Method of waste treatment

This disclosure sets forth a method for processing metal waste incorporating substantial zirconium as exemplified by nuclear fuel rods which include enriched uranium and other nuclear products. This process contemplates conversion of the zirconium and other constituents into oxides by mixing with an acid, subsequently forming a solution or a gel which is either dried or frozen, thereby yielding a green shaped body. The green body is thereafter sintered to form a dimensionally and structurally stable monolith for disposal.

BACKGROUND
 The present invention relates to a method of treating zirconium based metal
 waste particularly, though not exclusively, waste resulting from nuclear
 fuel reprocessing.
 Fuel rods for nuclear plants comprise a core of enriched uranium material
 having an outer can or cladding of a zirconium based alloy. Presently,
 when the spent fuel rods are reprocessed, they are chopped up into shorter
 lengths and treated with nitric acid to dissolve out the spent fuel core,
 leaving behind the cladding since it is not attacked by the nitric acid.
 The pieces of zirconium alloy constitute so-called intermediate level
 waste which needs to be contained and stored safely for many years. One
 current method of dealing with this waste is to crush the pieces and store
 it encapsulated as the metal in concrete grout in drums.
 A further problem with reprocessing irradiated fuel is that associated with
 isolating and dealing with the fission products generated during the
 nuclear reaction process. Normally, the fission products are separated
 from the uranium and plutonium, the latter two elements being reprocessed
 for further use. However, it is necessary to contain and safely store the
 fission by-products as they constitute so-called high-level waste. One
 method of dealing with this waste is by encapsulation by vitrification.
 Dealing with the zirconium waste and the fission product waste currently
 constitutes two separate stages of the reprocessing cycle and are both
 extremely costly in both plant and in running costs.
 It is an object of the present invention to provide a process for dealing
 more economically with zirconium waste. It is a further object to provide
 an alternative and more cost effective means of dealing with and storing
 the fission product waste.
 The present invention relates to a process for treating zirconuim based
 metal waste, the process including the steps of converting at least some
 of said zirconium based metal into an oxide (as herein defined). As
 hereafter described in more detail, the oxide is used in the production of
 a green body, for example by pressing, and the green body is sintered.
 According to a first aspect of the present invention there is provided a
 process for treating zirconium based metal waste, said waste comprising at
 least some of the zirconium based metal in solution, the process including
 the steps of converting at least said solution of said metal into an oxide
 of said zirconium based metal; and, sintering said oxide to form solid
 articles.
 One zirconium based mental alloy currently is use is known as "Zircalloy"
 (trade name) and comprises in excess of 95 wt % zirconium.
 The step of bringing the zirconium based metal into solution by chemical or
 electrochemical means is known in the prior art and provides a stable
 solution, e.g., a nitrate and oxide residues. See, for example, "Use of
 Electrochemical Processes in Aqueous Reprocessing of Nuclear Fuels" by F.
 Baumgarter and H. Schmeider, Radiochemical Acta, Vol. 25 pp 191-210
 (1978).
 In this specification the terms "zirconium oxide" and "oxide of zirconium"
 and similar terms are frequently used. The actual chemical compositions
 resulting from the processes described herein may not have chemical
 compositions which correspond exactly either to a pure zirconium oxide or
 to zirconia, ZrO.sub.2, since the sintered materials in question will
 contain impurities and/or intentionally added materials and contaminants
 which it is desired to encapsulate, and/or to stabilize the crystal phase
 and which may also modify the crystal structure. Examples of such
 stabilizing and modifying additions may include, for example, metal oxides
 such as yttria, Y.sub.2 O.sub.3 to stabilize the crystal phase of
 zirconium oxide. Furthermore, in embodiments to be described below,
 particles of zirconium oxide powder are embodied in a matrix also
 containing aluminum and/or silicon atoms. Therefore, any reference herein
 to "zirconium oxide" or similar terms are to be taken as generic terms
 encompassing the resulting matrix of the sintered product or intermediate
 material in all embodiments and variations of the invention described
 herein howsoever arrived at.
 The zirconium based metal may constitute waste resulting from irradiated
 fuel rods from nuclear plants for example.
 The zirconium based metal may be brought into solution by electrochemical
 dissolution wherein the metal is made anodic in a electrolyte or nitric
 acid so converting the metal to a nitrate. In this method, a substantial
 proportion, perhaps about 85% of the zirconium metal, is converted
 directly to the oxide which forms a sludge in the dissolution vessel. The
 remaining nitrate may be thermally treated to decompose the nitrate to the
 oxide in a known manner.
 The resulting oxide may be separated, dried and milled to break down
 friable flakes if necessary; the resulting powder being pressed, cast or
 extruded for example into "green" compacts and sintered to solid bodies in
 known manner at temperatures up to about 1800.degree. C.
 Those steps in the ceramics art normally associated with the pressing and
 sintering of refractory oxide materials may be employed as desired and
 include such steps as appropriate as mixing with resin binders and/or
 lubricating waxes and preliminary burn-off treatments to remove such
 resins and waxes for example prior to sintering. Such steps are described
 in standard texts such as "Enlargement and Compaction of Particulate
 Solids", Ed. Nayland G. Stanley-Wood, Butterworths & Co. Ltd. 1983,
 particularly chapters 7 and 11; "Principles of Powder Technology", Ed.
 Martin Rhodes, Wiley, 1994, chapter 10; and "Principles of Ceramic
 Processing", J S Reed, Wiley Inter-science 1995, chapters 12, 17, 20, 22,
 and 29.
 In practicing the present invention, where the zirconium based metal
 constitutes the cladding of a nuclear fuel rod, the whole fuel rod,
 including the irradiated uranium fuel, is preferable brought into solution
 in nitric acid. Thus, the solution will contain nitrates of uranium,
 plutonium, zirconium and also the fission products in the spent fuel. The
 uranium and plutonium may then be separated from the solution by one of
 the known so called "PUREX" processes which are essentially solvent
 extraction techniques. See for example, "The Chemistry of the Purex
 Process" by J. Malvyn McKibben, Radiochinica Acta 36 (1984) 3-15. This
 results in the solution retaining the fission by-products which are
 normally treated as a separate waste product. Again, the resulting
 nitrates may be thermally treated to decompose and convert them to oxide,
 including those of at least some of the fission products.
 The zirconium based metal may alternatively be brought into solution by a
 route other than one of the so-called "PUREX" processes. For example, the
 zirconium metal waste may by converted to ZrX, where "X" is a halide,
 using an intensified fluorination technique such as by a fluidised bed
 with hydrogen fluoride. Other fluorinating agents such as nitrofluor
 (NOF:3HF) may also be used. The zirconium halides thus prepared may be
 readily converted to oxides.
 Alternatively, oxides of the fission products may be separately treated and
 subsequently blended with the zirconium oxide powder in a preferred
 proportion.
 A major advantage of the latter option is that the fission products are
 effectively encapsulated in the resulting sintered zirconium oxide body
 and a separate treatment stage for the fission products is removed from
 the process with a consequently great cost saving. Zirconium oxide is a
 particularly stable ceramic and has the necessary chemical durability to
 allow it to form the matrix for encapsulation of the high-level waste
 fission products. Furthermore, the melting point of zirconium oxide is
 greatly in excess of glass which forms the matrix in current verification
 encapsulation processes. The sintered bodies may be stored in drums in
 concrete grout for example. A further advantage conferred by the nature of
 zirconium oxide compared with glass is that it may allow higher levels of
 fission product waste to be incorporated into the ceramic encapsulate than
 is achievable with glass.
 The sintered zirconium oxide material may also by used to encapsulate some
 or all of the plutonium arising from the nuclear reaction process in the
 same manner as described with reference to the fission products above.
 The irradiated fuel rods may be processed as complete units without prior
 dicing into shorter lengths so improving the efficiency and ease of
 automation of the process and also reducing the contamination attributable
 to the dicing or slicing process. This again improves the economics of the
 process as a complete plant dedicated to cutting and handling of the fuel
 rod pieces may be dispensed with.
 BRIEF SUMMARY
 In a first aspect, the present invention is characterized in that the
 zirconium oxide, either alone or including at least some of the fission
 products, is mixed with a sol or a solution of a gel forming chemical, and
 a green body is produced from the mixture and subsequently sintered.
