Patent Number: 046630856
Section: summary

BACKGROUND OF THE INVENTION This invention relates to an apparatus for the decontamination of radioactive metallic waste generated as in a nuclear power plant and required to be decontaminated for safe disposal. Generally, radioactive metallic waste generated as in a nuclear power plant is put to permanent storage within the plant's premises and thereby prevented from producing adverse effects upon the environment. When this practice of permanent storage is continued, however, the total amount of storage of such radioactive metallic waste will increase, eventually making it difficult to find a space for further storage. Pipes and other similar objects which are contaminated by radioactivity in the nuclear power plant have large dimensions and cannot be readily reduced in size by any of the existing treatments. These objects, therefore, are put in storage only with difficulty. Studies are now under way in search of a method which is capable of decontaminating such radioactive metallic waste and lowering its level of radioactivity to the level of radioactivity existing in nature, namely the background level of radioactivity, and thereby enabling the metallic waste to be handled as safely as ordinary industrial waste. The radioactive metallic waste, however, has contaminants adhered to its surface. Mere removal of contaminants loosely piled up on the surface hardly suffices for ample decontamination. Sufficient decontamination necessitates dissolution of surface layers of metallic matrices which are covered with adhered deposited contaminants. Prior art methods of decontaminating such radioactive metallic waste include, a method of electrolytic decontamination which accomplishes the decontamination by electrochemically dissolving the surface layers of the metallic matrices and a method of chemical decontamination which chemically dissolves the surface layers of metallic matrices by use of a decontaminating agent. The method of electrolytic decontamination is advantageous in that it effects decontamination at a high speed. It nevertheless entails the disadvantage that it cannot be applied effectively to metallic matrices of complicated shape because it requires the surface of the electrode to be opposed to the surface of the metallic matrix under treatment. The method of chemical decontamination enjoys high adaptability to metallic matrices of complicate shape. It has the disadvantage, however, that the decontamination proceeds at a slow speed and the decontaminating agent is irreversibly consumed and consequently gives rise to a large volume of spent decontaminating agent which constitutes secondary waste. Moreover, the decontaminating agents used in the apparatus for chemical decontamination and the apparatus for electrolytic decontamination are destined to be completely exhausted because of deterioration of performance and accumulation of radioactivity and inevitably give birth to secondary waste in large volumes. U.S. Pat. No. 4,217,192, for example, discloses a method and system for the decontamination of metallic objects radioactively contaminated in the nuclear industry by means of chemical etching. This chemical etching resorts to the effects of oxidation and reduction obtained by a procedure which comprises immersing, in an electrolytic solution formed of a nitric acid solution containing trivalent cerium ions and tetravalent cerium ions and held in one container, a given contaminated metallic object in conjunction with a pair of electrodes intended for regeneration of the electrolytic solution, allowing the metallic object to stand therein until it is relieved of radioactivity, and removing the decontaminated metallic object from the electrolytic solutiuon. Since this method is operated batchwise and not continuously, it has the disadvantage that the operation itself entails extra time and labor each time one round of operation is replaced by the next one and the period of exposure of operators to radiation is long. Since this method uses a concentrated nitric acid solution at elevated temperatures, it has the possibility that the solution will emit nitric acid gas and nitric acid mist and impair the working environment. Further, this method has the disadvantage that the residue of etching produced within the electrolytic solution will be suspended in the form of slurry to hasten wear of the electrolytic solution, deprive the electrolytic solution of its function, and even shorten its service life. In the disclosure, no measure for overcoming these disadvantages is not suggested at all. OBJECT OF THE INVENTION The first object of this invention is to provide an apparatus for the decontamination of radioactive metallic waste as the object of treatment, which is capable of thoroughly dissolving the surface layer of the object under treatment without reference to size and shape, the very factors that prevent the conventional methods of decontamination from fulfilling their functions indiscriminately, thereby lowering the level of radioactivity to a point where the object will be handled as safely as ordinary industrial waste. The second object of this invention is to provide an apparatus for the