The present invention relates to arrangements for containing waste material for long term storage and the invention is particularly applicable to immobilisation of high level radioactive waste material such as that produced by nuclear reactors.

Extremely long term safe storage of nuclear wastes is a major problem for the nuclear industry and various proposals have been made for dealing with this problem. One proposal concerns immobilising the waste in a suitable borosilicate glass which can then be deposited in a suitable geological formation. However, doubts concerning possible devitrification of the glass and consequent leaching of radioactive elements have founded criticism of the safety of this technique.

Another recent proposal involves the formation of a synthetic rock in which the nuclear reactor waste is immobilised, details of this method being described by A.E. Ringwood et al in NATURE March 1979. According to the disclosure, a selected synthetic rock is formed with the radioactive elements in solid solution. The constituent minerals of the rock or close structural analogues have survived in a wide range of geochemical environments for millions of years and are considered highly resistant to leaching by water.

The nuclear reactor waste is incorporated into the crystal lattices of the synthetic rock in the form of a dilute solid solution and therefore should be safely immobilised. A dense, compact, mechanically strong block of the synthetic rock incorporating the nuclear waste is produced by pressure and heat in a densification process and the block may then be safely disposed of in a suitable geological formation.

The following patent applications have been filed by the Australian National University based on the work by A.E. Ringwood et al:-

• European Patent Application No. 79301382.2 entitled "Safe Immobilisation of High Level Nuclear Reactor Wastes"; and

United States Patent Application 124953 entitled "A Process for the Treatment of High Level Nuclear Wastes".

The present application, in some embodiments, is concerned with making use of the synthetic rock arrangements of A.E. Ringwood et al and is concerned with an apparatus and method for producing disposable blocks of material which can include radioactive wastes in an immobilised form. However, the present application is not necessarily restricted to the particular classes of synthetic rocks of A.E. Ringwood et al and the apparatus and method described herein could be applied to other synthetic rocks in addition to those specifically described by A.E. Ringwood et al.

Other examples of synthetic rock systems which might be used with aspects of the present invention could include the following:

• 1. Supercalcine (G.J. McCarthy, Nuclear Technology, Vol.32, Jan.1977)
• 2. Product of Zeolite Solidification Process (IAEA Technical Report Series No. 176, page 51).
• 3. Product of Titanate Solidification Process (IAEA Technical Report Series No. 176, page 53).
• 4. Product of the Sandia Process (R.W. Lynch and

R.G. Dosch, US Report S_AND750255 (1975).

For the purposes of this specification, synthetic rock is defined as a material which consists chemically of one or more metal oxides (or compounds derived from metal oxides which have been formed into a rock- like structure by subjecting a mass of solid particles of the material to heat and pressure.

According to a first aspect of the present invention, there is provided a method for forming solid blocks (including synthetic rock in which nuclear reactor waste is immobilised), directly in the canister in which it will be disposed, the method comprising:-

• (a) establishing a quantity of supply material in a metal canister, means being provided for preventing gross outward deformation of the metal canister during the method, the metal canister being sufficiently heat and corrosion resistant to contain the supply material during and after the method has been effected and the supply material comprising material for forming the synthetic rock and a minor proportion of nuclear reactor waste capable of being immobilised in the synthetic rock when densified into a block;
• (b) applying pressure to compress the supply material along an axis of the canister and applying heat to cause densification and the formation of a block of synthetic rock including the nuclear reactor waste; and
• (c) either before or after said densification step, sealing the canister with a metal cap whereby the sealed canister is adapted to be removed and placed in a suitable long term storage location.

In one embodiment, the metal canister is mounted in a cavity in a refractory support element which prevents gross outward deformation. In another embodiment, the metal canister is formed so as to collapse in a bellows-like manner under axial pressure, the wall structure of the canister itself preventing gross outward deformation.

At least preferred embodiments of the invention provide a simple and effective method which can readily be practised in a "hot cell" and a relatively safe and easily handled product ensues. It is considered that during very long term storage radiation damage within the synthetic rock is likely to cause a small expansion perhaps of the order of 2% to 3%.

