product. The number of layers of solid waste feed received and treated by the system is limited by the size of the canister. When the solid waste feed deposited within the canister has been treated, the canister is sealed and stored or disposed of in accordance with appropriate regulations.
In some embodiments, the microwave system for vitrifying waste is combined with an inductive heating system that assists in heating the incoming waste material, pyrolyzing the waste material, and maintaining a molten layer of material that allows for the escape of gas from the molten mixture and the compaction of the waste before cooling into the final waste product. Generally, inductive heating is provided by heating coils surrounding the waste container near the zone within the container containing the molten layer of waste. In other embodiments, the microwave system is combined with a vitrification system that uses some other process other than inductive heating to achieve a vitrified final waste product.
In some embodiments, the waste container within which the microwaves pyrolyze the incoming waste material is a microwave chamber adapted to be emptied of vitrified final waste product after use and thereafter reused for treating more incoming waste material with microwaves. In other embodiments, the waste container is a one-use canister adapted to serve as the final storage vessel for the vitrified final waste. The canister is adapted to serve as a microwave vessel within which the incoming waste material is pyrolyzed through microwave treatment. In some such embodiments, the canister further includes materials selected to assist in the inductive heating of the waste material by heating coils surrounding the canister.
The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which:
a is a view of a section view of one embodiment of a modular vitrification system, showing the initial filling of the canister with radioactive waste material and pyrolysis and liquification of the first layer of waste material;
b is a section view of the same canister as shown in
c is a section view of the same canister as shown in
a is a view of one embodiment of a microwave-enhanced vitrification system that combines the microwave system and the modular vitrification system, with waste canisters being moved into position along a conveyor, showing a first step in the process of positioning a canister to receive waste material and treating the waste material to achieve a vitrified final waste product;
b is a view of a subsequent step in the process of using the embodiment shown in
c is a view of a subsequent step in the process of using the embodiment shown in
d is a view of a subsequent step in the process of using the embodiment shown in
Disclosed herein are a microwave-enhanced vitrification system and processes for treating radioactive waste material. In some embodiments, the microwave-enhanced vitrification system comprises a microwave system for treating waste material combined with a modular vitrification system that uses inductive heating to vitrify waste material. The final product of the microwave-enhanced vitrification system is a denser, compacted radioactive waste product.
The present invention, in some of its embodiments, provides a microwave system for treating radioactive waste material. In some embodiments, the microwave system comprises a microwave waveguide positioned to direct microwaves at radioactive waste in a waste container. The microwaves excite the waste material through coupled heating in order to pyrolyze and vitrify the waste material into a more compact form. In particular, where waste coming into the microwave system (“incoming waste material”) comprises media combined with radioactive isotopes in a non-dense mixture, the microwave system acts to reduce the volume of waste material by heating the incoming waste material with microwaves, pyrolyzing the waste material, destroying the crystalline structure of the incoming waste material, producing a molten mixture of the waste material components, allowing gases within the incoming waste material to escape the molten mixture, and allowing the molten mixture to cool into a dense, vitrified composition (the “final waste product”).
One embodiment of the microwave-enhanced vitrification system includes a microwave source, a waveguide, and a canister. The microwave source generates microwaves suitable for pyrolyzing and liquefying solid radioactive waste material for the purpose of stabilizing the waste material for safe storage and disposal in accordance with knowledge common to one skilled in the art. The waveguide directs and in some embodiments focuses the microwaves generated by the microwave source such that the microwaves travel toward the waste material within the canister. The canister is suitable for long term storage of treated radioactive waste material. In some embodiments, the canister is constructed of a suitable material for external decontamination and durability, such as stainless steel. The canister receives the unvitrified solid or slurry incoming waste. Initially, the canister receives a first layer of unvitrified incoming waste material. Each layer of incoming waste has a depth that is completely penetrable by the microwaves. The waveguide is positioned with respect to the first layer of solid waste feed such that the microwaves generated by the microwave source are directed toward and applied to the first layer. In some embodiments, the microwave-enhanced vitrification system supplements the first layer of solid waste feed with a “starter material,” such as silicon carbide, iron filings, iron powder, or similar substance, which facilitates coupling until the melt is self-sustaining.
After the first layer of solid waste feed is treated as discussed above, a second layer of incoming waste material is added to the canister such that the second layer is deposited on top of the first layer. The second layer is then treated in the same manner as the first layer. Each additional layer of solid waste feed is received by the canister and treated by the microwaves in accordance with the above discussion, which can be continuous or semi-continuous in nature. The pyrolyzed waste in the lower portions of the canister cools as additional waste material is received and treated. When the waste cools, it forms a stable vitrified final waste product. The number of layers of solid waste feed received and treated by the system is limited by the size of the canister. When the solid waste feed deposited within the canister has been treated, the canister is sealed and stored or disposed of in accordance with appropriate regulations.
In some embodiments, the microwave system for vitrifying waste is combined with an inductive heating system or other vitrification system that assists in heating the incoming waste material, pyrolyzing the waste material, and maintaining a molten layer of material that allows for the escape of gas from the molten mixture and the compaction of the waste before cooling into the final waste product. Generally, inductive heating is provided by heating coils surrounding the waste container near the zone within the container containing the molten layer of waste.
In some embodiments, the waste container within which the microwaves pyrolyze the incoming waste material is a microwave chamber adapted to be emptied of vitrified final waste product after use and thereafter reused for treating more incoming waste material with microwaves. In other embodiments, the waste container is a one-use canister adapted to serve as the final storage vessel for the vitrified final waste. The canister is adapted to serve as a microwave vessel within which the incoming waste material is pyrolyzed through microwave treatment. In some such embodiments, the canister further includes materials selected to assist in the inductive heating of the waste material by heating coils surrounding the canister.
One embodiment of the microwave system is illustrated generally by the block diagram in
One embodiment of the microwave system, illustrated generally in
The waveguide 400 is illustrated in more detail in
When in use, a microwave system configured in accordance with embodiments of the present invention can employ the waveguide positioned to direct microwaves at radioactive waste in the microwave chamber. The microwaves excite the waste material through dielectric heating in order to pyrolyze and vitrify the waste material into a more compact form. The microwave system acts to reduce the volume of waste material by dielectrically heating the incoming waste material with microwaves, pyrolyzing the waste material, destroying the crystalline structure of the incoming waste material, producing a molten mixture of the waste material components, allowing gases within the incoming waste material to escape the molten mixture, and allowing the molten mixture to cool into a dense, vitrified final waste product.
In experimental tests, a number of materials were pyrolyzed in a microwave chamber in a setup substantially similar to that described above and illustrated at
In subsequent tests, a number of test materials were treated in the microwave chamber for more extended periods to achieve complete or near-complete pyrolysis of the test materials. Temperatures ranged from 1200 to 1600 degrees Fahrenheit during these subsequent tests. Test results indicated appreciable volume reduction in the pyrolyzed material after it cooled.
It can be determined from the foregoing discussion that a microwave system according to example embodiments of the present invention has applicability in pyrolyzing incoming waste material, including a variety of waste media and admixtures, to achieve significant volume reduction of the total waste product. In some embodiments of the present invention, the microwave system is supplemented by a modular vitrification system that uses inductive heating to assist in pyrolyzing and melting the incoming waste material.
In the modular vitrification system, the waste material is pyrolyzed and melted within a canister that serves as waste container. The modular vitrification system employs a continuous or semi-continuous fill and sequential melting method. The canister is filled with incoming waste material loaded into canister through the top of the canister and allowed to fall toward the bottom of the canister and settle there, at first on the floor of the canister and then on top of the already loaded waste. In some embodiments, one or more admixture materials are added to the waste material to assist in inductive heating of the waste material or to assist in the formation of a vitrified final product from a molten intermediate product. As incoming waste material fills the canister, the walls of the canister above and immediately adjacent to the top-most level of incoming waste material are heated by the induction coils to form a radiant Hohlraum (black body radiation), which heats a shallow layer of top-most waste material, thereby pyrolyzing and liquefying the top-most layer of waste material. Heating of the waste material starts from the periphery of the waste material nearest the walls of the canister and proceeding inwards towards the center of the layer of waste material.
