AU600685B2

AU600685B2 - Preparation of particulate radioactive waste

Info

Publication number: AU600685B2
Application number: AU64634/86A
Authority: AU
Inventors: Wilhelmus Joseph Buykx; Eric John Ramm; Alfred Edward Ringwood
Original assignee: Australian Atomic Energy Commission
Current assignee: Australian Atomic Energy Commission; Australian National University
Priority date: 1985-11-04
Filing date: 1986-11-30
Publication date: 1990-08-23
Anticipated expiration: 2006-10-31
Also published as: AU6463486A

Description

iuu -r COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Form Short Title: Int. Cl: PH03244 4TH NOVEMBER 1985 Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: This do:ument contains the amendments made under Section 49 and is correct for printing TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: AUSTRALIAN ATOMIC ENERGY COMMISSION and THE AUSTRALIAN NATIONAL UNIVERSITY New Illawarra Road, Lucas Heights, New South Wales 2234 and Acton, Australian Capital Territory 2601 ERIC JOHN RAMM, WILHELMUS JOSEPH BUYKX AND ALFRED EDWARD RINGWOOD GRIFFITH HASSEL FRAZER 71 YORK STREET SYDNEY NSW 2000

AUSTRALIA

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C C c c CC ccr cE Complete Specification for the invention entitled: PREPARATION OF PARTICULATE RADIOACTIVE WASTE The following statement is a full description of this invention, including the best method of performing it known to me/us:- 9377A/SB 1103 1 LheIAuslralia Natio .nal .Uni versity...

for and on behalf of THE AUSTRALIAN NATIONAL UNIVERSITY.

GRIFFITH HASSEL FRAZER, G.P.O. BOX 4164, SYDNEY N.S.W 2001

AUSTRALIA

8246A q The present invention is related to a process of forming an intimate mixture of radioactive nuclear waste material and synthetic rock-forming materials. This mixture may then be transformed into synthetic rock incorporating radioactive nuclear waste by a sintering pr6cess at high temperature and pressure.

In a known process of the this type, as described in the applicants, Australian patent no. 524883, initial preparation of the mixture comprises forming a composite slurry in which the waste (in the form of high level radioactive nuclear waste) is mixed with synthetic rock-forming materials. This slurry is fed to a rotary kiln operating at about 750 OC in which the slurry is heated, the substantial quantity of water in the slurry vaporised and the mixture calcined to form a particulate material. The particulate material is an intimate mixture 0.00 of nuclear waste material and the rock-forming materials.

O C0 o o The rotary kiln initiates mineral formation of the 0 0 0 .0 synthetic rock and in one embodiment the synthetic rock 020 materials used are those which produce in the final 0-4 synthetic rock three titanate materials (h llandite o 00 o BaAl.Ti60 1, zirconolite CaZrTi207 and perovskite CaTiO plus rutile titanium oxide TiO 2 and i. small amount of titanium metal. The titanium metal 0--"025 is for reducing purposes and preferably is added in 0 00 00.6 particulate form and mixed with the calcined powder before the mixture is poured into the container in which subsequently hot uniaxial ,,ressing takes place.

0 a Preferably, the container has a bellows-like wall.

030 The present invention is directed to new and useful 0 0 alternatives which avoid disadvantages inherent in the use 0 of a wet slurry for mixing the materials initially; it has now been found that a sufficiently intimate mixture of nuclear waste material and synthetic rock-forming materials can be achieved without wet slurry mixing.

It is now pointed out that some of the disadvantages of wet slurry mixing include the following: 449 6SISB-2 -2-

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To enable the composite slurry to be pumped through the associated equipment, it is necessary for the slurry to have a high water content. Therefore, the process creates, as an undesirable by-product, large amounts of high level radioactive-contaminated water which must be treated further to diminish its hazardous nature.

Much larger and more complex handling equipment is required in an active cell. This results in significant expense and much greater capital costs for an active cell, the cost of which is dependent on the volume of the cell.

