1 2
copper powder fills the voids around the rods and
ENCAPSULATION OF WASTE MATERIALS within the canister. A lid is applied and the canister
placed in a thin walled container. Evacuation of air
FIELD OF THE INVENTION from within the canister takes place and the outer con
The present invention relates to the encapsulation of 5 tainer can then be sealed and Passed t0 a hot isostatic waste materials and is especially applicable to providing pressing step operating typically at 500° C. and the very a system whereby safe long-term storage can be ob- hiSh pressure of 150 MPa. The very high pressure is tained for dangerous waste materials such as radioactive necessary to bond the lid to the container by sintering, waste materials and toxic compounds. Radioactive The proposals described in the previous three parawaste materials include spent nuclear fuel rods as well 10 graphs are widely questioned as not viable on grounds as high level radioactive waste produced by reprocess- including feasability and safety because of the very high ing spent nuclear fuel rods. pressures involved.
„ „T „ „ ^^tm~„t Yet a further proposal is to encapsulate the unpro
BACKGROUND TO THE INVENTION cessed spgnt nu*J fud m a ^ graphite ^
Although in some embodiments the invention may be *5 sulphide bonded matrix. It appears such a matrix can be
applicable to waste materials other than radioactive formed at about 500° C. but it is questionable whether
waste, the present invention will be illustrated with the matrix would be considered safe for long-term stor
particular reference to high level radioactive waste and age because of an inherent brittle characteristic with the
spent nuclear fuel rod encapsulation. attendent risk of fracture and leaching of radioactive
High level radioactive waste usually is in the form of 20 components by ground water. Furthermore, the long
a solution of the waste. This solution can be calcined at term stability of the matrix in terms of its resistance to
600° C.-800" C. temperature to provide the waste in leaching does not appear to be established,
powder form. The waste must be safely encapsulated in Accordingly, over a period of many years there have
some system which is corrosion and heat resistent and been extensive investigations at considerable cost in to
will prevent the waste coming into contact with any 25 mny different of systems for safeiy immobilising
ground water so that the possibility of contamination of radioactive waste for disposal. With a view to provid
ground water through teaching is avoided. ing a system having commercial attractivness with in
Alternatively spent fuel rods could be simply dis- herent a table safet and the practicality to operate
posed of m a safe containment system. , TMa long-term basis, the present invention has been
Various prior proposals have been published in rela- 30 ^ ^ ^ ^ ^ ... tQ
tion to safe disposal of nuclear waste. One proposal is to ,, ■ , , ,
r . . * r previous published proposals,
immobilise the waste as a minor component of a syn- r r r r
thetic rock matrix which is produced from mineral SUMMARY OF THE INVENTION
precursor components and the waste in powder form. . ^ <r + ?it_
The matrix is produced under conditions of high tern- 35 According to a first aspect of the present invention,
perature and high pressure maintained for several there ,1S, P">v«ted a method of immobilising a waste
hours. Suitable synthetic rock structures have been material comprising
published by A. E. Ringwood et al, see for example the <a> takmS a generally cylindrical container having a
following- "ase and navm8 a slde wall extending around the axis of
Nature March 1979 40 ^e container and incorporating a bellows-like convolu
European Patent Application No. 79301382.2 tion to Permit tne container during the process to be
U.S. patent application Ser. No. 124953 substantially uniaxially compressed with substantial
Engineering techniques for production of synthetic reduction in axial length and relatively little change in
rock incorporating the high level waste include hot radial dimensions or distortion, the metal being highly
isostatic pressing processes (see for example European 45 resistant to corrosion and maintaining substantial
Patent specification No. 0044381 assigned to ASEA) strength at temperatures used in the process,
and hot uniaxial processes (see Australian patent appli- 0>) filling the container with solid material compris
cation No. 18163/83 and equivalent U.S. Pat. No. ing the waste material,
4,645,624 assigned to the assignees of the present appli- (c) providing protective particulate material for
cation. 50 forming a dense solid barrier around the waste material
Another publication by ASEA concerns a suggested during the method,
process in which a copper canister is filled with a mix- (d) closing the container,
ture of unprocessed spent nuclear fuel and copper pow- (e) heating the container to an elevated temperature
der. Hot isostatic pressing is specified for the purpose of and applying uniaxial pressure along the direction of
embedding the fuel in a-dense copper matrix. 55 said axis, the temperatures and pressures being selected
Yet another publication by ASEA (see U.S. Pat. No. such that the waste material becomes immobilised in a
4,209,420 H. Larker) concerns a suggested process in dense solid matrix of the protective particulate material,
which a ceramic canister (formed by hot isostatic press- and the method being characterised by at least a part of
ing) is filled with a unprocessed spent nuclear fuel waste the protective particulate material being in intimate
and a lid is placed in position. A gas-tight shroud of 60 contact with the waste material and at least a part of the
metal is placed around the canister. Hot isostatic press- protective particulate material comprising a metal pow
ing at very high pressure is specified for the purpose of der which is retained and disposed around the waste
joining the lid to the canister body. material for forming a dense shroud which is of a metal
Another proposal for nuclear fuel storage has been highly resistant to corrosion, whereby the waste matemade in a paper entitled "Final Storage of Spent Nu- 65 rial is immobilised.
clear Fuel" by Swedish Nuclear Fuel Supply Company The above described aspect of the invention may be
(KBS Division). A massive copper canister having rela- implemented by incorporating the metal powder (such
tively thick walls receives spent nuclear fuel rods and as copper powder) with the waste material in said con
