Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF T~IE INVENTION
-
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.
BACKGROUND TO THE INVENTION
. 10
Extremely long term safe storage of nuelear wastes is
a major problem for the nuclear industry and various
proposals have been made for dealing with this problemO
One proposal concerns immobilising the waste in a suitable
borosilicate glass which ean then be deposited in a
suitable geological formation. However, doubts coneerning
possible devitrification of the glass and consequent
leaehing of radioaetive elements have founded eritieism 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 de,scribed 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 environ~ents for
- millions of years and are considered highly resistant to
, ~
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3~.8
leaching by water.
The nuclear reactor waste i5 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 hea~ in a densification process
and the block may then be safely disposed of in a suitable
geological formationO
The following patent applications have been filed
by ~he Australian National University based on the work
by A.E. Ringwood e~ al:-
Australian Patent Specification 523,472 entitled
"Safe Immobilisation of High Level Nuclear Reactor
Wastes" and
United States Patent No. 4,320,248 entitled
"A Process for the Treatment of High Level Nuclear Wastes".
.The present ~pplication,-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. Ilowever, 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 ln 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)O
4. Product of the Sandia Process (R.W. Lynch and
R.G. Dosch, US Report SAND-75-0255 (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.
S~.RY OF THE INVENTION
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),
dirèctly 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 de~ormation 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 o being immobilised in the synthetic rock when
densi~ied into a block;
~b) applying pressure to compress the supply material along
: an axis o~ the canister and applying heat to cause
densification and the formation of a block o~
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 ~reventing gross
outward deformation.
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. ~
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 o~ 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
`" 15 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
~5 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 provi~e a selected oxygen potential to facilitate
the effective formation of the synthetlc rock with
S 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 m~tal 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
2~ fitting lid and the supply material could be poured or
cold pressed into the can before the lid is fitted.
Pxeferably 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 ~nown as Sandvik 253MA.
Typically heating to about 1260C and the application
of pressure of about 7MPA will be suitable sintering
conditions. TXe pressure could be increased, for
example, up to 14 MP~. ~owever, in order ~o 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.
Mos~ 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
matexial 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 an2 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
S 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 gxains and the grains can then fall
through the cavity in the suppcrt element to be collected
- and recycled.
The method can include vibrating the refractory
1~ 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
2~ 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,
2~ 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
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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,
S 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 whlch 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 diameteri 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 o~ properly densified material in the canister at
the end of the pEocess.
~5 According to a third aspect of the invention there
is provided a disposable element comprising a sealed
metal canister containing a densified s~nthetic 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
5 invention.
BRIEF DESCRIPTION OF THE DRAWINGS
.
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 apparatusarranged 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 ~ 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
~5 embodiment before compressing; and
Figure 7 is a view of the canister of Figure
after compression.
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.B
DETAILED DESCRIPTION OF THE DRAWINGS AND PREFERRED
EMBODIM~NTS __
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, ~he
bottom ram 3 has a head adapted to support the bottom
of the canister, the head having a suitable refractory
block 1~ 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 ~ 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 g. When the process
is ~inished 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
3 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 is capable of
being moved upwardly through the cavity 9 for ejecting
the final product.
A typical method of operation comprises the following
steps:
(i) ~ith 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
2~ proportion with the components for forming the synthetic
rock and readily poured granules are formed. A
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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 th~ hydraulic rams 2
and 3 to compact the supply material 6 in the metal
canister. Typically a pressure of about 7MP~
is applied.
(v) Heating in the bottom zone only of the ~upply 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 1260C 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 cani,ster. Each
2~ induction coil segment is activated in turn for a
time of about 3 hours until only a small segment
of sùpply material exists bet~een the zone being
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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 facin~ 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 300C. 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.
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Conveniently the metal is stainless steel of Sandvik
grade 253 M~.
In this embodiment the supply material is introduced
into the metal canister in thin-walled cans 1~ 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 nic~el or iron or the like.
To ~orm 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 opération.
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 techni~ue. It
has been found that the cans do not significantiy
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
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~~.8
product with blocks of synthetic rock 18 within the
thin walled metal cans 14. A refractory spacer 19_ 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 s~ructure, 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 synihetic
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 ZrO2. The components are mixed with nitrate
solutions of the other components, coprecipitated by
addition of sodium hydroxide and then washed.
Typical Compositions of SYNROC and Constituent Phases
"Hollandite" Zirconolite Perovskite Bulk SYNROC
40~ 35~ 25~ Composition
_ _ _ _ _ _. _ _
TiO2 71.0 50.3 57.8 60.3
Zr2 0.2 30.5 0.2 10.8
A123 12.9 2.5 1.2 6.3
CaO 0.4 16.8 40.6 16.2
BaO 16.0 - - 6.4
... _ _ . ................. .
Total 100.5 100.1 99.8 100.1
The precursor matexial 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 ~orm a thick homoaeneous 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 850C 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.
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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 o~ operation at temperatures up to 1200C. A
radio frequency induction coil 31 is then used to raise
the temper~ture of the ends of the pistons 30 and the
canister and its contents to about 1150C.
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 6 and 7 is a variation on
the embodiment of Figure 5, the steps up to the mixer 24
of Figure 5 beins 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
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' ~
compression step, thus obviating the need for handling
of the canlster after compression to insert it into the
cylinder and the pourlng of lead to fill the cavity
around the canister in the embodiment of Figure 5.
~s shown in Figure 6, the cylinder 40 is supported
on a base 43 and the canister 41 inserted with an open-
ended metal cylinder 41_ 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 4~
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 ris~ of insufficient compression
in the bellows zone and improperly formed synthetic
roc~ occuring between the bellows corrugations. In
pxactice the adjacent corrugations of the bellows will
-21-
come together in the compression step.
Figure 7 also illustrates how the induction coil 45
can be ~oved upwardly to the next location ready for
treating the next canister which is to be inserted on
top of the canister 41.
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