A lengthy rotary kiln is required since a substantial part of the kiln is devoted to removing water in vapour form from the slurry before the calcining step in the process commences. Capital cost penalties are inherent in this feature.

d) A relatively expensive off-gas treatment system is required to deal with the considerable volumes of 20 gases emerging from the rotary kiln.

i In general terms the present invention is directed at least partially towards avoiding disadvantages now noted in I the slurry process and is aimed at providing a simplified process for producing the particulate material for loading into the containers in which the particulate material is to be hot pressed. Although application of the present I invention to the production of particulate material for a i hot uniaxial pressing is an important application of the d present invention, the present invention is not limited in this regard and may equally be applied to the production of

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j material for the formation of synthetic rock incorporating nuclear waste by other processes such as hot isostatic processes.

According to one aspect of the present invention, there is provided a process for providing particulate material suitable for the formation of synthetic rock under heat and pressure, the process comprising supplying 4496S/CF I C 4 matsng synthetic rock-forming components in dry particulate form to a continuous mixer adapted to advance particulate material along a mixer path from an inlet to an outlet; supplying radioactive nuclear waste in liquid phase to the particulate components at locations spaced along the mixer path and operating the mixer to cause the waste to be subitantially uniformly distributed through the synthetic rock components and at the same time heating the synthetic rock components and waste to maintain a substantially dry t particulate material along said mixer path; calcining the particulate material to produce in powder form a precursor comprising synthetic rock-forming components and radioactive nuclear waste distributed therethrough, the precursor being adapted to be used in a hot pressing process to form synthetic rock throughout which the radioactive nuclear waste is distributed and immobilised.

In this specification the nuclear waste (such as high level radioactive nuclear waste from spent fuel rods of a nuclear reactor) is referred to as being added in the liquid t phase. The expression "liquid phase" is to be understood as including not only solutions but also other dispersions in a liquid carrier such as suspensions.

Various further inventive features may be included in different embodiments of the invention as described hereinafter. Broad,ly, however, the invention permits the use of synthetic rock components already in a dry particulate form, these components being prepared by any convenient process outside an active cell thereby minimising expensive active cell space, removing restrictions concerning processes operated by remote manipulators and facilitating economic production of the components. Preferably the components are in a readily pourable form. It has been found that a highly uniform 4496S/MR I OA* L uj lu

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c- 4A-- intimate dispersion of the nuclear waste through the synthetic rock can be achieved and furthermore the spacial requirements and off-gas requirements within the active cell are substantially reduced.

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i -r 5 I In a preferred embodiment of the invention, the mixer includes an off-gas system for collecting and processing water vapour and other vaporised components and entrained particulate matter from the radioactive waste synthetic rock mixture.

The form of the mixer may, with advantage, be chosen from the group comprising a screw conveyor mixer and a rotating paddle mixer having an approximately horizontally directed mixing path.

Heating in the mixer can be effected by any suitable means including induction heating, hot gas, oil or v microwave heating.

Preferably, the calcining step is effected in a calcining stage downstream of the mixer at which S 15 temperatures substantially greater than those prevailing in the mixer are used; for example, in the mixer temperatures i of about 300 0 C may be achieved whereas in the calcining !i stage temperatures of about 750 C are required.

The calcining stage may be conducted in a rotary kiln or alternatively may be effected in an apparatus selected i from the group consisting of a screw conveyor, a i Spaddle-type conveyor (which may be an extension of the i mixer portion of the apparatus), a downwardly directed i multiple parallel tube device leading to a receiving hopper S 25 and a downwardly directed tube device having one or just a small number Df relatively large diameter tubes in which heating takes place.

Advantageously the calcined particulate material is received from the calcining stage and is conveyed by a screw conveyor to a filling station for containers for the hot pressing operation. Since the particulate material must be poured into an open topped container, it is very desirable to maintain the form of the particulate material to facilitate very reliable pouring bearing in mind that the process must be conducted with manipulators in an active cell. Embodiments of the invention may facilitate the production of the particulate material in a convenient 4496S/MR A A

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T ILI~ I form for this purpose.