At least in preferred embodiments, such long term expansion can be accommodated without increased risk of contamination of the environment, for example through leaching with ground water.

Another important factor from an economics point of view is that the process is relatively simple and therefore can be readily conducted in a hot cell. Apparatus having a long working life is required as inevitably contamination of the apparatus will occur in the method and decontamination and disposal of worn apparatus is therefore an expensive and inconvenient operation.

Further advantages can be achieved with various embodiments of the invention including preferred or optional features discussed below.

Preferably, the metal canister has a sealed bottom end wall and only the final step of welding or otherwise permanently fixing a metal cap to the top of the canister is required in the hot cell.

At least for the formation of some types of synthetic rock it is considered that the present method is best implemented by including in the supply material or in contact therewith a suitable metal in a suitable quantity to provide a selected oxygen potential to facilitate the effective formation of the synthetic rock with radioactive waste immobilised therein. Suitable metals to consider for providing the desired oxygen potential are nickel, titanium and iron. The metal could be provided in the form of a lining to the metal canister or as an inner can for the supply material or alternatively the metal could be provided in fine particulate form mixed with the supply material.

Most advantageously the present invention includes the additional step of initially forming the supply material into a granulated form which can be easily poured. This should minimise spillage and contamination in the hot cell. The granules can be formed in a cold pressing operation, by disc granulation, by a spray drying/calcination or by fluidised bed/calcination process.

In a preferred and important embodiment of the invention, the supply material is initially charged into thin walled metal cans which will remain solid at the sintering temperatures used which are typically of the order of 1200°C. The metal can may have a close fitting lid and the supply material could be poured or cold pressed into the can before the lid is fitted. Preferably the lid is tight fitting so as largely to retain any components of the nuclear waste which are somewhat volatile at the high sintering temperatures. This step can be very important to the economics of operation since contamination of the hot cell by such volatile components can be largely minimised.

The thin walled metal can could have a close fitting lid rather like a paint tin and can be made of nickel or iron and indeed the choice of such metals can provide the preferred oxygen potential.

One useful material for the metal canister is stainless steel which is sufficiently corrosion resistant and has sufficient high temperature strength to be readily used in the present method.

One such steel is that known as Sandvik 253MA.

Typically heating to about 1260°C and the application of pressure of about 7MPA will be suitable sintering conditions. The pressure could be increased, for example, up to 14 MPA. However, in order to cause effective sintering and densification of the supply material, a practical limit exists as to the maximum height of a column of supply material. Therefore in a preferred embodiment of the invention the method includes using an apparatus in which the refractory support element includes a series of separate electrical induction heating coils disposed to apply selectively heat to regions extending respective distances along the axis of the metal canister, whereby a series of densification steps occur commencing at one end of the canister, the induction coils being utilised in sequence after the densification and sintering of the previous section of the supply material.

Most conveniently, water cooled induction coils in partially overlapping relationship are provided. During the method a constant pressure is applied to the supply material by means of a refractory faced plunger inserted into the open end of the canister and gradual densification occurs. At least prior to the final step of sintering it is most economic to top up the canister to compensate for the densification which has occurred up to that stage.

An additional quantity of supply material or an additional small can of supply material may be inserted before the final step. A close fitting refractory spacer is then inserted on top of the supply material to prevent the refractory faced plunger from entering the final heat zone.

The pressure most conveniently is applied from a lower supporting hydraulic ram and from a refractory faced metal ram in contact with the supply material. The refractory facing protects the metal ram from overheating. Water cooling of the metal ram may also be desirable.

The invention is best implemented in a manner which carefully minimises outward deformation of the metal canister and yet provides a long working life for the apparatus. In one advantageous embodiment a refractory, support element having a slightly tapered bore in which the canister is a clearance fit is used together with refractory grains which are poured into and compacted the space between the metal canister and the tapered bore so as to provide a relatively dense buffer to restrain substantially deformation of the metal canister during the densification step. The ejection step can simply comprise operating a bottom ram to press upwardly the canister which can slide relative to the grains and the grains can then fall through the cavity in the support element to be collected and recycled.

The method can include vibrating the refractory grains in order to provide a good density and resistance to deformation of the metal canister.

According to a second aspect of the invention there is provided an apparatus for use in the method as described in any one of the embodiments above in accordance with a first aspect of the invention; the apparatus comprises a refractory support element with a bore in which the metal canister containing the supply material is adapted to be placed with a clearance between the walls of the canister and the walls of the cavity, means being provided for introducing granular refractory material into the space between the metal canister and the wall of the cavity, means for compacting the granular material therein whereby outward deformation of the metal canister under heat and pressure is substantially restrained, means for applying heat in the sintering process to the supply material within the metal canister, means for applying densifying pressure in the axial direction of the canister, means being provided for removing the canister after the densification step and means being provided for collecting and reusing the granular material after removal of the canister.

Most preferably the apparatus includes induction heating coils which are water cooled.

In a commercially advantageous embodiment, the apparatus is adapted to handle a relatively long canister which might be up to approximately 3.6 metres long and up to approximately 375 mm in diameter; the apparatus in this embodiment should include a series of separate induction coils to permit densification and sintering of the supply material zone by zone from one end of the metal canister in separate steps thereby ensuring effective densification and sintering along the entire mass of supply material in the metal canister.

Preferably the zones overlap to ensure a continuous mass of properly densified material in the canister at the end of the process.

According to a third aspect of the invention there is provided a disposable element comprising a sealed metal canister containing a densified synthetic rock mass including, in the crystal structure, a minor proportion of nuclear reactor waste, the element being produced by the method of the first aspect of the invention or the apparatus of the second aspect of the invention.

For the purposes of exemplification only, embodiments of the invention will now be described with reference to the accompanying drawings, of which:-

• Figure 1 is a schematic elevation of an apparatus arranged for practising an embodiment of the invention;
• Figure 2 is a view on an enlarged scale of the central part of the apparatus of Figure 1 taken in axial cross sectional elevation;
• Figure 3 is a schematic view on an enlarged scale of a processed disposable element formed by using an embodiment of the invention;
• Figure 4 is a graph illustrating a typical applied pressure and temperature cycle;
• Figure 5 is a schematic representation of a second embodiment;
• Figure 6 is a schematic representation of a third embodiment before compressing; and
• Figure 7 is a view of the canister of Figure 6 after compression.

Referring first to Figure 1, the apparatus comprises a steel framework 1 supporting top and bottom hydraulic rams 2 and 3 and an electrical induction furnace 4 arranged to receive a metal canister 5 containing supply material 6 for densification and sintering within the metal canister. As shown in Figure 1, the bottom ram 3 has a head adapted to support the bottom of the canister, the head having a suitable refractory block 12 thereon and the top ram is adapted to have a plunger 7 with a refractory facing 7a which extends within the canister for applying pressure in the axial direction of the canister to the powdered supply material 6.

The induction furnace 4 comprises a block 8 of refractory material having sufficient tensile strength to withstand the substantial applied pressures and to absorb the forces tending to expand radially outwardly the metal canister. The refractory block 8 has a tapered central bore 9 for receiving a refractory granular in-fill for supporting the metal canister. Furthermore the block 8 includes a series of internally water cooled electrical induction coils 10.

Granular refractory material is poured from a hopper 20 when valve 21 is opened to fill the tapering space between the canister 5 and bore 9.

When the process is finished by ejection of the canister, the granular refractory material falls down into a collecting bin 22 from which it is pumped by pump 23 through line 24 back to the hopper 20.

Referring now to Figure 2, it will be seen that , compacted granular refractory in-fill 11 is disposed in the tapered annular space between the exterior of the circular cross section metal canister 5 and the bore 9 in the block 8.

