EP0044692B1

EP0044692B1 - Arrangements for containing waste material

Info

Publication number: EP0044692B1
Application number: EP81303221A
Authority: EP
Inventors: Eric John Ramm; Alfred Edward Ringwood
Original assignee: Australian Atomic Energy Commission; Australian National University; Australian Nuclear Science and Technology Organization
Current assignee: Australian Atomic Energy Commission; Australian National University
Priority date: 1980-07-15
Filing date: 1981-07-14
Publication date: 1986-10-08
Also published as: DE3175445D1; EP0044692A2; JPS57118200A; JPH0219920B2; EP0044692A3

Description

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 SAND750255 (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.
One method for containing radioactive waste is disclosed in EP application 81103570.8 (published after the filing data of this application as EP-A-0044381) of ASEA AB. This discloses a method for treating radioactive particulate or piece-formed material by enclosing the material in a gas-tight container and ompressing the material into a solid body at a necessary temperature and a necessary pressure, characterised in that the material is enclosed in a container with a corrugated sheath and that the material is converted into a solid body by compression at a high temperature. The conversion into a solid body involves isostatic pressing.
This invention relates to a method for forming solid blocks which include synthetic rock in which nuclear reactor waste is immobilised, the method comprising:-

(a) preparing a 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) establishing a quantity of the supply material in a metal canister which is sufficiently heat and corrosion resistant to contain the supply material during the method, providing means for preventing gross outward deformation of the metal canister during the method.
(c) applying heat and pressure to compress the supply material and to cause densification and the formation of a block of synthetic rock including the nuclear reactor waste; and
(d) either before or after said densification step, sealing the canister with a metal cap whereby the sealed canister is adapted to be placed in a suitable long term storage location. According to the invention, the means for preventing outward deformation comprises substantially only a bellows-like wall structure in the metal canister, the metal canister being of generally cylindrical form. The pressure is applied along the axis of the canister.

The bellows-like wall structure of the metal canister allows it to collapse 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 economic 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 high 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.
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 another 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 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 graph illustrating a typical applied pressure and temperature cycle;
Figure 2 is a schematic representation of an embodiment of the invention;
Figure 3 is a schematic representation of another embodiment before compressing; and
Figure 4 is a view of the canister of Figure 3 after compression.

The embodiment shown in Figure 2 is characterized 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 2 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 2 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 Ti0₂ 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 850°C 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 Ni₃Fe. 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 3 and 4 is a variation on the embodiment of Figure 2, the steps up to the mixer 24 of Figure 2 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 2.
As shown in Figure 3, the cylinder 40 is supported on a base 43 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 4, 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 occurring between the bellows corrugations. In practice the adjacent corrugations of the bellows will come together in the compression step.
Figure 4 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.

Claims

1. A method for forming solid blocks which include synthetic rock in which nuclear reactor waste is immobilised, the method comprising:-

(a) preparing a supply material (28) 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) establishing a quantity of the supply material (28) in a metal canister (20) which is sufficiently heat and corrosion resistant to contain the supply material (28) during the method, providing means (41) for preventing gross outward deformation of the metal canister during the method.

(c) applying heat and pressure to compress the supply material and to cause densification and the formation of a block of synthetic rock including the nuclear ractor waste; and

(d) either before or after said densification step, sealing the canister with a metal cap (29) whereby the sealed canister is adapted to be placed in a suitable long term storage location, characterized in that said means for preventing outward deformation comprises substantially only a bellows-like wall structure (41) in the metal canister (20), the metal canister being of generally cylindrical form and in that pressure is applied along the axis of the canister.

2. A method as claimed in Claim 1 and further characterized by locating a tubular screen (26) within the metal canister (20) and locating thermally insulating powder (27) between the screen and the interior wall of the canister.

3. A method as claimed in Claim 1 and further characterized by inserting an open ended metal cylinder (41a) within the metal canister (20) whereby to prevent substantial ingress of synthetic rock material into said bellows-like structure (41).

4. A method as claimed in Claim 1, and characterized by locating the metal canister (20) in an outer cylinder (40) and then applying said pressure to compress the metal canister, the compressed canister being looser than an interference fit in said outer cylinder.

5. A method as claimed in any one of Claims 1 to 4, characterized in that the bellows-like structure of the side wall comprises a series of convolutions extending from one axial end of the metal canister to the other.

6. A method as claimed in any one of the preceding Claims characterized in that the canister used in the method is formed of austenitic stainless steel.

7. A method as claimed in any one of the preceding Claims characterized in that the pressure applied to the supply material is in the region of 7 MPa.

8. A method as claimed in any one of the preceding Claims and further characterized by providing, in contact with the supply material, nickel, titanium, iron or other metal capable of providing a suitable oxygen potential to facilitate the effective incorporation of the waste into the synthetic rock.

9. A method as claimed in any one of the preceding Claims, and further characterized by forming the supply material into granulated form before loading the supply material into the metal canister (20).

10. A method as claimed in Claim 9 and further characterized by loading the supply material (28) into thin-walled metal cans which will remain solid at the densification temperature but will deform upon densification of the supply material as it forms synthetic rock, the metal cans being loaded in sequence into the metal canister (20).

11. A method as claimed in any one of the preceding Claims and further characterized by the metal canister (20) having substantially greater length than diameter and said densification is effected in a series of zones in sequence extending from one end of the metal canister by utilising a series of separately tapped electrical induction heating coils (31, 45) or heating in turn each zone.

12. A method as claimed in Claim 11 characterized in that the heating zones overlap one another.

13. A disposable element comprising an exterior sealed metal canister (20) containing a densified synthetic rock mass including in the crystal structure a minor proportion of nuclear reactor waste, the element being produced by a method as claimed in any one of the preceding Claims.