[0001] The present invention relates to removal of gases from a compressible, substantially
closed container during hot uniaxial pressing and, in particular but not exclusively,
is related to such a method for use in a process for immobilising high level radioactive
nuclear waste material in a synthetic rock formed under heat and high pressure from
an intimate mixture of such a waste material and synthetic rock-forming material.
These materials may be poured into a compressible, bellows-type canister which is
closed and then subjected to hot uniaxial pressing such as described in our co-pending
European Patent Application Nos. 81303221.6 and 83304974.5.
[0002] A known alternative to the present applicants' hot uniaxial pressing process is a
hot isostatic processing in which the particulate waste material and synthetic rock
forming material is placed in a metal container which needs to be evacuated and completely
sealed. This metal container is then subjected to high temperatures with a very high
surrounding gas pressure to cause compaction of the material within the canister as
it forms a synthetic rock. Thus the canister is supported on all sides by the gas
pressure and the very nature of the process is such that any gaseous material within
the container must be retained therein. When a canister is filled with the particulate
mixture (for forming the synthetic rock incorporating radioactive waste) even if a
high filling density is achieved there will be a considerable quantity of gas in the
interstices of the mixture, unless the gas is completely evacuated, a time consuming
and complex process in an active cell.
[0003] The present invention concerns a development of the process of hot uniaxial pressing
of the present applicants and in contrast to the hot isostatic pressing process proposes
an arrangement whereby gases occuring within the container are removed in a controlled
manner.
[0004] According to a first aspect of the invention, there is provided a method of forming
synthetic rock incorporating radioactive waste wherein precursor materials for the
synthetic rock mixed with radioactive waste are placed in a metal canister (1), the
wall of which includes a bellows-like wall structure (3), the method comprising heating
the canister (1) and its contents and maintaining a sufficiently elevated temperature
during the application of axial pressure to the canister to cause the formation of
synthetic rock and the discharge of gases, and characterised by using a canister including
a discharge duct (8) connected to an exhaust gas processing system for discharging
gases from within the canister (1).
[0005] The method preferably extends to connecting the discharge duct of the canister to
an exhaust gas processing system whereby any necessary processing steps such as filtering
of radioactive gasses can take place.
[0006] According to a second aspect of the invention there is provided a metal canister
for use in a hot pressing process for immobilising high level radioactive nuclear
waste material, the canister (1) being adapted to be filled by a particulate material
comprising radioactive waste and precursor materials for forming synthetic rock, the
canister (1) being adapted to be closed after the particulate material has been poured
into the canister (1), the canister (1) having a bellows-like wall (3) structure and
characterised by the canister (1) having a gas discharge duct (8) adapted to be connected
to a gas processing system when the canister, after being filled with particulate
material, is subjected to high temperatures and/or axial compression.
[0007] Preferably, the discharge duct is arranged to co-operate with a filter structure
capable of retaining good gas permeability at high temperature, with the filter being
arranged to prevent any solid material escaping from the canister during densification
of material being compressed therein.
[0008] The filter structure advantageously comprises a cap-like structure having apertures
therein and co-operating with a base end wall of the canister which has an aperture
therein leading to the discharge duct, a cavity being defined between the aperture
and the cap and incorporating a filter material whereby ingress of synthetic rock
forming materials into the filter during compression of the canister is substantially
avoided.
[0009] The discharge duct is preferably in the form of a bore extending through the base
end wall of the canister and terminating in a pipe adapted to be connected to a gas
processing system. Alternatively, the discharge duct could be provided by a slot-like
recess in the bottom of the base end wall of the canister, the duct in an operating
position being closed by co-operation with an upper face of a pressure pad located
on the hydraulic ram.
[0010] The canister optionally may include a cylindrical screen confining the particulate
rock forming material and radioactive waste to a central zone of the canister and
preventing the ingress of this material into the region of the convolutions of the
bellows-like structure in the cylindrical side wall. The zone between the exterior
of the screen and the convoluted side wall could be left free of solid material or
alternatively could receive granulated Zircaloy from spent nuclear fuel rods. In either
case removal of gas from the region between the screen and the convoluted wall portion
can be provided by apertures in the base end wall of the canister connecting to the
discharge duct.
[0011] In one important embodiment, the discharge duct terminates in a pipe which communicates
with a gas extraction manifold, for example by the aperture at the end of the pipe
being disposed adjacent the opening to the manifold, a suction being maintained to
cause reliable scavenging of all discharged gases.
