AU592669B2 - Encapsulation of waste materials - Google Patents

Description

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ii 592669 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 Form COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: PH08262 Lodged: 30 September 1986 Complete Specification-Lodged: Accepted: Lapsed: Published: I I .1 I t

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Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventor: AUSTRALIAN NUCLEAR SCIENCE AND TECHNOLOGY ORGANISATION New Illawarra Road, Lucas Heights, New South Wales, 2234, Australia ERIC JOHN RAMM Address for Service: GRIFFITH HASSEL FRAZER 71 YORK STREET SYDNEY NSW 2000

AUSTRALIA

Complete Specification for the invention entitled: "ENCAPSULATION OF WASTE MATERIALS" The following statement is a full description of this invention, including the best method of performing it known to me/us:- 2339A/MB rI i i I I -sc~l ,ai a 09 a a a a The present invention relates to the encapsulation of radioactive products such as unprocessed spent fuel or high level nuclear waste calcine.

High level radioactive waste usually is in the form of a solution of the waste. This solution can be calcined at 600°C 800 0 C temperature to provide the waste in powder form. The waste must be safely encapsulated in some system which is corrosion and heat resistent and will prevent the waste coming into contact with any ground water so that the possibility of contamination of ground water through leaching is avoided.

Alternatively, spent fuel rods could be simply disposed a of in a safe containment system.

a, Various prior proposals have been published in relation .15 to safe disposal of nuclear waste. One proposal is to immobilise the waste as a minor component of a synthetic rock matrix which is produced from mineral precursor components and the waste in powder form. The matrix is produced under conditions of high temperature and high 20 pressure maintained for several hours. Suitable synthetic rock structures have been published by A.E. Ringwood et al, see for example the following:- Nature March 1979 iEuropean Patent Application 79301382.2 U.S. Patent Application 124953 noi c.S. pc~e r 42q s Engineering techniques for the production of synthetic rock incorporating the high level waste include hot isostatic pressing processes (see for example European Patent specification 0044381 assigned to ASEA) and hot uniaxial processes (see Australian patent application 18163/83 and it is equivalent USA patent 4,645,624 assigned to the present applicants and The Australian National University).

Another publication by ASEA (see United States Patent Specification 4209420 H. Larker) concerns a suggested process in which a ceramic canister (formed by hot isostatic pressing) is filled with a unprocessed spent nuclear fuel ~3T\ waste and a lid is placed in position. A gas-tight shroud Ca a a a a

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-2of metal is placed around the canister. Hot isostatic pressing at very high pressure is specified for the purpose of joining the lid to the canister body.

Another proposal for nuclear fuel storage has been made in a paper entitled "Final Storage of Spent Nuclear Fuel" by Swedish Nuclear Fuel Supply Company (KBS Division). A massiva copper canister having relatively thick walls receives spent nuclear fuel rods and copper powder fills the voids around the rods and within the canister. A lid is applied and the canister placed in a thin walled container.

Evacuation of air from within the canister takes place and the outer container can then be sealed and passed to a hot isostatic pressing step operating typically at 500 0 C and the very high pressure of 150 MPa. The very high pressure is necessary to bond the lid to the container by sintering.

SThe proposals described in the previous two paragraphs are widely questioned as riot viable on grounds including feasability and safety because of the very high pressures involved.

unprocessed spent nuclear fuel in a dense graphite nickel S, sulphide bonded matrix. It appears such a matrix can be formed at about 500 0 C but it is questionable whether the matrix would be considered safe for long-term storage because of an inherent brittle characteristic with the attendent risk of fracture and leaching of radioactive ,tit components by ground water. Furthermore, the long-term stability of the matrix in terms of its resistance to leaching does not appear to be established.

Accordingly, over a period of many years there have been extensive investigations at considerable cost in to many different kinds of systems for safely immobilising radioactive waste for disposal. With a view to providing a system having commercial attractivness with inherent acceptable safety, and the practicality to operate on a long-term basis, the present invention has been devised and provides new and useful alternatives to previous published proposals.

5170S/MB -3- -1 According to a first aspect of the present invention, there is provided a method of immobilising a waste material comprising taking a first metal container, the metal being highly resistant to corrosion and maintaining substantial strength at temperatures used in the process, filling the first container with the waste material or a combination of the waste material and protective powder material which is either a ceramic powder or metal powder of a metal which is highly resistant to corrosion, said ceramic powder or metal powder being such that the waste material can be distributed therethrough and immobilised therein, substantially evacuating the first container and closing the first container, placing the first container in an outer second container and introducing a protective powder material to support said first container within the second container S and spaced from the walls of the second container, the S ,20 protective powder material comprising either a ceramic 4! Npowder or a metal powder, i substantially evacuating the second container and closing the second container, and heating said containers and their contents to an l 25 elevated temperature an applying external pressure, the i temperatures and pressures being selected such that the Swaste material becomes bound in a dense solid matrix of the ceramic powder or metal powder and a dense protective Sshell is provided by the first container, the second 30 container and a solid dense matrix formed therebetween rr from said protective powder.

