Apparatus and method for decontamination of radioactive metallic waste by electrolysis

Abstract: An insulating shield plate (1) divides a room of an electrolysis bath (2) into an anode chamber (13) and a cathode chamber (14). An anode (5) is disposed in the anode chamber (13), and a cathode (6), metallic waste (7) and heater (4) are disposed in the cathode chamber (14). An electrolyte (3) stored in the electrolysis bath (2) flows in a circulation path from a circulating pump (10) through a circulating line (12) and filter (11) to return the electrolysis bath (2). An exhaust gas processing device (9) is connected to the electrolysis bath (2). When a voltage is supplied to the anode (5) and cathode (6), a surface of the metallic waste (7) is charged in a positive polar to dissolve a base metal of the metallic waste (7). Further, a cylindrical anode (33) is arranged in an electrolyte (32) in an electrolysis bath (31), and a cylindrical metal (34) as a metallic waste is arranged in the cylindrical anode (33). After a bar-shape cathode (35) is arranged in the cylindrical metal (34), a DC voltage is supplied to the cylindrical anode (33) and the bar-shape cathode (35). At this time, the system may be constructed in that oxygen occurs by charging in a positive polar in an inner surface of the cylindrical metal (34) and the base metal of the cylindrical shape is dissolved.

Description: The present invention relates to an apparatus and method for decontaminating a radioactive metallic waste for the purpose of reducing a radioactivity occurring during operation, outage for inspection and decommission of the nuclear facilities and included in the metallic waste, and more specifically, to an apparatus and method for decontaminating a radioactive metallic waste for the purpose of reducing a radioactivity included in the metallic waste having shapes of a pipe, plate and the like. In particular, the invention relates to a system for decontaminating radioactivity of a metallic waste by performing a bipolar electrolysis with non-contact in an electrolyte in an electrolysis bath with respect to a metallic waste contaminated by radioactive material and by dissolving a base metal by dielectric function so as to eliminate radioactivity, comprising: said electrolysis bath having a predetermined shape and being arranged to be filled up by said electrolyte which has a predetermined component, density and temperature for performing said electrolysis; an anode arranged in said electrolysis bath with a predetermined shape along said metallic waste and arranged to be charged in a positive polarity by a direct current (DC) voltage supplied from a DC power source; and a cathode arranged in said electrolysis bath with a predetermined shape and arranged to be charged in a negative polarity by said DC voltage supplied from said DC power source.. Such a system is disclosed in DATABASE WPI, Section Ch, Week 9350, Derwent Publications Ltd., London, GB; AN 93-397806; JP-A-05 297 192 (TOSHIBA KK), PATENT ABSTRACTS OF JAPAN, Vol. 016, No. 046 (P-1307); JP-A-03 249 600 (TOSHIBA CORP) and FR-A-2 565 021. FR-A-2 565 021 further discloses dissolving contaminated metal by a chemical, as opposed to an electrochemical, process. Various methods are provided for completely decontaminating a radioactivity included in a radioactive metallic waste and occurring during operation of the nuclear power establishment, at an outage for inspection of the nuclear facilities and at decommissioning the waste. For example, Japanese Patent Laid-open No. 62-46297 and No. 60-186799 disclose an electrolysis decontamination using acid and neutral salt (chloride) solution, which have been developed and utilized. The electrolysis decontamination is effective with respect to a metallic waste having a comparatively simple shape such as a plate, cylindricality and the like. A system of the electrolysis decontamination, comprises an anode as a metallic waste, and a cathode arranged in front of a surface to be decontaminated on the metallic waste as the anode, in which a direct voltage is supplied between the metallic waste (anode) and the cathode to polish a base metal on the surface to be decontaminated, thereby decontaminating the radioactivity from the metallic waste. The electrolysis decontamination mentioned above, however, has the problems as follows: i) Since a contamination remains behind a connection portion between the metallic waste and the anode because the connection portion is not dissolved, it is necessary to change the manner of holding the anode to decontaminate again so as to complicate the decontamination; ii) When large scale device and apparatus are decontaminated, since a current value becomes large in proportion to a surface area, it is necessary to provide an anode clamper under the consideration of a contact area. Accordingly, it is also necessary to frequently change the anode clamper to match the shape with the device and apparatus; and iii) When large number of device and apparatus are treated, since it is necessary to change the manner of holding the anode and to change the anode clamper, it is possible for an operator to receive an increased exposure. By the way, we file a patent application to the JPO as a Japanese patent application laid-open No. 5-297192, and No. 6-242295 for decontaminating the radioactivity of the metallic waste by using a bipolar electrolytic with non-contact in the same manner of the present invention. The present invention makes these previous methods receive further a high function and high performance. In view of the above-mentioned condition, an object of the present invention is to provide a system and method for a decontamination of radioactive metallic waste, capable of removing radioactivity or decreasing radioactive level of the metallic waste in a short time and that it is unnecessary to change a clamp of an electrode and perform an attach and taking out of the electrode before and after a decontamination. Previously