Abstract:
A method and system for increasing the waste loading of vitrified nuclear waste includes a plasma mass filter and a heating apparatus. The plasma mass filter is used first to collect radioactive particles from a multi-species plasma. The radioactive particles are then placed, together with a frit, in crucibles. The crucibles are then induction heated to fuse the radioactive elements with the frit to form a melted mixture which is then cooled to form vitrified waste having relatively high waste loading.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains generally to systems and methods for the remediation of nuclear waste. More particularly, the present invention pertains to systems and methods for vitrifying relatively high concentrations of a radioactive nuclear waste. The present invention is particularly, but not exclusively, useful as a system and method for using induction heating to vitrify the output of a plasma mass filter.  
         BACKGROUND OF THE INVENTION  
         [0002]    One particularly effective technique for disposing of nuclear waste involves a process known as vitrification, or glassification. In a vitrification process, nuclear waste is incorporated into glass by first heating and fusing the nuclear waste with frit. This is typically done in a large conventional melter having electrodes which heat and fuse the nuclear waste with frit. The melted mixture of nuclear waste and frit is then poured into storage canisters for subsequent cooling and disposal.  
           [0003]    Present day vitrification techniques, however, face several significant problems. For one, the electrodes of a conventional joule-heated melter can be shorted out by certain components, such as chromium, noble metals or spinel crystals when they are present in the nuclear waste. Unfortunately, electrodes which have shorted out will render the melter inoperable, and essentially shut down the entire vitrification operation. For another, it is always a concern to avoid melt processing accidents such as have occurred when conventional melters have been used.  
           [0004]    Another important concern with current vitrification techniques is that, once the waste has been vitrified, heat from the encapsulated radionuclides may cause the glass to fracture, degrade and release radionuclides. Thus, there is a thermal limit as to the amount of radioactive material that can be encapsulated in a volume of glass. Finally, the process of vitrifying unprocessed nuclear waste using conventional techniques may require decades to accomplish because of the large volume of nuclear waste that may be involved.  
           [0005]    Insofar as nuclear waste itself is concerned, nuclear waste generally contains a mix of radioactive material, non-radioactive material and glass contaminants. Obviously, it is desirable to differentiate between radioactive material, which requires special handling (i.e. vitrification), and the nonradioactive material and glass contaminants, which can be disposed of in a more conventional manner. Thus, if the radionuclides can somehow be effectively concentrated and separated from the non-radioactive material and contaminants of nuclear waste, the handling and disposal of the radioactive material can be accomplished much more efficiently.  
           [0006]    There are many different types of devices that can separate particles of a mixed material. One example is the device disclosed in U.S. Pat. No. 6,096,220 (the Ohkawa Patent), which issued on Aug. 1, 2000 to Ohkawa, for an invention entitled “Plasma Mass Filter” and which is assigned to the same assignee as the present invention. Briefly, the plasma mass filter, as disclosed in the Ohkawa Patent, relies on crossed electric and magnetic fields that are established in a chamber to place charged particles on different predictable orbital paths. This is done to separate the charged particles from each other. Specifically, charged particles having relatively low mass to charge ratios are confined inside the chamber during their transit of the chamber. On the other hand, charged particles having relatively high mass to charge ratios are not so confined. Instead, these high-mass particles (M 2 ) are collected inside the chamber wall before completing their transit through the chamber.  
           [0007]    It happens that under present vitrification practices, i.e. using conventional melters which do not incorporate the teachings of the Ohkawa Patent, only a relatively small mass percentage of radionuclides will be encapsulated in a given mass of glass. By way of example, consider the fact that presently, only about 15% by weight of the Hanford nuclear waste that is being processed is of mass greater than 90. However, when this waste is vitrified using conventional vitrification techniques, it constitutes about 33% by weight of the vitrified waste. The result here is that less than 5% of the atoms in the vitrified waste are radioactive. Thus, recognizing higher waste loadings may be possible, and keeping in mind that the thermal limit is the best that can be done, it is apparent that more highly concentrated nuclear waste and more efficient vitrification techniques are desirable. An objective here is to achieve effectively higher waste loadings (i.e. the amount of radioactive material in a volume of glass).  
           [0008]    In light of the above, it is an object of the present invention to provide a system and method for using induction heating to vitrify relatively high concentrations of radioactive nuclear waste. It is another object of the present invention to provide a system and method for increasing the waste loading of vitrified waste. Still another object of the present invention is to provide a system and method for separating radioactive elements from non-radioactive elements to obtain a relatively high concentration of radioactive nuclear waste. Yet another object of the present invention is to provide a system and method for vitrifying relatively high concentrations of radioactive nuclear waste by using a plurality of disposable crucibles that can be heated to higher temperatures for a shorter period of time to accelerate a vitrification process. Still another object of the present invention is to provide a system and method which is easy to use, relatively simple to implement, and comparatively cost effective.  
         SUMMARY OF THE INVENTION  
         [0009]    A system and method for increasing the waste loading of disposable radioactive nuclear waste relies on the general notion that, in addition to other constituents, radioactive waste contains radionuclides having relatively high atomic weights. Accordingly, when nuclear waste is vaporized and ionized to create a multi-species plasma, the resultant multi-species plasma will include charged particles having relatively low mass to charge ratios (M 1 ) and charged particles having relatively high mass to charge ratios (M 2 ). This multi-species plasma can then be injected into the chamber of a plasma mass filter, such as the one disclosed in the Ohkawa Patent, which is incorporated herein by reference. In accordance with the teachings of the Ohkawa Patent, the multi-species plasma interacts with the crossed magnetic and electric fields inside a chamber to eject high-mass particles (M 2 ) into the wall surrounding the chamber. On the other hand, the low-mass particles (M 1 ) are confined inside the chamber. They do not collide with the chamber wall and, instead, are allowed to transit the chamber. Thus, the particles are separated. After the high-mass particles (M 2 ) have been separated from the low-mass particles (M 1 ) in this manner, the high-mass particles (M 2 ) are collected from the chamber wall of the filter for subsequent vitrification. For the present invention, it is envisioned that more than 70% of the material collected from the chamber wall will be radioactive high-mass particles (M 2 ).  
           [0010]    In accordance with the present invention, once the high-mass particles (M 2 ) have been collected from a plasma mass filter chamber, they are vitrified at a sufficiently high temperature to ensure high mass loadings can be achieved. Two attractive approaches to this high temperature vitrification are available: cold wall crucibles or single use crucibles. By using a plurality of crucibles, the design can be simplified since multiple usage is not required. For the individual crucibles, each crucible is structurally shaped as a cup or receptacle which has a compartment or hollow cavity for receiving the nuclear waste. In this structure, the cavity is surrounded by a wall which has an inner layer of alumina, an intermediate layer of graphite and an outer layer of stainless steel. Additionally, at least one induction coil is mounted on the outer surface of each crucible.  
           [0011]    In the operation of the present invention, along with the high-mass particles (M 2 ), frit is placed in each crucible. Each crucible is then heated by its induction coils to approximately 1600° C. In this process, the high-mass particles (M 2 ) fuse with the frit to form a melted mixture having a relatively high waste loading (as much as 66%). This mixture is then cooled in the respective crucibles to form vitrified nuclear waste. These crucibles containing vitrified nuclear waste can then be subsequently discarded.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0013]    [0013]FIG. 1 is a perspective view of a plasma mass filter with portions broken away for clarity;  
         [0014]    [0014]FIG. 2 is a exploded perspective view of a heating apparatus in accordance with the present invention with a disposable crucible of the present invention separated therefrom; and  
         [0015]    [0015]FIG. 3 is a cross-sectional view of a crucible as seen along line  3 - 3  in FIG. 2. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Referring to FIG. 1, a plasma mass filter for use with the present invention is shown and generally designated  10 . As shown, the filter  10  includes a substantially cylindrical shaped wall  12  which surrounds a chamber  14 , and defines a longitudinal axis  16 . The filter  10  also includes a plurality of magnetic coils  18  which are mounted on the outer surface of the wall  12  to surround the chamber  14 . In a manner well known in the pertinent art, the coils  18  can be activated to create a magnetic field in the chamber  14  which has a component B z  that is directed substantially along the longitudinal axis  16 . Additionally, the filter  10  includes a plurality of voltage control rings  20 , of which the voltage rings  20   a - c  are representative. As shown these voltage control rings  20   a - c  are located at one end of the cylindrical shaped wall  12  and lie generally in a plane that is substantially perpendicular to the longitudinal axis  16 . With this combination, a radially oriented electric field, E r , can be generated.  
         [0017]    For the plasma mass filter  10  of the present invention, the magnetic field B z  and the electric field E r  are specifically oriented to create crossed electric magnetic fields. As is well known to the skilled artisan, crossed electric magnetic fields cause charged particles (i.e. ions) to move on helical paths, such as the path  22  shown in FIG. 1. The plasma mass filter  10  for the present invention requires that the voltage, along the longitudinal axis  16 , V ctr , be a positive voltage, compared to the voltage at the wall  12  which will normally be a zero voltage.  
         [0018]    In the operation of the plasma mass filter  10 , a rotating multi-species plasma  24  is injected into the chamber  14 . Under the influence of the crossed electric magnetic fields, charged particles confined in the plasma  24  will travel generally along helical paths around the longitudinal axis  16  similar to the path  22 . More specifically, as shown in FIG. 1, the multi-species plasma  24  includes charged particles which differ from each other by mass. Due to the fact that the elements of the nuclear waste may not be known, it is contemplated for the present invention that the plasma  24  includes at least two different kinds of charged particles, namely high-mass particles  26  (radioactive elements) and low-mass particles  28  (non-radioactive elements and glass contaminants).  
