Patent Number: 050733052
Section: summary

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a method of evacuating treating containers to a vacuum for use in compacting radioactive wastes by an HIP (hot isostatic press), hot press or the like. Such radioactive wastes include metals and bricks contaminated with plutonium or a transuranium element having a long half life. In recent years, attention has been directed to treatments by HIP or the like for compacting radioactive wastes into solid blocks for stabilization before storing the wastes (see, for example, Examined Japanese Patent Publication SHO 57-959). For example, the treatment of hulls will be described which are sheared cladding tubes. Hulls are hollow, have a low bulk density of 1.0 and are therefore precompressed to a true density ratio of at least about 70% by a press first. During the compression, a highly radioactive oxide formed by zircalloy on the surfaces of the hulls and having a thickness of about 10 .mu.m partly separates off. The compressed waste is then placed into a treating container of stainless steel or the like, which is then filled with a metal powder, stainless steel powder or the like to eliminate the space or clearances remaining in the container. A closure is then welded to the container, piping (hereinafter referred to as an "evacuating pipe") is thereafter attached to the closure for connection to a vacuum pump, and the interior of the container is evacuated to a degree of vacuum, e.g., about 10.sup.-2 torr. The container thus evacuated is completely sealed off to hold the vacuum therein, and the container is compressed by HIP or hot press under an external pressure with heating, whereby the container is compacted. The container is evacuated to prevent the container itself from breaking owing to the presence of air or like gas confined in the container when the container is compressed under a high pressure. However, if the container is thus evacuated after the waste has been placed thereinto, a particulate radioactive substance separating off the waste is led out of the container via the evacuating pipe to contaminate the vacuum pump and the inner surface of the evacuating pipe. The spillage of the radioactive substance due to aspiration can not be prevented completely even at a reduced evacuation rate. Further even if a filter is removably installed in the evacuating pipe and the like, the filter becomes contaminated and is therefore extremely difficult to replace, hence inconvenience is caused. SUMMARY OF THE INVENTION The main object of the present invention is to provide a method of evacuating treating containers to a vacuum free of the foregoing problem. To fulfill this object, the present invention provides a method of evacuating a container to a vacuum for use in treating radioactive wastes by placing the waste into the container, and evacuating, sealing off and thereafter compressing the container, the method being characterized by placing the waste into the container, forming over the waste a filter layer of particulate material fulfilling one of the following requirements, and thereafter aspirating a gas through the filter layer from thereabove to evacuate the container and sealing off the container. (1) A layer having a thickness of at least 5 mm and formed of a particulate material not smaller than 40 .mu.m to less than 105 .mu.m in mean particle size. PA0 (2) A layer having a thickness of D mm and formed of a particulate material not smaller than 105 .mu.m to not greater than 210 .mu.m in mean particle size d .mu.m, the thickness D and the mean particle size d having the relationship represented by: EQU D.gtoreq.(20/105).times.d-15 With the method described above, the gas within the container is drawn out through the interstices between the particles of the particulate material, whereas the radioactive substance separating off the waste is blocked by the filter layer fulfilling the specified requirement and is prevented from being led out of the container. Accordingly, the present method satisfactorily evacuates the container while reliably preventing the release of the radioactive substance from the container. The filter layer is subjected to the compacting treatment along with the container and therefore need not be replaced. The above and other objects, features and advantages of the present invention will become apparent upon a reading of the following detailed description and the accompanying drawings.