Patent Number: 06297419&
Section: description

DETAILED DESCRIPTION The process according to a first embodiment of the present invention may be described as follows and with reference to the flow chart shown in FIG. 1. A source of zirconium alloy fuel rod cladding is indicated at 10. The cladding is brought into solution by electrochemical dissolution 12 by making the cladding anodic and passing a current through the metal under a nitric acid electrolyte. This step results in the metal being converted to zirconium nitrate 14. However, during the dissolution step 12, a substantial quantity of the zirconium alloy is converted directly to an oxide which forms a sludge at the bottom of the dissolution tank and is subsequently removed to be added back into the process at a later stage. The zirconium nitrate is then thermally decomposed in the step 16 to the oxide 18 by one or more of the techniques including direct heating, fluidised bed, plasma-arc or microwave assisted heating The oxide 18 is then mixed in the step 20 with a sol of a gel forming chemical which in this case is aluminium secondary butoxide which is diluted with alcohol and modified with an alkanolamine, which is in this case, tri-ethanolamine. The modifying agent causes cross linking of the aluminum secondary butoxide in a controlled and time dependent manner on hydrolysis resulting in the onset of gelling. The zirconium oxide mixed with the gelling aluminum secondary butoxide forms a slurry 22 to which may optionally be added material 24 such as oxides of fission products and/or plutonium 26 which have been extracted from the dissolved spent uranium fuel by a known so-called "PUREX" process, the fission products and plutonium constituting high level waste which must be encapsulated and stored in a repository for many years. The slurry 22 continues to gel and is cast or extruded 28 into molds or self supporting shapes at the steps 30 where it is allowed to fully gel and solidify. After setting, the shaped "green" bodies are demolded 32, if appropriate, to form free-standing, handleable bodies 34 which are then dried slowly 36 to prevent excessive cracking during shrinkage. The dried green bodies 38 are then sintered 40 at a substantially lower temperature than that required for physically mixed oxides to form durable refractory material monoliths 42 which may then be stored 44 in a repository 46 in a known manner. During the drying step 36, the water is driven off and the hydroxyl groups in the chemical matrix are decomposed to leave only aluminum and oxygen present in the structure on a substantially molecular scale and binding together the powder particles of zirconium oxide and also the particles of other constituents; the sintering rate during the sintering step 40 is very high and can be accomplished at relatively lower temperatures in the region of about 1400.degree. C. compared with the higher temperatures conventionally used to sinter pressed green bodies of zirconium oxide. Therefore, the preferred embodiment of the present invention has many advantages over known techniques in that the resulting monoliths of refractory zirconium oxide are chemically both very stable and very durable and able to encapsulate the high level waste directly within the matrix. Furthermore, the low sintering temperature which the preferred embodiment of the process of the present invention permits reduces hazards associated with high vapor pressures of some elements and consequently further reduces contamination and plant costs. FIG. 2 shows a flow sheet of an alternative process according to the present invention utilizing the technique of freeze casting. A freeze castable silica or zirconium oxide sol 50 is mixed with a ceramic filler powder 52 and zirconium oxide waste 54 to form a slurry 56. The starting materials 52, 54 may be milled to improve homogeneity and mixing prior to forming the slurry 56. The zirconium oxide waste 54 may include fission products incorporated therein but, high-level fission product waste 58 may alternatively be added separately or additionally as a constituent of the slurry 56. The slurry 56 is cast 60 into a mold (not shown) having a cavity of any desired shape and freeze-cast to form a frozen body 62. The mold may be vibrated to assist packing of the slurry material within the mold and to assist mold filling by the elimination of air bubbles. The slurry 56 may alternatively be freeze extruded 64 to form an alternative frozen body 66. The freeze casting process causes the slurry constituents to form chemical bonds such that when the frozen body 62 or 66 is warmed at the steps 68 and the body demolded, it forms a relatively strong, free-standing and handleable monolith 70. The thawed body 70 is dried slowly to avoid too rapid shrinkage and consequent cracking and, once dried, it is sintered to form a high density, durable ceramic body 72 containing high level fission product waste material. A first example of the second preferred embodiment of the process of the present invention is to form a zircon, ZrSiO.sub.4, monolith. The process comprises making a mixture of a castable silica sol which is mixed with a zircon filler powder and waste zirconium oxide formed from the electrochemical dissolution of zirconium metal fuel cans. The mixture may also contain fission products from the zirconium metal waste stream or, fission product waste may be added as a separate component of the mixture. The process comprises the steps of vibro-energy milling the powder constituents to homogenize and thoroughly mix thus, breaking up zirconia flakes from electrochemical dissolution. The milling may take place wet so as to reduce dust and contamination hazard so the milled and homogenised powder is added to silica-sol to form the ceramic slurry 56, the slurry being capable of being poured into a mold (not shown) or at least capable of being so transferred to a mold. The mold may be connected to a vacuum system so as to remove entrained air or may be provided with a vibratory system for the same purpose and also to assist mold filling. The filled molds are rapidly cooled to about -50.degree. C. to freeze them and aged for a suitable period which may range from about 10 minutes to longer times. Once aged, the filled molds are rapidly warmed to room temperature and the now solid monoliths are removed from the molds and dried in air. The dried monoliths are then sintered at a minimum temperature of 1400.degree. C. The free silica from the sol reacts with a stoichiometric amount of zirconium oxide waste on sintering to form zircon. The small particle size of the sol and filler particles ensures lower sintering temperatures than normal ceramic forming processes used heretofore. Chemical bonds are formed during the freeze casting process between the silica and zirconium oxide and other constituents which are reinforced and serve to accelerate the sintering process at a low sintering temperature. An alternative to silica-sol is the use of zirconium oxide sol. This process involves the mixing of a zircon filler powder with zirconium waste and optionally fission product waste which is then mixed with a zirconium oxide sol. The process steps for producing a sintered zircon and stabilised zirconium oxide monolith are essentially as described above with reference to the formation of a monolith using the silica-sol route. As noted above, oxides of zirconium generally refer to the oxide plus a wide range of other materials which are not pure oxide; the other materials are mixed in the feed and therefore become part of the finished product, namely, the sintered disposable monoliths. While the foregoing is directed to the disclosed embodiment, the scope is set forth in the following claims.