decontamination of radioactive metallic waste, which is capable of regenerating the decontaminating agent used in the form of electrolytic solution thereby minimizing the occurrence of secondary waste. The third object of the invention is to provide an apparatus for the decontamination of radioactive metallic waste, which effects decontamination at a high speed, curbs the occurrence of spent electrolytic solution by ensuring reclamation thereof, and warrants safe attendance of operators. The fourth object of the present invention is to provide an apparatus for the decontamination of radioactive metallic waste which is capable of efficiently curbing the occurrence of radioactive secondary waste in the disposal of used electrolytic solution having dissolved therein metallic ions contaminated by radioactivity. SUMMARY OF THE INVENTION The first aspect of the present invention resides in an apparatus for the decontamination of radioactive metallic waste by the electrolytic oxidation-reduction with an aqueous nitric acid solution containing trivalent cerium ions, i.e. the step of converting the trivalent cerium ions into tetravalent cerium ions through electrolytic oxidation and the step of dissolving the radioactive metallic waste with the electrolytic solution now vested with the oxidative power of the freshly produced tetravalent cerium ions and, at the same time, effecting regeneration of the tetravalent cerium ions, which apparatus is characterized by a device disposed inside an electrolytic cell and adapted to stir the electrolytic solution therein, a filter adjoined to the electrolytic cell and adapted to screen oxides such as of iron, and a condenser, a demister, and a waste gas blower all adapted to recover the mixed vapor and mist of water and nitric acid (H.sub.2 O--HNO.sub.3). A second aspect of this invention resides in an apparatus for the decontamination of radioactive metallic waste, characterized by disposing an electrolytic cell for holding the electrolytic solution of cerium nitrate (Ce.sup.3+ --Ce.sup.4+ --HNO.sub.3) and a decontamination cell for decontaminating the radioactive metallic waste independently of each other and interposing a filter between the electrolytic cell and the decontamination cell. A third aspect of the present invention resides in an apparatus for the decontamination of radioactive metallic waste, characterized by having a waste gas treating device for recovery of nitric acid vapor communicate with the decontamination cell and the electrolytic cell in the apparatus of the second aspect of the invention. In the decontamination cell, the pipe for feeding the electrolytic cell is shaped in the form of a nozzle so constructed that the electrolytic solution will be discharged in a sprayed form from the tip thereof. A fourth aspect of this invention resides in an apparatus for the decontamination of radioactive metallic waste, comprising the aforementioned electrolytic cell, a decontamination cell for dissolving the surface layer of the object under treatment thereby effecting the decontamination of the object, an overflow tube disposed on the lateral wall of the electrolytic cell, an overflow tube disposed on the lateral wall of the decontamination cell, a decontamination drain tube disposed on the bottom side of the decontamination cell, a drain tank communicating with the two overflow tubes and the decontamination drain tube and serving to admit the electrolytic solution overflowing from the electrolytic cell, the electrolytic solution overflowing from the decontamination cell, and the electrolytic solution drained from the decontamination cell, an electrolytic solution transfer tube for returning the electrolytic solution from the drain tank to the electrolytic cell, a decontaminant transfer tube for returning the electrolytic solution from the drain tank to the decontamination cell, a waste gas duct disposed in the space overlying the electrolytic cell, the decontamination cell, and the drain tank, a nitric acid vapor recovery device connected to the waste gas duct, a waste gas treating device consisting of a mist recovery device and a gas exhauster, a flush water tube for delivering rinse water from a rinse water tank to the decontamination cell, and a rinse return tube for returning the rinse water used in the decontamination cell from the decontamination cell to the aforementioned rinse water tank. A fifth aspect of this invention resides in an apparatus for the decontamination of radioactive metallic waste, characterized by immersing in the electrolytic cell having dissolved therein metallic ions including radioactivated metallic ions an anode made of an inactive metal and a cathode made of a metallic material possessing hydrogen overvoltage of a larger absolute value than the absolute value of the hydrogen overvoltage of iron. This aspect of the invention is further characterized by causing flow of direct current between the anode and the cathode, causing the metallic ions including radioactive metallic ions in the electrolytic solution to be precipitated and deposited on the cathode, then separating the deposited precipitate from the cathode, and discarding the separated deposit as radioactive waste.