         [0019]    Due to the configuration of the crossed electric magnetic fields and, importantly, the positive voltage V ctr  along the longitudinal axis  16 , the plasma mass filter  10  causes charged particles in the multi-species plasma  24  to behave differently, according to their mass, as they transit the chamber  14 . Specifically, charged high-mass particles  26  are not able to transit the chamber  14  and, instead, they are ejected into the wall  12 . On the other hand, charged low-mass particles  28  are confined in the chamber  14  during their transit through the chamber  14 . Thus, the low-mass particles  28  exit the chamber  14  and are, thereby, effectively separated from the high-mass particles  26 . After the high-mass particles  26  (radioactive elements) are separated from the low-mass particles  28  (non-radioactive elements and contaminants) they are then collected from the wall  12  of the plasma mass filter  10  for subsequent vitrification.  
         [0020]    The demarcation between low-mass particles  28  and high-mass particles  26  is a cut-off mass, M c , which can be established for a parabolic voltage profile by the expression:  
           M   c   =zea   2 ( B   z ) 2 /8 V   ctr .  
         [0021]    In the above expression, “ze” is the charge on an electron, “a” is the radius of the chamber  14 , “B z ” is the magnitude of the magnetic field, and “V ctr ” is the positive voltage which is established along the longitudinal axis  16 . Of these variables in the expression, “ze” is a known constant. On the other hand, “a”, “B z ” and “V ctr ” can all be specifically established for the operation of plasma mass filter  10 .  
         [0022]    Referring now to FIG. 2, a heating apparatus for vitrifying the high-mass particles  26  after they have been removed from the chamber  14  is shown and generally designated  30 . As shown, the heating apparatus  30  includes a plurality of induction coils  32 , of which the induction coils  32   a - d  are only exemplary. Further, each induction coil  32   a - d  is connected via a respective conductor  34   a - d  to a power source  36 .  
         [0023]    In FIG. 2, it is also seen that the heating apparatus  30  includes a plurality of crucibles  38 , of which the crucibles  38   a - d  are exemplary. Referring now to FIG. 3, and using the crucible  38   a  as an example, it is to be appreciated that each crucible  38  has a wall  40 . Further, the wall  40  is made of three components that include an inner layer of alumina  42  (or some other material that withstands high temperature and is chemically inert), an intermediate layer of graphite  44  (or some similar material that withstands high temperature and absorbs inductive power), and an outer surface layer of stainless steel  46 . As shown, the wall  40  of crucible  38  defines a hollow cavity (compartment)  48 . Additionally, the crucible  38  can include a lid  50  which will enclose the hollow cavity (compartment)  48 . As intended for the present invention, and indicted in FIG. 2, each induction coil  32  is dimensioned for selectively receiving a respective crucible  38 , such that the coil  32  is positioned in a surrounding relationship relative to the crucible  38 . Consequently, upon activation of the power source  36 , the induction coils  32   a - d  will heat the respective crucibles  38  and whatever contents are in the hollow cavity (compartment)  48  of the crucible  38 .  
         [0024]    Still referring to FIG. 2, it is seen that a crucible  38  (exemplified by the crucible  38   a ) can be water-cooled. Specifically, for this purpose, a water source  52  can be provided to feed water through a supply line  54  and through a coil  56  that surrounds the crucible  38   a . A return line  58  can also be provided to return water to the water source  52  for recycling. As intended for the present invention, this embodiment of the present invention allows the crucible  38  to be operated as a so-called “cold crucible” to achieve elevated temperatures.  
         [0025]    In the operation of the present invention, the high-mass particles  26  are collected from the chamber  14  of the plasma mass filter  10 , as indicated above, and placed in the crucibles  38 , along with a frit  60 . Power source  36  is then activated, and the induction coils  32  are heated to approximately 1600° C., or higher (nearer 2000° C. for a “cold crucible”). During the heating of the crucibles  38 , the high-mass particles  26  will fuse with the frit  60  to form a melted mixture. This mixture is then cooled. As a result, the nuclear waste is encapsulated in solidified glass and the crucibles  38  containing this vitrified nuclear waste are subsequently discarded.  
         [0026]    Using the “cold crucible” embodiment as disclosed and indicated above, higher melt temperatures may be attainable within the crucible  38 . Specifically, this may be desirable as optimal conditions will exist when the melted mixture is near the thermal limit. For this condition, the thermal limit can be defined as the point where the vitrified high-mass particles  26  in the melted mixture begin to break down the glass. The point being that (for higher efficiencies) it is desirable to operate as close to the thermal limit as is possible.  
         [0027]    While the particular System and Method for Radioactive Waste Vitrification as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.