 Examples of suitable chemicals are aluminum secondary butoxide and
 aluminum iso-propoxide which form complementary phases with the zirconium
 oxide. The gel forming chemical may be treated with a modifying agent such
 as an alkanolamine, an example of which is triothanolamine, to stabilize
 them. This is due to metal alkoxides being readily precipitated in the
 presence of moisture. When stabilized a polycondensation reaction occurs
 promoting gelation on hydrolysis. This results in a stable cross-linked
 inorganic polymer gel. The modified chemical is mixed with the zirconium
 oxide to form a slurry, the proportion of zirconium oxide being added such
 that the resulting mixture is still workable and the density is as high as
 possible to minimize shrinkage during subsequent processing. The
 proportion of water added to the slurry controls the gelation time. The
 slurry so formed is then cast into a mold or otherwise formed into desired
 shapes such as by extrusion for example, and allowed to set. Once set, the
 green bodies are removed from their mold, if appropriate, and slowly dried
 so as to minimize cracking during shrinkage. The dried green bodies are
 then sintered to densify and increase the strength of the articles for
 long term storage in a repository.
 A hydrolyzed zirconium salt or other metal salt such as a chromium salt may
 be used instead of the aluminum alkoxide.
 In a second aspect, the present invention is characterized in that the
 oxide is mixed with a material which is a chemical or a combination of
 chemicals which gels and hardens by heat (rather than by hydrolysis), and
 a green body is produced from the mixture and subsequently sintered.
 Examples of suitable materials include zirconium acetate, zirconium
 acetate/citric acid, zirconium nitrate/citric acid and zirconium
 acrylamide.
 A particular advantage of the present invention in its second aspect is
 that after the drying process bonds are formed between the zirconium oxide
 particles and the residual material resulting from either the aluminum
 alkoxide or from the zirconium sol for example; this residual material
 comprises aluminum, zirconium and oxygen, as appropriate, on a molecular
 scale. Due to this residual material being on a substantially molecular
 scale, the necessary sintering reaction is greatly enhanced. It is
 expected that the temperature will be generally lower than those normally
 needed for sintering similar zirconia powder bodies.
 In a third aspect, the present invention is characterized in that the
 zirconium oxide, either alone or including at least some fission product,
 is mixed with a gel which is freeze castable, and a green body is produced
 from the mixture and subsequently sintered. That is, the oxide is
 initially bound together by the so-called freeze-casting technique
 utilizing a sol-gel method. Gelation takes place by dehydration of the sol
 during freezing and, at a critical concentration, the sol particles form
 chemical bonds. The result of this is that when an oxide mass which was
 previously a slurry, is brought back to room temperature from its freezing
 temperature, it remains in the green state in a stable solid and
 handleable form. Due to the formation of ice crystals as a result of the
 freezing process, the ceramic particles take up space between the ice
 crystals and form a continuous matrix around the crystals. On sintering of
 the thawed and dried green body, very little shrinkage occurs.
 Furthermore, due to the strong bonding produced during sol-gel
 freeze-casting, sintering temperatures are relatively low thus promoting
 relatively little shrinkage and consequent cracking.
 A particular advantage of the freeze-casting technique is that it is
 essentially solvent free thus, reducing the hazardous and consequent
 on-costs by way of more complex plant usually associated with the use of
 solvents. Prior technology related to the use of the freeze-casting
 technique is applicable to the second preferred embodiment.
 In the third aspect of the present invention utilizing the freeze-casting
 technique, the zirconium oxide and fission products may be combined with a
 silica sol, or alternatively, with a zirconia sol.
 In the invention, in any of the first, second and third aspects, filler
 powders such a zircon (ZiSiO.sub.4) for example may be added to the
 zirconium oxide waste and sol to control shrinkage on sintering. Other
 ceramic filler powders may also be added as appropriate. The role of
 filler powder may be fulfilled by suitable content levels of the zirconium
 oxide waste powder itself. The use of filler powders also applies to the
 process of the first preferred embodiment.
 The present invention also provides a process for the disposal of nuclear
 waste comprising converting at least said zirconium based metal into a
 sintered body according to any of the first second and third aspects and
 storing said body.
 The present invention also contemplates the encapsulation of fission
 product oxides within the sintered zirconium oxide body.

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.