[0012] One form of this outlet pipe is an L-shaped pipe fitting having a horizontal limb
rotatably mounted in sealing engagement in the base end wall of the canister and connected
to the discharge duct; an arm of the L-shaped pipe fitting extending at right angles
to this horizontal limb is adapted to be rotated from a upwardly directed transport
position to a downwardly directed location by pivotal action whereby the open tip
of the pipe is inserted through a slot in a side wall of an upwardly directed tube
forming the manifold for the extraction system. This tube is conveniently attached
to the side of the pressure pad structure of the hydraulic ram. Other configurations
can be used.
[0013] Embodiments of the present invention will now be described by way of example and
with reference to the accompaning drawings in which;
Figure 1 is a plan view of a compressible, bellows- type container incorporating a
first embodiment of the present invention;
Figure 2 is an elevation, in partial section, of the container shown in Figure 1;
Figures 3A and 3B are respective elevations showing in detail alternative filter arrangements
for the filter structure shown in Figures 1 and 2;
Figures 4A and 4B are respective elevations of the arrangements shown in Figures 1
and 2 but incorporating a further inventive feature concerning a gas discharge system.
[0014] Referring firstly to Figures 1 and 2 of the drawings, there is shown a compressible,
bellows-type metal canister 1, for use in a hot pressing process for immobilising
high level radioactive nuclear waste material in the form of a synthetic rock. The
canister typically is generally as described in co-pending application no. 45384/80.
The canister includes a gas filter and discharge arrangement constituting one embodiment
of the invention. The canister 1 comprises a base wall 2 and a corrugated bellows-like
side wall 3 of generally circular cross-section. Concentrically arranged within the
corrugated side wall 3 is a cylindrical liner 4. In the centre of the base wall 2
is located a conically tapered aperture 5 provided with a filter plug shown diagrammatically
at 6. Between the corrugated side wall 3 and inner liner 4 of the canister are provided
two further, diametrically-opposed apertures 7. All three apertures 5, 7 are connected
by an outlet pipe 8 extending diametrically across the base wall 2 and exteriorly
of the canister. This outlet pipe 8 is connectable to any suitable waste disposal
system, as will be described hereinafter with respect to a preferred embodiment.
[0015] Referring now to Figures 3A and 3B, there are shown two alternative embodiments of
filter plug 6 which may be used in association with the central aperture 5 in the
base wall 2 of the compressible canister 1.
[0016] The filter plug 6 in Figure 3A comprises an inverted castellated cap 9 with which
is associated a filter mass 10 made of alumina or titania fibre. This filter material
is packed into the conically-tapered aperture 5 and into the gaps between the castellations
of the cap 9. The projecting lugs of the castellated cap 9 rest on the upper surface
of the base wall 2 around the periphery of the conical-aperture 5 and thus compressive
forces in the axial direction of the canister are absorbed and ingress of synthetic
rock forming components into the filter structure are substantially avoided.
[0017] The filter plug 6 shown in Figure 3B differs from that of Figure 3A only in that
it has a filter disc 10' made of Hastalloy in place of the mass of alumina or titania
fibre. The filter disc 10' is welded around its periphery as shown at 16 to the conical-aperture
5. Furthermore, in the embodiment of Figure 3B the outlet duct 8 is formed by the
co-operation of a slot in the underside of the base wall 2, the duct being closed
on its lower side by co-operation with the upper face of pressure pad 12 resting on
a hydraulic ram.
[0018] The discharge of gases through the outlet duct 8 can be to a gas processing system
of the type described below with reference to Figures 4A and 4B. The gases will comprise
the gas in the interstices of the particulate material in the canister and any volatile
components produced from the particulate material during the heating stage.
[0019] As shown in Figures 4A and 4B. the outlet pipe 8 (or outlet duct) is connected to
an outlet tube 11. In Figure 4A, the compressible canister 1 is shown in a free-standing
position upon a lower pressure pad 12 of a hydraulic press associated with an induction
furnace (not shown) in which the canister is to be heated to a high temperature and
then compressed axially. In this arrangement. the outlet tube 11 is L-shaped and has
its horizontal limb rotatably but sealingly mounted in a side of the base wall 2;
the terminal limb in the illustrated loading position extends upwardly, with its open
end free to the atmosphere.
[0020] In the process, as shown in Figure 4B. the compressible bellows-type canister 1 is
raised by the hydraulic ram to place the upper wall 17 of the canister against a fixed
refractory abutment pad 13. The canister is thus positioned so as to be heated in
the induction furnace (not shown) which surrounds the canister. However, before heating
can commence, the outlet tube 11 is rotated through 180° into a downwardly extending
position, such that the terminal limb extends into a manifold arrangement 14 communicating
with an exhaust tube 15. which is connected to a low pressure gas filtration system.
It is to be noted that the manifold arrangement 14 and associated down pipe 15 are
mounted on the lower pressure pad 12. so that they can move in unison with the exhaust
tube 11 and canister 1 supported on that pad.
