Patent Number: 048083372
Section: summary

FIELD OF THE INVENTION 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. BACKGROUND OF THE INVENTION Radioactive waste materials and synthetic rock precursor may be poured into a compressible, bellows-type canister which is closed and then subjected to hot uniaxial pressing such as described in our U.S. patent application Ser. No. 524,841 now U.S. Pat. No. 4,645,624, assigned to the assignee of the present application. 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 quanitity of gas in the interstices of the mixture, unless the gas is completely evacuated, a time consuming and complex process in an active cell. SUMMARY OF THE INVENTION 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. 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, the wall of which includes a bellows like wall structure, the method comprising heating the canister 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 connected to an exhaust gas processing system for discharging gases from within the canister. The invention also extends to a method of forming synthetic rock incorporating radioactive waste wherein precursor materials for the synthetic rock mixed with radioactive waste are placed in a generally cylindrical metal canister, the cylindrical wall of which includes a bellows-like structure and the canister includes a discharge duct for discharging gases therefrom, the method comprising heating the canister 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. PREFERRED FEATURES OF THE INVENTION 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. 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. 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 incorporating a filter material whereby ingress of synthetic rock forming materials into the filter during compression of the canister is substantially avoided. 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. 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 bellow-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. 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. 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; the arm of the L-shaped pipe fitting extending at right angles to this horizontal arm 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.