One embodiment of a modular vitrification system according to the present invention is illustrated in
As shown in the cut-away view and close-up view in
In some embodiments, the topmost layer or upper zone—i.e., the molten layer of waste—is approximately 5 cm thick, but persons of skill in the art will recognize that the thickness of the molten layer will vary depending upon a number of factors, including the type of waste material being added and the rate at which incoming waste material is added to the canister. In general, incoming waste material is added at a rate calibrated to allow for the thorough pyrolysis and liquification of each new topmost layer before the next topmost layer is added. Further, as the waste material undergoes pyrolysis, liquification, and vitrification, the waste material ejects gaseous products, including gases trapped in the crystal structure of the pre-pyrolysis incoming waste. It is important for the melt zone to remain sufficiently thin and to remain molten for a sufficient period of time to permit gases escaping the cooling lower zone to permeate through the melt zone.
In embodiments where the outermost layer 512 of the canister 510 is fabricated from stainless steel, the frequency of the excitation energy emitted by the induction coils 520a-d need not be a very high frequency; for example, frequencies as low as 30 Hz are sufficient to ensure that the inductive field penetrates the canister 510 to heat the graphite crucible layer 514.
a, 9b, and 9c show one embodiment of the progressive filling and sequential melting of the rising level of waste material within the canister 510. As in
Turning first to
In many embodiments, the outside of the canister 510 is air-cooled during the filling and vitrification process, and the induction coils 520a-d are cooled by circulating water around the induction coils 520a-d.
Heating of the waste material starts from the periphery of the waste material nearest the walls of the canister and proceeding inwards towards the center of the layer of waste material. However, a faster and more even pyrolysis and liquification of the waste material is possible when the inductive heating of the modular vitrification system is combined with microwave treatment of the incoming waste material within the canister, according to the microwave system discussed above.
By combining microwave heating of waste material with inductive modular vitrification (or other vitrification methods), several advantages are realized. In a system such as that described in the preceding paragraph and illustrated in
a through 12d illustrate one embodiment of a microwave-enhanced vitrification system that combines the microwave system and the modular vitrification system, with waste canisters being moved into position along a conveyor. In the illustrated embodiment, the canister 1510 is carried by a conveyor 1600 into a position beneath the lid 1512 and induction coils 1520. (In the illustrated embodiment, framing arms 1525 hold the induction coils in place.) At the designated position on the conveyor 1600 beneath the lid 1512 and induction coils 1520, an elevator or hydraulic lift 1650 lifts the canister 1510 into an elevated and “locked” position so that the lid 1512 makes contact with the canister 1510 and the induction coils 1520 surround the canister on its sides. Once the canister 1510 is in the locked position, the canister 1510 is filled with waste from the waste feed tube 1545, and the waste material within the canister is pyrolyzed, liquefied, and vitrified by microwave treatment and inductive heating, as described above. When the canister 1510 has been filled to its maximum safe capacity and all of the waste within has been vitrified, the elevator or hydraulic lift 1650 lowers the canister 1510, which then moves along the conveyor 1600 to its next destination. Those of skill in the art will recognize that alternative means for moving the canister 1510 into position are contemplated and encompassed by the present invention; for example, the conveyor 1600 could alternatively take the form of a track system or a bogie system.
A microwave-enhanced vitrification system according to the present invention provides for a homogenous vitrified product with a reduced volume compared to the incoming waste material. In some embodiments as described above, the microwave-enhancing vitrification system vitrifies a batch of waste material using a single canister—i.e., without using both a melt and a storage container. This reduces decontamination and decommissioning costs. Additionally, the system is able to increase the scale of a project by merely adding additional canisters. Other benefits of the microwave enhanced vitrification system include eliminating complex and capital-intensive refractories, water-cooled crucibles, or sand refractories that could fail, leak volatiles, or require maintenance.
While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.
This application is a continuation of and claims priority from U.S. Patent Application No. 61/312,019, U.S. Patent Application No. 61/320,511, and U.S. Patent Application No. 61/321,623.
Number | Date | Country | |
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61312019 | Mar 2010 | US | |
61320511 | Apr 2010 | US | |
61321623 | Apr 2010 | US |