[0021] Although the high level radioactive nuclear waste incorporated into the synthetic
rock materials includes elements volatile at the typical temperatures to which the
material is heated (about 1150°C) it has been found that little, if any of these components
are infact exhausted from the canister; it is thought these volatile components are
absorbed into the synthetic rock materials. However, in order to maximize safety aspects
it is proposed to collect all gases discharged through the outlet duct 8. The filter
structure has a filter material for preventing the ejection of any particulate matter
from the canister which might be entrained with the gases. Due to the gas collection
system shown in Figures 4A and 4B the gaseous stream can be filtered and any radioactive
components removed.
[0022] Figure 4A shows the loading postion. For transportation the terminal limb 11 of the
outlet duct is directed upwardly to prevent damage or catching on any objects. After
positioning of the canister 1 on the pressure pad 12, the limb is rotated downwardly
to engage in the slotted open end of manifold 14 which together with discharge pipe
15 are fixed to the side of the pressure pad 12.
[0023] Other configurations for discharge pipe connections could be utilised. Simply, reliable
connections are important and one useful alternative is to provide a V-shaped slot
in opposite walls at the end of manifold 14 and to raise the manifold and orientate
it so that it engages a side wall of a fixed discharge tube 11 and bridges across
a portion of the side wall of the discharge tube having a gas discharge aperture.
1. A method of forming synthetic rock incorporating radioactive waste wherein precursor
materials for the synthetic rock mixed with radioactive waste are placed in a metal
canister (1), the wall of which includes a bellows-like wall structure (3), the method
comprising heating the canister (1) and its contents and maintaining a sufficiently
elevated temperature during the application of axial pressure to the canister to cause
the formation of synthetic rock and the discharge of gases, and characterised by using
a canister including a discharge duct (8) connected to an exhaust gas processing system
for discharging gases from within the canister (1).
2. A metal canister for use in a hot pressing process for immobilising high level
radioactive nuclear waste material, the canister (1) being adapted to be filled by
a particulate material comprising radioactive waste and precursor materials for forming
synthetic rock, the canister (1) being adapted to be closed after the particulate
material has been poured into the canister (1). the canister (1) having a bellows-like
wall (3) structure and characterised by the canister (1) having a gas discharge duct
(8) adapted to be connected to a gas processing system when the canister, after being
filled with particulate material, is subjected to high temperatures and/or axial compression.
3. A metal canister according to claim 2, characterised in that upstream of the discharge
duct (8) a filter structure (6) is provided within the canister (1) and is capable
of retaining good gas permeability at high temperature, with the filter (6) being
arranged to prevent any solid material escaping from the canister (1) during densification
of material being compressed therein.
4. A metal canister according to claim 3. characterised in that the filter structure
(6) comprises a cap-like structure (9) having apertures therein and co operating with
a base end wall (2) of the canister which has an aperture (7) therein leading to the
discharge duct (8), a cavity being defined between the aperture and the cap and incorporating
a filter material (10) whereby ingress of synthetic rock forming materials into the
filter (6) during compression of the canister (1) is substantially avoided.
5. A metal canister according to any one of claims 2 to 4, characterised in that the
discharge duct (8) is in the form of a bore extending through a base end wall (2)
of the canister (1) and terminating in a pipe (11) adapted to be connected to a gas
processing system.
6. A metal canister according to any one of claims 2-to 4, characterised in that the discharge duct (8) is in the form of a slot-like recess
in the bottom of a base end wall (2) of the canister (1). the discharge duct (8) in
an operating position being closed by co-operation with an upper face of a pressure
pad (12) located on an hydraulic ram.
7. A metal canister according to any one of claims 2 to 6, characterised in that a
cylindrical screen (4) is provided confining the particulate material to a central
zone of the canister (1) and preventing the ingress of this material into the region
of the convolutions of the bellows-like structure in the wall (3).
8. A metal canister according to any one of claims 2 to 7, characterised in that the
discharge duct (8) terminates in a pipe (11) which communicates with a gas extraction
manifold (14,15). and a suction is applied to enable reliable scavenging of all discharged
gas.
9. A metal canister according to claim 8, characterised in that the pipe is an L shaped
pipe fitting (11) having a horizontal limb rotatably mounted in sealing engagement
in the base end wall (2) of the canister (1) and connected to the discharge duct (6);
an arm of the L-shaped pipe fitting (11) extending at right angles to this horizontal
limb is adapted to be rotated from an upwardly directed transport position to a downwardly
directed location by pivotal action whereby an open end of the pipe is inserted through
a slot in a side wall of an upwardly directed tube, which forms the manifold (14.15)
for the extraction system.
10. A method as claimed in claim 1, wherein the metal canister is as claimed in any
one of claims 2 to 11.