Patent Number: 055704689
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and an apparatus for decontaminating substances contaminated with radioactivity that are to be used in a nuclear power station or the like for the purpose of decontaminating such contaminated substances. 2. Description of the Prior Art Heretofore, in a nuclear power station or the like, various parts contaminated with radioactivity are produced in the power generating equipment, the attendant equipment thereof, etc., as a result of a long-term operation. To decontaminate such parts, a shot blasting method is generally employed. As a result, a great amount of shot blasting grit contaminated with radioactivity is produced. The amount of such contaminated grit, which is usually stored in metal drums, is enormous, so that a vast storage place is required. However, an attempt to expand the storage place will meet with objections from the inhabitants of the area, thereby causing a social problem. It is accordingly an object of the present invention to provide a method and an apparatus for decontaminating substances contaminated with radioactivity which help to decontaminate grit contaminated with radioactivity to thereby make it possible to reduce the requisite area for storing the grit. While in this invention the object of decontamination is mainly shot blasting grit contaminated with radioactivity, as stated above, the method and the apparatus of this invention are not restricted to the decontamination of such grit, but are also applicable to the decontamination of other substances contaminated with radioactivity. In such a decontaminating apparatus, a problem generally experienced is the necessity to move the contaminated substance from one place to another for each decontamination process. That is, the contaminated substance is first decontaminated by using a liquid, rinsed in the same liquid, and then dried. Then, the substance is decontaminated with another liquid, rinsed in the same liquid, dried, and so on. Thus, the contaminated substance must be moved from one decontaminating apparatus to another. As a result, great space is required for installing these apparatuses. Further, it is necessary to provide a step and a device for performing the bothersome operation of moving the substance from one decontaminating apparatus to another and for mechanically and reliably grasping and releasing the substance each time it is moved. It is an object of this invention to provide a method and an apparatus for decontaminating substances contaminated with radioactivity which require no such large space, do not necessitate any movement of the contaminated substance for each decontaminating step, and do not require any complicated apparatus for mechanically grasping and releasing the contaminated substance, whereby all of the decontamination processes can be performed in a single apparatus. In some cases, for convenience sake, a plurality of apparatuses according to the present invention may be provided, the contaminated substance being moved between these apparatuses. Such arrangement is also included in the scope of this invention for the purpose of achieving an improvement in operational efficiency, without any difference to the fact that a single apparatus can perform all the decontaminating processes. Another object of this invention is to provide a method and an apparatus for decontaminating substances contaminated with radioactivity which make it possible to decontaminate the contaminated substances effectively and to a sufficient degree. BRIEF SUMMARY OF THE INVENTION To achieve the above objects, this invention provides a method for decontaminating substances contaminated with radioactivity, comprising the steps of: decontaminating a substance contaminated with radioactivity by using a chelate liquid, removing the chelate liquid from the contaminated substance, drying and heating the contaminated substance by hot air at a temperature not lower than the boiling point of a solvent, adding the above-mentioned solvent to the contaminated substance to rapidly vaporize the solvent to thereby separate the remaining chelate liquid from the contaminated substance, and removing the thus separated chelate liquid from the contaminated substance together with the solvent. There is also provided an aspect of the method for decontaminating substances contaminated with radioactivity, wherein methylene chloride is used as a solvent. There is further provided another aspect of the method for decontaminating substances contaminated with radioactivity, wherein the substance contaminated with radioactivity is shot blasting grit. In accordance with this method, constructed as described above, the remaining chelate liquid adhering to the contaminated substance and containing contaminated metal ions is separated from the contaminated substance by the rapid vaporization of the solvent, and then drained along with the solvent subsequently fed. Thus, the remaining chelate can be removed effectively to thereby effect decontamination. Further, the draining of the solvent immediately results in the substance being brought to a dried state, so that there is no need to perform the bothersome operation of removing the chelate liquid by drying. This can be executed very effectively when the solvent is methylene chloride. Further, when the contaminated substance is shot blasting grit, which consists of fine particles, it is possible to effectively perform the difficult separation and removal of the remaining and adhering chelate liquid, bringing the grit in a dried state. This invention also provides an apparatus for decontaminating substances contaminated with radioactivity, comprising: a washing device including a spray device for ejecting liquid, a solvent filtering device, and a chelate liquid filtering device; a solvent supply device communicating with the spray device and the solvent filtering device and provided in such a way as to allow circulation of liquid; a rinse solvent supply device connected to the solvent supply device through the intermediation of a solvent purifying device and communicating with the spray device; a chelate liquid supply device provided in such a way as to allow circulation successively through the spray device, and the chelate liquid filtering device; an electrolytic processing device communicating with the chelate liquid supply device and adapted to electrolyze the chelate liquid; a precipitation device for supplying a precipitant to the chelate liquid which has lost its chelating property by being electrolyzed by the electrolytic device, to thereby form floes i n the liquid; a filtering device communicating with an ion exchange device and adapted to filter the flocs; a chelating agent supply device communicating with the ion exchange device and adapted to supply chelating agent to water; and a hot air supply device and a gas recovery device which communicate with the washing device. There is also provided an aspect of the apparatus for decontaminating substances contaminated with radioactivity, wherein the washing device comprises a main vessel provided to a frame and having an opening and a lid; a decontamination vessel rotatably provided in the main vessel and having a large number of pores in its outer periphery, an opening and an opening/closing lid; and a driving device connected with the decontaminating vessel and adapted to rotate the decontaminating vessel. There is provided another aspect of the apparatus for decontaminating substances contaminated with radioactivity, wherein the pores have a width or size that is smaller than that of the shot blasting grit, so that the shot blasting grit cannot pass therethrough. There is provided still another aspect of the apparatus for decontaminating substances contaminated with radioactivity, wherein the decontamination vessel has a polishing-cleaning material combined with the decontamination vessel. There is provided yet another aspect of the apparatus for decontaminating substances contaminated with radioactivity, wherein the decontamination vessel is detachably formed with respect to the main vessel, and a plurality of decontamination vessels and a plurality of main vessels are formed. Next, the operation of the apparatus will be described. In the apparatus of this invention, constructed as described above, a single apparatus can perform all of the following processes: decontamination using a solvent and decontamination using a chelate liquid; rinsing using a solvent, and separation of the remaining chelate liquid rapidly resulting from the rinsing; drying of a contaminated substance, and so on. Further, the chelate liquid can be reproduced and recycled through the steps of: electrolysis in a electrolytic device, addition of a precipitant in a precipitation device to form and precipitate flocs, filtration, processing in an ion exchange device to make clean water, and supplying thereto of a chelating agent from a chelating agent supply device. Also the pores of the decontamination vessel having a size or width that is smaller than that of the grit enable the grit to be effectively decontaminated. Further, since the decontamination vessel has a polishing-cleaning material, decontamination is performed effectively by joint use of polishing and cleaning during the so-called running-liquid washing using the solvent and the chelate liquid. Further, in an aspect where the decontamination vessel is detachably formed with respect to the main vessel, and a plurality of main vessels and a plurarity of decontamination vessels are formed, a time-consuming process and a non-time consuming process can be conducted separately, thereby achieving an improvement in operational efficiency.