In the calcining stage, heat may be applied by any convenient means such as resistance heating. inductive heating, hot gas heating or microwave heating.

Particularly if a relatively wide downwardly directed tube calciner is used, microwave heating may be an effective means of providing substantially uniform calcining temperatures through the body of material and since a relatively wide tube structure is utilized this can be advantageous in maintaining flow of the particulate material so that reliable operation of the apparatus is achieved.

There are various preferred embodiments with respect to the initial preparation of the synthetic rock components outside the active cell. The advantageous embodiments involving further inventive features include the following: S The synthetic rock components are initially formed in GO 0 O o. an aqueous slurry which is dewatered to form a damp cake of solid particles which is then dried at relatively low temperature 130 C) to form a *Ott dry powder. This powder is conditioned if necessary tit to be in a flowable form for supply to the mixing apparatus.

The powder produced by the process in above may be 25 additionally processed by precompaction to take on a granulated form and this product is then presintered at a low temperature e.g. about 300 C to improve the mechanical strength of the granules which then S provides the feedstock to the mixer. This feedstock has a readily flowable form yet can be produced S relatively easily and economically.

The process of above may be further modified to provide the granules in spheridised form to further enhance the flow characteristics of the product.

A slurry of synthetic rock components may be spray dried to form fine particles which are then subjected to a presintering at low temperature e.g. about 4496S/CF -6t- 300 C to provide a powder feedstock for the mixer.

A highly pourable powder may be formed by a sol-gel process followed by a presintering.

Yet a further important and additional inventive feature now proposed concerns the control of the gases in contact with the mixture of radioactive waste and synthetic rock component during the calcining stage. This inventive feature can be incorporated in a preferred embodiment of the dry mixing invention described herein but may also be applicable as an improvement to the process of applying a mixed slurry to a rotary kiln as described in the applicants' previous patent 524883. More particularly the control of the gaseous atmosphere concerns the maintenance of a reducing gas of a non-explosive constitution which is passed over the material during calcining. It is thought that during calcining, nitrates of the radioactive waste I edecompose to provide some nitrous oxide gas in sufficient "ao0, quantities during the high temperature portion of the calcining step so that adverse effects may occur in the synthetic rock structure.

0000o Preferably this inventive aspect is implemented by "O Ii. having a gas which comprises substantially pure hydrogen; the gas is preferably pure hydroge' but could be a mixture o of hydrogen with an inert gas sucrf as nitrogen in which 25 case the mixture must comprise at least 80% hydrogen to avoid an explosive mixture being utilized.

$1 ,This gas is passed continuously through the calciner "i and mixer and takes away any off-gases including any radioactive gases that might be produced, the gases then being processed through a filtration system to remove any

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radioactive components.

*One advantage of using pure hydrogen is a reduction of residence time (from 60 minutes to 10 minutes) in the calciner, thereby reducing the length of the calciner.

Other advantages are the maintenance of low oxygen partial pressure in the calciner and minimum ruthenium volatilisation in drying and calcination leading to 4496S/CF improved synthetic rock quality and less contamination of the off-gas system.

According to a second aspect of the invention, there is provided apparatus having means for implementing the process steps described above in any one of the embodiments and in a third aspect of the invention there is provided a particulate material produced by the process or apparatus.

The invention will be further described, by way of example, with reference to the accompanying drawings of which:- Figure 1 is a schematic diagram of one form of inventive apparatus for carrying out a process according to a first embodiment of the present invention; Figure 2 is a schematic diagram of a second form of 15 inventive apparatus for carrying out a process according to o0° a second embodiment of the present invention; o o Figure 3 is a schematic diagram of a third embodiment 00 a 0064' of the invention; and "00 Figure 4 is a schematic view of a fourth embodiment of 0a 0 '20 the invention.

Each figure shows an apparatus for use in a process for immobilising high level radioactive nuclear waste in a S synthetic rock. Figure 1 shows an apparatus having a horizontal screw conveyor 1 for mixing and calcining high level nuclear waste and synthetic rock forming components which are introduced through hopper 10, and for supplying the product to a vertically downwardly directed second screw conveyor 2 for mixing in a small proportion of titanium powder through an inlet 12 and then discharging the resultant mixture into a receiving hopper 13.

Subsequent stages, described in more detail below, cause the powder to be subjected to high temperature and pressure to form a synthetic rock in which the high level radioactive waste is immobilised.

At the input end of the first conveyor 1, a motor 3 is arranged to drive an elongated mixing and conveying screw 4 which co-operates with a housing of the conveyor.

4496S/CF 8-- 7 Similarly, and at the top of the second conveyor 2, a second motor 5 is arranged to drive a second screw 6 which conveys material from the top of the conveyor 2 to the hopper 13.

At a location approximately halfway along the first conveyor 1i, a gas outlet 7 is provided for the removal of gases emitted within the conveyor 1 along with a purging gas flow of hydrogen and nitrogen (or more preferably pure hydrogen) which is introduced in a counter-current manner through an inlet 8 at the downstream end of the conveyor 1. Substantially pure hydrogen is the preferred purging gas since it is non.-explosive and has an advantageous reducing effect on the particles of material during the calcining step.

*od 15 The pure hydrogen or substantially pure hydrogen allows the use of a short screw conveyor in the calcining region.

Along the length of the first conveyor 1 a heating element 9 (such as an induction coil) is provided. This 1 20 serves a dual function. Firstly, the heating element 9 causes evaporation of any water present in the supply ^powder, which consists of synthetic rock forming components and nuclear waste. This particulate material normally will be initially damp. Secondly, the heating element 9 causes calcination of the powder particularly towards the downstream end of the conveyor so that a dry particulate structure exists in the powder discharged; during calcination initiation has taken place of the formation of mineral phases in the synthetic rock components.

In the drawing the calcining portion 11 of the conveyor i' indicated near the downstream end.

Particulate material discharged into the second conveyor 2 is moved gradually downwardly and intimately mixed with titanium powder added through an inlet 12.

The particulate material received in hopper 13 is available for supply to the ho, pressing operation which in this preferred embodiment is conducted generally as 4496S/CF described in the applicants' abovementioned prior patent specification using a metal container 14 having a bellows-like cylindrical wall. A~s shown in Figure 1, each container 14 is filled with mixture from hopper 13 and then transported from below the discharge hopper 13 by a conveyor 16 through a series of stations A, B and C in which successively each filled container receives a lid and is welded closed, decontaminated and weighed. A manipulator 15 then transfers each container 14 in turn to a pre-compaction press 17 where the container is partially uniaxially compressed. Subsequently, each partially-compressed container 1.4 is transferred to a hot uniaxial pressing station 18 at which the bellows-type container J.4 is heated to about 1200 0Cand then uniaxial pressed at 14-21 MPa to form in the container synthetic rock with high level radioactive nuclear waste immobilised therein. The pressing stage has an induction heating coil 18a, a metal susceptor sleeve l8b, a shroud 18c, a fixed upper abutment 18d and a hydraulic ram 18c.

Figure 2 illustrates an embodiment which includes modifications only upstream of the container filling stage. Like reference numerals have been used for parts corresponding to those shown in Figure 1 and only the distinctions will now be described.

Referring now to Figure 2, the two chamber apparatus of Figure 1 is modified to provide a three chamber arrangement in which the calciner section 1.1' is formed in a vertical parallel tube calciner 20. The heating element 9 associated with the first conveyor 1' simply has an effect of drying the feedstock and a downstream end portion 4' of the screw conveyor distributes the feedstock through a series of apertures 21 for distributing the feedstock into the vertical tubes 22. The vertical tubes are spaced from one another and surrounded by respective heating coils 23 imbedded in refractory material 24.

Calcined powder is discharged through the calciner section 111 into a discharge hopper 25, the downstream 4496S/CF 0-

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throat of which incorporates the purge gas inlet The purge gas thus passes up through the calciner section ii' and into the downstream end of the conveyor 1' In Figure 2. the discharge screw conveyor instead of being vertical, as in Figure k .s a horizontal conveyor 2' having an inlet 12' for titanium powder and a downward discharge 13' for discharging the mixed powder.

In the embodiment of Figure 2, the form of the inlet manifold 21 can be altered such that the conveyor 1' discharges into a narrow neck of an inlet manifold which flares outwardly to a width corresponding to that of the vertical tube assembly.

Referring now to Figure 3, there is schematically shown an alternative embodiment in which, instead of using a screw conveyor, a paddle-type conveyor 40 is used. The conveyor has a central shaft 40a, paddes 40b spaced therealong for mixing and advancing powder in the conveyor, and a cylindrical housing 40c. The housing has at its upstream end an inlet hopper 41. for receiving synthetic rock forming components in the form of granules or in a form derived from a sol--gel process, this material being conveyed progressively downstream to be mixed gradually with high level nuclear waste admitted in liquid phase through a series of spaced inlets 42. An intimate mixure is provided and operation of a heater 43 causes water components of the high level waste to be vaporised and discharged through gas outlet 44 which is connected to a low pressure gas filtration system.

!I The intimate dry mixture of high level waste and i 30 synthetic rock components is discharged into a rotary kiln in which calcining takes place, the product being discharged into a hopper 46. A reducing gas flow is passed from inlet 47 through the calciner to discharge through outlet 44.

Yet a further embodiment is shown in Figure 4. This embodiment is similar to Figure 3 except that the rotary kiln is replaced by a wide single tube vertical calciner 4496S/SB -11- 3 o having suitable windows 51 connected to respective microwave energy guides 52 for calcining the product as it moves down in a column through the calciner. At the base of the calciner, an inlet 53 is provided for a purge gas (of pure hydrogen or a mixture of approximately 3% hydrogen in'nitrogen). In Figure 4, there is a horizontal screw conveyor 54 for discharging the granulated product into a receiving hopper 55 from which bellows containers may be filled for the subsequent stages of the process. The conveyor 54 preferably has a feed hopper for titanium metal powder corresponding to the inlet 12' shown in Figure 2.

The above described embodiments advantageously provide for intimate mixing of high level radioactive nuclear waste (normally supplied as a suspension/solution) with synthetic o..I 15 rock forming powders which can be economically and conveniently handled in dry form and excellent uniformity of dispersion of the nuclear waste has been found in the resultant synthetic rock structure. Since a substantially dry process is involved, much less water is present initially, and because all water requires evaporation in a stage immediately prior to the calcining step there is the advantage of little waste to evaporate. The relatively dry process permits simplification and a reduction of demand placed on an off-gas filtration system. Most importantly a relatively simple process and apparatus can be designed and this is very important for successful long-term active cell operations.

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Claims

2. A process as claimed in claim 1 wherein an off-gas system is provided and operating for collecting and processing water vapour and other vaporised components and entrained particulate matter from the radioactive waste synthetic rock mixture.
3. A process as claimed in claim 1 or claim 2 wherein the mixer is a screw conveyor mixer or a rotating paddle mixer having an approximately horizontally directed mixing path.
4. A process as claimed in any one of the preceding claims, and wherein the calcining is effected in a calcining stage downstream of the mixer at temperatures substantially greater than those prevailing in the mixer.
5. A process as claimed in claim 4 and wherein temperatures of the order of 300 0 C are used in the mixer and temperatures of the order of 750 0 C are used in the 4496S/MR N u 7 14 calcining stage.
6. A process as claimed in any one of the preceding claims wherein the calcining stage is effected in a rotary kiln downstream of a separate apparatus for effecting the mixing stage.
7. A process as claimed in any one of claims 1 to and wherein the calcining stage is provided at the downstream region of a screw conveyor or a paddle-type conveyor, the upstream portion of which provides the mixing stage.
8. A process as claimed in any one of claims 1 to wherein the calcining stage is effected in a tubular r calciner having a substantially vertically downwardly directed duct.
9. A process as claimed in any one of claims 1 to or claim 8, and wherein the calcining stage includes the ?7- use of microwave heating means. A process as claimed in any one of the preceding claims and including the pLliminary step of forming the synthetic rock components as an aqueous slurry dewatered to form a damp cake of solid particles, drying said damp cake at relatively low temperatures compared with those used in the subsequent process stages, and manipulating the powder to be in a flowable form.
11. A process as claimed in claim 10, and wherein the process includes precompaction said damp cake to form the synthetic rock components into granulated form, and presintering at a temperature of the order of 300 0 C to improve the mechanical strength of the granules, whereby a flowable product is provided.
12. A process as claimed in any one of claims 1 to 9 and including the preliminary step of forming the synthetic rock components into a slurry and spray drying the slurry to form fine particles, and presintering the fine particles at temperatures of the order of 300 0 C to provide a feedstock in powder form for said mixer.
13. A process as claimed in any one of claims I to 9 and wherein in a preliminary stage the synthetic rock, 6 S /MR components are formed in a sol-gel process, rollowed by sintering the material at temperatures of the order of 300 0 C to produce a flowable powder.
14. A process as claimed in any one of the preceding claims and including maintaining a reducing gas atmosphere of non-explosive constitution in the calcining stage and circulating the gas through an off-gas system and operating means to remove any radioactive components in the off-gas. A process as claimed in claim 14 and wherein the reducing gas comprises substantially pure hydrogen, a mixture of hydrogen and nitrogen and comprising at least hydrogen, or a mixture of hydrogen and nitrogen comprising approximately 3% hydrogen, all percentages being by weight. 15 16. A process as claimed in any one of the preceding 6 claims in combination with additional stages comprising S filling bellows-like containers with the calcined o: product in a pouring process, the containers each having a S l bellows-like cylindrical side wall, 20 closing each bellows-like container in turn with a lid and welding the container closed, and effecting hot uniaxial pressing of each container in turn to cause densification of the contents of the container and the formation of synthetic rock with the radioactive waste immobilised therethrough.
17. Apparatus for practising the process as claimed in claim 1 and comprising mixing conveyor means for receiving in particulate form synthetic rock forming components and for receiving nuclear waste, the conveyor means being arranged for conveying the rock components and waste while mixing the waste and said components, the conveyor means comprising a generally cylindrical housing extending substantially horizontally, heating means being provided for drying the mixture in the conveyor means, a calcining conveyor arranged to receive the mixture of said waste and rock forming components and having heating means for calcining the mixture as it is advanced to produce calcined particulate material, an<1 means for discharging 4496S/SB A the calcined particulate material into containers for disposal purposes.
18. An apparatus as claimed ill claim 17 and including gas take -off means disposed substantially at the upstream end of the calcining conveyor and a reducing gas inlet means disposed in the region of the downstream end of the calcining conveyor, the calcining conveyor conveyor being part of a gas circulation system.
19. Apparatus as claimed in claim 17 or 18 and indcluding a screw conveyor for handling the discharged particulate material from the calcining conveyor and having means for admixing a minor proportion of titanium powder into the final mixture. Apparatus as claimed in any one of claims 17 19 15 and wherein the calcining conveyor comprises a vertically S4"sP downwardly extending tube arrangement down which the particulate material falls under gravity while being heated. S, 21. Apparatus as claimed in claim 20 and wherein the tube arrangement comprises a relatively wide diameter tube 20 with windows spaced along the sides thereof and being transparent to microwave energy, the windows being connected to wave guides for guiding microwave energy into S. the interior of the tube for heating and calcining of the particulate material.
22. A process of immobilising radioactive waste material substantially as herein described with reference to any one of the accompanying drawings.
23. Apparatus for immobilising nuclear waste substantially as herein described with reference to any one of the accompanying drawings. Dated this 31st day of October 1986 AUSTRALIAN ATOMIC ENERGY COMMISSION AND THE AUSTRALIAN NATIONAL UNIVERSITY By their Patent Attorneys GRIFFITH, HASSEL FRAZER 4496S/SB -16--