The invention is particularly significant where the waste material is high level nuclear waste calcine or unprocessed spent nuclear fuel rods.

Heating at least partially up to the level required for the pressing step be effected as a preliminary step in separate apparatus such as a continuous preheating furnace.

s/HM -4- 5'170S/HM -4- The method can be carried out advantageously at pressures of the order of 20 MPa, eg. 20-25 MPa.

SPreferably, the first metal container is generally cylindrical and has a base and side walls incorporating a bellows-like convolution to permit the container during the i process to be compressed with substantial reduction in axial length and relatively little change in radial dimensions or distortion.

Preferably, the outer second container is of generally cylindrical form having a cylindrical side wall incorporating a bellows-like convolution.

i Surprisingly, effective encapsulation of waste materials can be achieved when said external pressure is in the form of a hot isostatic pressing process, which may utilise I 15 surprisingly low temperatures and pressures, at least with preferred embodiments of the invention.

In a preferred embodiment, the metal powder is copper or :Ill a copper alloy which should flow effectively to form a highly ,corrosion resistant and safe sheath around waste material with isostatic applied pressures of about 20MPa at a temperature of i about 800 0 C, which is maintained for about 1 hours for a Scommercial size pressing e.g. in the order of 800 mm diameter and initial length of about 1800 mm.

A most significant embodiment of the present invention is one wherein there is provided a method of immobilising a nuclear waste material comprising: i taking a relatively thin walled first metal container, the metal being of an alloy which is highly resistant to corrosion (such as Inconel 601), the first container being shaped and dimensioned to be deformable under the temperature and pressure conditions used in the process, filling the first container with the nuclear waste material and a metal protective powder material which is copper or a copper alloy highly resistant to corrosion, substantially evacuating the first container and closing the first container, 5170S/HM i1 i

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placing the first container in an outer relatively thin walled second container of metal alloy which is highly resistant to corrosion such as Inconel 601 and introducing further protective powder material to fill the space between the first container and the second container and spacing the containers apart, the protective powder material being copper powder or powder of a copper alloy highly resistant to corrosion, substantially evacuating the second container, and closing the second container, and heating said containers and their contents to an elevated temperature and effecting hot isostatic pressing, the temperatures and pressures being selected such that the respective containers retain sufficient strength to fulfil a containment function for the nuclear waste during the process and to permit under the applied pressures flow of the protective powder material to form a dense solid matrix enshrouding the spent nuclear waste.

Advantagously it has been found that the invention can be 20 practiced at pressures of about 15 to 30 MPa and most preferably about 20 MPa and temperatures in the region of 800 to 850 0 C are considered effective.

Preferably, the invention is implemented in corrosion resistant metal containers having relatively thin walls of the order of 3 mm thick and with such a wall thickness commercial size pressings in the order of 800 mm diameter and initial length of 1,800 mm can be formed. The spacing between the inner container and the outer container is at least an order of magnitude greater than the wall thicknesses of either the first container or the outer container and typically would be about 10% of the diameter of the second container e.g. about mm. Thus this embodiment of the invention results in a dense copper matrix surrounding the nuclear waste material, the dense copper shroud being at least 75 mm thick and thus providing an extremely safe long term effective containment.

Most preferably each of the first container and the outer second container has rigid plane end walls and cylindrical io5170S/HM -6- ~r ~Y rr Yi~ ii-. side walls incorporating a bellows-like convolution. This arrangement facilitates progressive compression and in particular axial compression during the pressing stage.

Of great significance is the relatively low operating pressure which may be compared with prior art proposals which require of the order of 150 MPa to deform the primary container and to bond the lid onto tho container. Embodiments of the invention can greatly reduce the major problems of safely incorporating the equipment into an active cell.

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Because the pressures are lower, the design of the sealing arrangements is so much easier and servicing and maintainance may be considered viable with remote handling equipment.

Thus, use of embodiments of the invention permits a viable and effective system to be designed for use in an active cell with radioactive waste and results in a capsule which is highly resistant to the escape of the waste mnaterials over very extended storage periods.

