Patent Number: 060375175
Section: summary

TECHNICAL FIELD OF THE INVENTION This invention relates to waste treatment systems, and more particularly to an apparatus and method for treating waste materials which include beta radiation emitting constituents. BACKGROUND OF THE INVENTION Radioactive waste materials are generated at a number of different sources, including nuclear power plants, nuclear weapons facilities, and nuclear fuel processing facilities. There are also a number of less obvious sources of radioactive wastes. For example, the fly ash of coal power plants may include radioactive constituents. Also, radioactive materials are used in certain medical procedures. Thus, medical facilities are major producers of radioactive waste materials. The radioactive wastes produced at medical facilities include equipment and clothing which may be contaminated by the radioactive material used in medical procedures. The three types of radiation emitted from radioactive materials are alpha, beta, and gamma radiation. Alpha and beta radiation comprise particles which are emitted from the nucleus of an atom, while gamma radiation comprises short-wavelength photons of nuclear origin. Alpha particles are doubly ionized helium nuclei, and thus have a net positive electrical charge. Beta radiation comprises primarily electrons, although some radioactive isotopes emit positrons which are also referred to as beta particles. Both the charged alpha particles and beta particles may be deflected by an electromagnetic field, although beta particles are deflected more easily due to their lower mass. Gamma radiation is either emitted from a radioactive material directly or emitted as the result of a collision between an alpha or beta radiation particle and some other particle. Radioactive materials may emit one or more of the three different types of radiation, alpha, beta, of gamma radiation. Many radioactive materials emit primarily only alpha particles and/or beta particles, but produce gamma radiation indirectly as the high-energy alpha and beta particles collide with other particles. Wastes which include radioactive materials may be treated in a molten metal process to remove organic materials and to tie up the radioactive material. U.S. patent application Ser. No. 09/096,617, filed Jun. 12, 1998 by the present inventor, discloses an apparatus and method for treating waste streams which include radioactive constituents. The apparatus and method disclosed in Application Ser. No. 09/096,617, which is incorporated herein by reference, removes organic constituents from the mixed waste stream and contains the radioactive constituents. Organic materials in the waste stream react with the molten reactant metal to produce primarily elemental carbon, hydrogen, nitrogen, and metal salts. Radioactive materials in the waste stream are alloyed in the molten metal for eventual storage. The molten metal process disclosed in Application Ser. No. 09/096,617, utilizes radiation absorbing metals such as lead and tungsten, for example, in the molten reactant metal in order to safely absorb radioactive emissions from the alloyed radioactive materials. SUMMARY OF THE INVENTION It is an object of the invention to provide an apparatus and method for treating wastes which include radioactive constituents, particularly beta radiation emitters. The apparatus according to the invention includes a molten metal reactor and an electromagnetic field generating arrangement. The molten metal in the reactor reacts with any organic constituents in the waste material, and alloys the radioactive constituents. The electromagnetic field generating arrangement produces a unidirectional electromagnetic field extending through the molten metal and through at least one target area preferably within the molten metal. This unidirectional electromagnetic field directs or deflects beta radiation toward the target area and into a replaceable radiation absorbing module positioned in the target area. It is also believed that the intense electromagnetic field may enhance the beta emissions. In any event, radiation absorbing material included in the module absorbs the beta radiation in a stable form. The radiation absorbing material in the module also absorbs gamma radiation produced as beta particles and alpha particles are absorbed by the radiation absorbing material. The molten metal reactor includes a reactor chamber charged with a suitable reactant metal. A heating arrangement is included in the molten metal reactor for heating the reactant metal and maintaining the reactant metal in a molten state at a desired reaction temperature. A waste input structure is preferably included for introducing the waste material into the reactor in position for a submerging arrangement to submerge the material in the molten reactant metal. A circulating arrangement may be included for circulating the molten metal within the reaction vessel and ensuring that the radioactive constituents circulate through the area of the molten metal traversed by the electromagnetic field. The preferred molten metal reactor also includes an arrangement for removing reaction products from the molten metal reactor and also an arrangement for adding additional reactant metal. The reactant metal used in the molten metal reactor may comprise any suitable reactant metal. The primary constituent of the reactant metal preferably comprises aluminum although magnesium and/or lithium may be used with or instead of aluminum. In one preferred form of the invention, the molten metal comprises primarily aluminum along with lesser fractions of other constituents such as iron, copper, zinc, and calcium, for example. The reactant metal also preferably includes one or more radiation absorbing metals such as lead, tungsten, palladium, cadmium, dysprosium, and europium. The electromagnetic field generating arrangement comprises at least one coil of electrically conductive material. The coil or coils are preferably located within the molten metal in position to produce a highly focused electromagnetic filed in at least one target area. Each coil is encased within a protective material to protect the coil material from reacting or alloying with the molten metal. In the preferred form of invention, each coil is made from a tubular conductor material such as copper. The invention includes a cooling system comprising a coolant fluid supply and pump for circulating the coolant fluid through each tubular conductor and cooling the conductor material. A heat exchanger may be used for cooling the coolant fluid prior to returning the fluid to the coolant supply. The electromagnetic field produced through the molten reactant metal according to the invention is unidirectional, that is, the field does not alternate directions. The direction of the field is such that it directs beta particles to the target area or areas and thus to the radiation absorbing modules positioned in each target area. The field generating arrangement includes a voltage supply for inducing a current through each coil to produce the desired field. In the preferred form of the invention the voltage is pulsed to produce a pulsed unidirectional electromagnetic field. The pulsed electromagnetic field creates a pumping or circulating action within the molten reactant metal. Each radiation absorbing module includes a material suitable for absorbing the radiation which is directed to the target area by the electromagnetic field. In the preferred form of the invention, each module includes alternating layers of tungsten and lead with a spacing arrangement to maintain the spacing between tungsten layers. The spacing arrangement may comprise tungsten spacer extensions formed on each tungsten layer. In any case, all of the radiation absorbing material in each module is preferably encased in a protective material. The protective material protects the radiation absorbing material so that the module may be positioned in contact with the molten reactant metal without losing the radiation absorbing metals to the melt. In the preferred form of invention, one radiation absorbing module is mounted on a positioning structure which allows the module to be positioned in a target area within the molten reactant metal. The preferred target area is an area at which the electromagnetic field strength is greatest. One or more additional radiation absorbing modules may also be positioned in different target areas traversed by the electromagnetic field. The apparatus according to the invention directs beta radiation emissions from radioactive waste materials in the molten reactant metal to the replaceable radiation absorbing modules. Thus, beta radiation may be absorbed in the module without allowing the radioactive materials to commingle and alloy with the radiation absorbing material of the module. Absorbing the beta radiation with these replaceable, isolated modules effectively increases the capacity of the system to handle beta emitting materials without increasing the volume of the reactant metal, and thus the volume of material which is then contaminated with the radioactive material. These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings.