Patent Number: 054611857
Section: description

DETAILED DESCRIPTION OF THE INVENTION As shown in FIG. 1, a suitable glass forming material is heated to the melting point of the material and mixed with a sacrificial oxide in zone 1, forming a molten glass mixture containing at least substantially totally dissolved sacrificial metal oxide. Molten glass mixture is forwarded to zone 2 where radioactive waste, such as spent nuclear reactor fuel rods and their metal cladding, is added to the molten mixture. It is an important feature of the invention that the sacrificial oxide is present in the molten glass bath when the feed of radioactive waste is introduced, so that the sacrificial oxide may readily oxidize the metal of the waste and aid in dissolution of metal oxides. The amount of sacrificial metal oxide provided in the molten glass mixture is selected so that the ratio of sacrificial metal oxide to the elemental metal and any carbon of the waste material is sufficient to insure that any metal from the waste or its cladding will be in the oxide state or converted thereto and thus soluble in or dissolved into the molten glass. The sacrificial oxide (MO) reacts with the metal Me, e.g., plutonium, of the waste and/or cladding (and with any oxidizable carbon present) according to the following general reaction: EQU MO+Me.fwdarw.M+MeO (1) Reaction mixture may then be forwarded to zone 3, where the metal M is separated from the sacrificial oxide. If the sacrificial oxide chosen comprises a metal M having a relatively low elemental melting point, the separation may be achieved simply, such as by density or decantation. If the metal M has a high melting point, decantation or filtration may be used. Molten glass mixture containing the converted radioactive waste is forwarded to zone 4 where it is transferred to containers and allowed to solidify. The metal M, separated in zone 3, is forwarded to zone 5 where it is reoxidized and returned to use in zone 1. In the illustration of FIG. 2, a ceramic lined (e.g., fired alumina) furnace 10 contains a molten bath or mass of glass (designated generally as A) generated and maintained by a heat source (induction heater) 11. As alternate construction, furnace 10 may employ a cold wall, wherein a coating of solidified glass from the bath lines the cooled inner wall surface and protects the furnace, or both a ceramic liner and cold wall construction may be utilized. The volume of glass, which may, for example, comprise or consist of lead borosilicate glass, is maintained in furnace 10 by addition of glass frit via tube 12. The optimum formula for dissolution, in the case of this glass, will range from about 1 mole of PbO per mole of B.sub.2 O.sub.3 to about 3 moles of PbO per mole of B.sub.2 O.sub.3. Sacrificial metal oxide, in this case PbO, is supplied to the bath via tube 13. Spent fuel rods (uranium oxide clad with zirconium) are fed to the melted glass bath through an entry 14. Unmelted frit (and perhaps waste) forms a crust on the melt. The stoichiometric ratio of PbO to the elemental metal of the fuel cladding is maintained at about 2. The PbO reacts with the zirconium according to the reaction: EQU 2PbO+Zr.fwdarw.2Pb+ZrO.sub.2 (2) The uranium oxide and zirconium oxide are soluble in the glass bath, while the lead, molten at the temperature of the bath (about 1000.degree. C.) forms a separate lower layer in the furnace 10. The separate layer serves several functions in the bath. Normally, the glass and all expected wastes have lower densities than lead; waste heavier than the glass will float at the glass-lead interface while being dissolved. This feature serves to protect the bottom liner from physical and chemical attack by the unprocessed waste or-the sacrificial oxide. The lead layer also provides a thermal and corrosion barrier between glass and liner. The molten lead is removed from the furnace through outlet 15 and transferred by line 16 to oxidation zone 17. Oxygen is blown through the lead, forming solid lead oxide, which is easily separated from the surface of the molten lead, as shown. The separated solid lead oxide is then sent via line 18 for use in furnace 10 through entry 13. Concurrently, molten lead borosilicate glass containing converted radioactive waste values is removed through line 19 to canister 20 and allowed to cool and solidify. In a further embodiment, not shown, one or more additional glass baths, connected serially, may be provided. In such case, the first bath serves as a rough oxidizing and dissolution zone. Periodically or continuously, molten glass containing dissolved and/or oxidized waste is removed from a suit, able location in the first glass bath and sent to a second bath (optionally smaller) where waste, glass, glass precursors, PbO, or B.sub.2 O.sub.3 may be added as desired to produce a glass product having predetermined desired composition or characteristics. A second bath thus serves to optimize or fine tune the concentrations of the components of the glass-waste composite. Elemental metal from the superficial oxide may or may not be separated from the second bath. If separated therefrom, the metal may be sent to the first bath or the oxidation zone. Additional serial glass baths may be employed as desired or needed to produce good glass. In FIG. 3, a sacrificial oxide, such as Fe.sub.2 O.sub.3, is selected and mixed with a suitable glass, such as sodium borosilicate glass, and the mixture is heated to fusion in zone 50, the ferric oxide dissolving in the molten glass. The molten glass containing the ferric oxide is passed to zone 51 where the waste material is added, with additional heat being supplied. The ferric oxide is provided in an amount sufficient to allow oxidation of all the elemental metal of the waste material, including any assembly or cladding, while insuring that little, if any, of the ferric oxide is converted all the way to iron. Stated differently, enough Fe.sub.2 O.sub.3 is employed so that the iron in the mixture is not totally reduced but remains as FeO. Since the FeO is soluble in the molten borosilicate glass, separation of the iron values is not necessary, and the molten glass mixture containing metal oxides derived from waste and/or cladding values and FeO is transferred in zone 52 to suitable containers. In any embodiment herein where CO.sub.2 is generated from oxidation of carbon, it may simply be removed as off-gas, as illustrated in FIG. 2. As mentioned, the invention has particular application to the treatment of weapons grade plutonium. Preferably, the plutonium is fed, along with other waste material, e.g., spent light-water reactor fuel, in the procedure of FIG. 2. For illustration only, the molar ratio of spent fuel to plutonium should be at least about 6, preferably from about 10 to about 100, e.g., 20 to 1, and the final glass concentration of plutonium as oxide in the glass composite may be from about 0.1 percent to about 5.0 percent by weight. Addition of spent fuel with weapons grade plutonium serves to isotopically dilute the weapons grade plutonium with reactor grade plutonium, combines the weapons grade plutonium with highly radioactive materials, and produces a final glass material having fewer safety, security, and proliferation concerns.