An important embodiment of the invention is to the encapsulation of spent nuclear fuel rods which are straight or coiled into a spiral. The fuel rods afe placed in the inner container and surrounded with the powder and it has been found X' by the use of copper powder at temperatures as low as about 800 0 C effective metal flow and encapsulation can be achieved.

0#15 Furthermore, the inner container by virtue of its surrounding sheath structure is heavily reinforced and provides a very *~secure containment structure both during the encapsulation process and for subsequent storage.

Preferably, the method includes using a particulate filter and off-gas connection to said inner container to t Ift facilitate the step of evacuating the inner container through t *I the off-gas system and filter before sealing the off-gas pipe.

Preferably, a similar arrangement is also provided for the outer container, whereby the outer container is also evacuated and the off-gas pipe sealed before the hot pressing step.

list A suitable metal for use in forming both the inner and outer containers is Inconel 601, which is highly corrosion resistant and has good strength at elevated temperatures.

Another application is to the case where the waste material is a high level radioactive waste calcine.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings which are schematic sectional side elevations illustrating the method embodying the present invention and comprising; Figure 1A illustrates filling an innev container in which the waste material is in the form of spent fuel rods; Figure lB illustrates an alternative in which waste material in particulate form is processed; 517 OS/MB-7 -7- Figure 2 illustrates evacuation and closure of the inner container of Figure lA; Figure 3 illustrates placing the inner container in an outer container; Figure 4 illustrates evacuation and closure of the outer container; Figure 5 illustrates hot isostatic pressing of the container combination; and Figure 6 illustrates the hot, isostatically pressed combination.

The method utilises an inner bellows container 110 of Inconel 601 of cylindrical form with a convoluted bellows-like peripheral wall and rigid flat ends. The base end wall 111 is welded to the bellows-like wall, to the top of which an annular top flange 112 is welded. In the illustrated S embodiment of Figure lA and Figures 2-6, spent nuclear fuel a a rods 113 are inserted into the inner container 110 with a protective powder 114 separating the fuel rods from the walls of the inner container 110 and from adjacent rods. In a preferred embodiment copper powder is used and vibration is S applied during the filling operation to acheive maximum Spacking density.

Figure lB illustrates an alternative in which calcined I aq *fit a a high level nuclear waste in particulate form is poured into the container or mixed with the protective powder and poured into the container 110; thus particulate radioactive material 114A fills the container.

,l As shown in Figure 2, in the second step of the method a lid 115 is welded onto the upper flange 112, the lid in its central region having a short upstanding off-gas pipe 116 in communication with the downstream side of a rigid particle filter 117. In the method, the lid 115 is welded into position and the interior of the container evacuated through pipe 116. The pipe is then closed by welding. The closed container then advances to step 3, which is shown in Figure 3, wherein the container is placed within an outer similarly shaped container 118 with protective powder 119 (which again is preferably copper powder) separating the inner container from the outer container. The preferred method is to pour copper powder into the outer container 118 to a level slightly 5170S/MB -8- -1 below an internal collar 124 and then to place the inner container 110 in the collar. Further copper powder is then poured in to fill the container 118 and vibration again is used to achieve high packing density. Any space can then be filled with further copper powder to top-up the container.

From Figure 3 it will be seen that the inner container 110 is supported on a transverse perforated support base 123 with collar 124 attached to its top face.

ii As shown in Figure 4, a similar lid 120 is welded onto the outer container and evacuation takes place through its off-gas pipe 121, the gas being filtered through the particle filter 122.

The final step of the method is shown in Figure 5. A hot '#isostatic pressing furnace 125 is schematically shown in Figure 5 and comprises a rigid, cylindrical wall 126, a rigid j I. top 127 to which the cylindrical wall is sealed and a removable base closure pad 128 which can be sealingly engaged with the cylindrical wall to form a pressure vessel. A cylindrical array of heating elements 128A are arranged within the pressure vessel for raising the temperature to that which 4 t P t is desired and the pressure vessel has an inlet for I I t aft~ pressurised inert gas which is disposed in the zone 129 surrounding the outer bellows-like container 118.

t It is preferred to preheat the container 118 and its contents in a separate preheating furnace and then to place the preheated container on pad 128 and then insert into the isostatic pressing furnace. The furnace is then pressure sealed and the gas within the furnace raised to the required temperature and pressure e.g. 800 0 C and 20 MPa. The powder 114 and 119 densifies and flows to form a dense mass and the inner and outer containers 110 and 118 are compressed generally in a uniaxial fashion as the containers have substantial radial stiffness but little axial stiffness.