It will also be seen that the induction coil 10 comprises a series of separate induction coil tappings which overlap one another, the respective end tappings being labelled A-A, B-B etc,

Figure 2 also shows the bottom ram 3 i^g capable of being moved upwardly through the cavity 9 for ejecting the final product,

A typical method of operation comprises the following steps;

• (i) With the top ram 2 retracted, the metal canister 5 having a closed bottom end is placed in the cavity 9 on top of the refractory block 12 which is in the position shown in the drawings.
• (ii) The nuclear waste material is mixed as a minor proportion with the components for forming the synthetic rock and readily poured granules are formed. A quantity of the granulated supply material is then poured into the metal canister 5 until it is substantially filled and the top ram 2 is lowered.
• (iii) The refractory granular material 11 is then poured into position and compacted for example by vibrating so that the metal canister is well supported against radially outward deformation.
• (iv) Pressure is applied by activating the hydraulic rams 2 and 3 to compact the supply material 6 in the metal canister. Typically a pressure of about 7MPA is applied.
• (v) Heating in the bottom zone only of the supply material is effected by connecting terminals A-A of the induction coil 10 to a power supply. A typical power supply operates at 3 KHz. Over a period of typically 45 minutes the temperature of the supply material in the zone A-A is brought up to a sintering temperature of about 1260°C and power is maintained for about 3 hours whilst maintaining the pressure.
• (vi) The induction coil portion A-A is then disconnected and the induction coil portion B-B connected to the power supply. It will be seen that a degree of overlapping occurs so that a continuous densified solid phase is produced in the metal canister. Each induction coil segment is activated in turn for a time of about 3 hours until only a small segment of supply material exists between the zone being densified and the ram facing 7a. The ram 2 is then withdrawn and the metal canister topped up with supply material and the method continues until just prior to the step of activating the induction coil segment G-G. Prior to this the refractory facing 7a is inserted to space and insulate the ram from the heated material.
• (vii) After densification of the top portion of the supply material has been completed, pressure is maintained and the element is allowed to cool to about 300°C. Pressure is then removed and the top refractory faced ram 2 is withdrawn.
• (viii)The bottom ram 3 is activated to eject the metal canister 5 from the induction furnace, simultaneously permitting the refractory granular material 11 to fall down to be collected in a recycling device.
• (ix) The excess top wall portion of the metal canister 5 is removed and a metal cap welded to close the canister. The canister can then be disposed of in a suitable geological formation.

Reference will now be made to Figure 3. Figure 3 illustrates a preferred embodiment of canister but is not to scale. In the preferred embodiment the metal canister 5 is formed with an integral bottom wall 6 and is typically of a 6 to 8 millimetres wall thickness and a diameter of 100 mm or more. Figure 3 illustrates the final unit after a cap 13 has been welded into position. Conveniently the metal is stainless steel of Sandvik grade 253 MA.

In this embodiment the supply material is introduced into the metal canister in thin-walled cans 14 having an integral base 15 and a press-fit lid 16. The cans could be similar to conventional paint tins and are preferably of a metal which provides the suitable oxygen potential to facilitate the incorporation of the waste into the synthetic rock. Thus the cans could be of nickel or iron or the like.

To form the unit of Figure 3 it is preferable initially to cold press or otherwise form the supply material into granules which are poured into the cans. Lids 16 are then press fitted. The cans are then inserted into the metal canister 5 when disposed as shown in Figure 2 prior to the densification operation. During the densification operation the cans, which conveniently correspond in height to each induction coil segment A-A, B-B etc. are compressed with the contained supply material thereby aiding in the retention of any volatile components in the supply material. Furthermore contamination of the apparatus of Figure 2 can be minimised by using this thin can technique. It has been found that the cans do not significantly buckle in their wall section but are compressed and come into intimate engagement with the interior of the metal canister 5. Figure 3 illustrates the final product with blocks of synthetic rock 18 within the thin walled metal cans 14. A refractory spacer 19a is left in the canister to fill the space.

The second embodiment of Figure 5 is characterised by the use of a metal canister 20 formed of stainless steel and having a bellows-like structure, the bellows-like structure preventing gross outward deformation of the canister during the pressing step. Figure 5 illustrates schematically the overall process and the apparatus which is to be used.