These conditions are maintained for about 1 hour until a uniform structure is achieved in which, in this example, the spent fuel rods are safely encapsulated in a dense copper sheath which also incorporates the corrosion resistant inner and outer containers 110 and 118.

Figure 6 shows the final container combination after the 5170 S/MB -9- I_ _J hot isostatic pressing process. This unit is of substantially cylindrical form and adapted to be deposited in a safe subteranian installation, the protective material around the nuclear waste providing an effective and very long term shroud resistant to decay, corrosion and leaching by ground water.

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Claims (10)

1. A method of immobilising a waste material comprising taking a first metal container, the metal being highly resistant to corrosion and maintaining substantial 5 strength at temperatures used in the process, filling the first container with the waste material or a combination of the waste material and protective powder material which is either a ceramic powder or metal powder of a metal which is highly resistant to corrosion, said ceramic powder or metal powder being such that the waste material can be distributed therethrough and immobilised therein, substantially evacuating the first container and closing the first container, placing the first container in an outer second 15 container and introducing a protective powder material to support said first container within the second container and spaced from the walls of the second container, the protective powder material comprising either a ceramic powder or a metal powder, substantially evacuating the second container and closing the second container, and heating said containers and their contents to an elevated temperature and applying external pressure, the temperatures and pressures being selected such that the waste material becomes bound in a dense solid matrix of the ceramic powder or metal powder and a dense protective shell is provided by the first container, the second container and a solid dense matrix formed therebetween from said protective powder.

2. A method as defined in claim 1, wherein said external pressure is applied in an isostatic process.

3. A method as defined in any one of the preceding claims wherein protective powder material is used in said first and second containers and comprises copper powder.

4. A method of immobilising a nuclear waste material comprising: 5170S/HM -11- i I:1 j! j j! i j 1 i rl t it i ;i i t .4 taking a relatively thin walled first metal container, the metal being of an alloy which is highly resistant to corrosion, the first container being shaped and dimensioned to be deformable under the temperature and pressure conditions used in the process, filling the first container with the nuclear waste material and a metal protective powder material which is copper or a copper alloy highly resistant to corrosion, substantially evacuating the first container and 10 closing the first container, placing the first container in an outer relatively thin walled second container of an alloy which is highly resistant to corrosion and introducing further protective powder material to fill the space between the first container 15 and the second container and spacing the containers apart, the protective powder material being copper powder or powder of a copper alloy highly resistant to corrosion, substantially evacuating the second container, and closing the second container, and V tt heating said containers and their contents to an elevated temperature and effecting hot isostatic pressing, the temperatures and pressures being selected such that the respective containers retain sufficient strength to fulfil a containment function for the nuclear waste during the process 25 and to permit under the applied pressures flow of the protective powder material to form a dense solid matrix enshrouding the spent nuclear waste.

5. A method as defined in any one of the preceding claims wherein the first metal container is generally iii' cylindrical and has a base and side walls incorporating a bellows-like convolution to permit the container during the process to be compressed with substantial reduction in axial length and relatively little change in radial dimensions or distortion.

6. A method as defined in any one of the preceding claims, wherein the second container is of generally cylindrical form having a cylindrical side wall incorporating a bellows-like convolution. 470S/HM -12-

7. A method as defined in any one of the preceding claims wherein the process is carried out at about 800 0 C and at about 20 MPa.

8. A method as claimed in any one of the preceding claims wherein said step of applying pressure is conducted for about one hour and the second container has dimensions in the order of 800 mm diameter, 1800 mm in length and wall thickness of the order of 3 mm, the first container also having a wall thickness of the order of 3 mm.

9. A method as claimed in any one of the preceding claims wherein said application of pressure is conducted in the range 15 to 30 MPa and at a temperature in the range 800 to 850 0 C. I i c i0. A method as claimed in any one of the preceding 1 15 claims wherein the spacing between the first and second li I containers after the process is at least an order of magnitude greater than the wall thickness of either of the first or "1 second containers and is approximatly 10% of the diameter of 2 the second container. 120 11. A method as claimed in any one of the preceding claims wherein the waste material comprises nuclear fuel rods configured to be placed in the inner container and surrounded by protective powder material which is a metal powder.

12. A method of encapsulating waste material substantially as herein described with reference to Figures 1A and 2 to 6 of the accompanying drawings or with reference to 141i Figure 1B and Figures 2 to 6 of the accompanying drawings. I 5th day of October, 1989 AUSTRALIAN NUCLEAR SCIENCE AND TECHNOLOGY ORGANISATION B their Pa nt Attorneys IFFITH H K CO. 5170S/HM -13-