Outside the hot cell, non-radioactive synthetic rock precursor is produced as indicated by the step shown in Figure 5 labelled "SYNROC precursor". The synthetic rock has a composition as indicated in the table set out below and is produced using tetraisopropyl titanate and tetrabutyl zirconate as ultimate sources of TiO₂ and Zr0₂. The components are mixed with nitrate solutions of the other components, coprecipitated by addition of sodium hydroxide and then washed.

The precursor material is a product which possesses a very high surface area and functions as an effective ion exchange medium, which is mixed with additives and high level nuclear waste (HLW) in the form of nitrate solution to form a thick homogeneous slurry at mixing stage 21 which is located in a hot cell. Typically up to about 20% of the slurry may comprise the high level wastes.

The slurry is then fed by line 22 to a rotary kiln 23 operating at about 850⁰C in which the slurry is heated, devolatilised and calcined. The resulting calcine is mixed in mixer 24 with 2% by weight of metallic titanium powder supplied from hopper 25. The mixer 24 then supplies the powder to a primary canister 20 of stainless steel and of bellows-like form as illustrated. It will be noted from the drawings that the canister can be compressed by a factor of about 3 and does not have gross outward deformation. As illustrated in the drawing, before the mixer supplies powder to the canister 20, a thin perforated metal liner 26 is located within the canister and the space between the liner and the canister wall is filled with zirconium oxide powder 27 or alternatively any other powder possessing low thermal conductivity properties may be used. The canister can then be filled with powder 28 from the mixer 24.

A stainless steel plug or cap 29 is then used to seal the canister and the canister placed between a pair of pistons 30 which are of molybdenum-based alloy and capable of operation at temperatures up to 1200°C. A radio frequency induction coil 31 is then used to raise the temperature of the ends of the pistons 30 and the canister and its contents to about 1150 C.

When sufficient time has elapsed for a uniform temperature to exist in the synthetic rock powder, compressive forces are then applied through the pistons 30 causing the canister wall to collapse axially like a bellows.

The resultant sealed compressed canisters containing the synthetic rock structure are then removed and stacked in a disposable cylinder 31a which is fabricated from highly corrosion resistant alloy such as that based on Ki₃Fe. The space between the primary canisters 20 and the internal wall of the cylinder 31a is filled with molten lead 32 and the cylinder finally is sealed for disposal.

The embodiment of Figures 6 and 7 is a variation on the embodiment of Figure 5, the steps up to the mixer 24 of Figure 5 being the same. However in this embodiment the outer cylinder 40 and the bellows-like canister 41 are respectively dimensioned so that the clearance between the envelope of the canister 41 and the interior of the cylinder 40 is substantially taken up after the compression step, thus obviating the need for handling of the canister after compression to insert it into the cylinder and the pouring of lead to fill the cavity around the canister in the embodiment of Figure 5.

As shown in Figure 6, the cylinder 40 is supported Qn a base 4_'3 and the canister 41 inserted with an open-ended metal cylinder 41a located within the canister. Mix from mixer 24 is then poured into the canister to fill the zone within the cylinder 41a and a top cap 44 secured in position. The whole mass is then heated by a radio frequency induction coil 45 which surrounds the outer cylinder and after sufficient time has elapsed for a uniform temperature to be reached, a ram 46 having a piston-like face 47 is used to apply compression to the canister 41.

As shown schematically in Figure 7, the canister collapses with slight outward expansion of the canister but the arrangement is such that the walls of the cylinder 40 do not have any significant constraining effect on outward expansion of the bellows-like canister 41. During this collapsing, in practice the cylinder 41a crinkles somewhat but prevents substantial ingress of synthetic rock material into the zone of the bellows, thereby obviating the risk of insufficient compression in the bellows zone and improperly formed synthetic rock occuring between the bellows corrugations. In practice the adjacent corrugations of the bellows will come together in the compression step.

Figure 7 also illustrates how the induction coil 45 can be moved upwardly to the next location ready for treating the next canister which is to be inserted on top of the canister 41.