mentioned JP-A-3 249 600 also in the name of the Toshiba Corporation, describes an electrolysis decontamination system wherein a perforated basket for containing the object to be decontaminated is interposed between the electrodes placed in the electrolysis bath. Whereas this prior art system is excellent from the point of view of reducing exposure to radiation for an operator, ions in the electrolyte are free to flow through the perforated basket between the anode and cathode, such that electrolysis occurs preferentially at or near the electrodes, thereby slowing the dissolution of the object to be decontaminated. A further object of the present invention is therefore to overcome this problem. According to the invention from one aspect, there is provided a system for decontaminating radioactive metallic waste by performing bipolar electrolysis without contact between said metallic waste and either of a pair of electrodes in an electrolyte in an electrolysis bath, and by dissolving a base metal from said waste by an electrochemical reaction so as to eliminate radioactivity therefrom, said system comprising: an electrolysis bath for containing said electrolyte; an anode in said electrolysis bath arranged to be charged in a positive polarity by a direct current (DC) voltage supplied from a DC power source; a cathode in said electrolysis bath arranged to be charged in a negative polarity by said DC voltage supplied from said DC power source; and an insulating shield for dividing said electrolysis bath into a chamber containing said anode and a chamber containing said cathode, said insulating shield having a plurality of faces for preventing ions in said electrolyte from flowing therethrough. According to the invention from a second aspect, there is provided a method for decontaminating radioactive metallic waste, comprising the steps of providing an electrolysis bath containing an electrolyte and a pair of electrodes present in chambers and separated by an insulating shield having a plurality of faces for preventing ions in said electrolyte from flowing therethrough; placing a metallic waste in one of the chambers such that it does not contact either of the electrodes; performing bipolar electrolysis in said bath by supplying a direct current from a DC power source to said pair of electrodes to charge one of them positively as an anode and the other negatively as a cathode, to either positively or negatively charge a decontamination surface of the metallic waste respectively facing the cathode or anode, so as to dissolve base metal from the metallic waste by an electrochemical reaction, thereby eliminating radioactivity from said waste. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: FIG. 1 is a system diagram showing a decontamination system for decontaminating a radioactivity of a metallic waste according to first and fourth embodiments of the present invention; FIG. 2 is a plan view showing an electrolysis bath in the decontamination system according to the first and fourth embodiments shown in FIG. 1; FIG. 3 is a longitudinally sectional view showing the electrolysis bath in the decontamination system according to the first and fourth embodiments shown in FIG. 1; FIG. 4 is an experimental/theoretical view for explaining a system for decontaminating a radioactivity of a metallic waste according to a second embodiment of the present invention; FIG. 5 is an experimental/theoretical view for explaining a system for decontaminating a radioactivity of a metallic waste according to a third embodiment of the present invention; FIG. 6 is a characteristic diagram showing a relationship between a solubility and a reciprocal of an absolute temperature of an electrolyte for explaining a system for decontaminating a radioactivity of a metallic waste according to a fifth embodiment of the present invention; FIG. 7 is a longitudinally sectional view showing a system for decontaminating a radioactivity of a metallic waste according to a sixth embodiment of the present invention; FIG. 8 is a longitudinally sectional view showing a system for decontaminating a radioactivity of a metallic waste according to a seventh embodiment of the present invention; FIG. 9 is a system diagram showing a system for decontaminating a radioactivity of a metallic waste according to an eighth embodiment of the present invention; FIG. 10 is a plan view showing an arrangement relationship between a bar-shape cathode in an electrolysis bath and a curved metallic waste shown in FIG. 9; FIG. 11 is a perspective view showing a first blind-shape cathode set in an electrolysis bath shown in FIG. 9; FIG. 12 is a front view showing a set condition of the first blind-shape cathode in the curved metallic waste shown in FIG. 11; FIG. 13 is a front view showing a set condition of a second blind-shape cathode in the curved metallic waste shown in FIG. 9; FIG. 14 is a system diagram showing a schematic constitution of a system for decontaminating a radioactivity of a metallic waste. FIGS. 15A and 15B are a cross sectional view and a longitudinally sectional view respectively and schematically showing an important portion of an electrolysis bath shown in FIG 14; FIG. 16 is a model view for explaining an electrolysis reaction shown in FIG. 15B; FIG. 17 is a system view showing a schematical constitution of a system for decontaminating a radioactivity of a metallic waste according to a ninth embodiment of the present invention; FIGS. 18A and 18B are a cross sectional view and a longitudinally sectional view respectively and schematically showing an important portion of an electrolysis bath shown in FIG 17; FIG. 19 is a model view for explaining an electrolysis reaction shown in FIG. 18B; FIG. 20 is a bar graph showing both dissolution ratio of systems respectively according to the system shown in Fig. 14 and the tenth embodiment of the present invention under a comparison with the prior art; FIG. 21 is a bar graph showing a dissolution ratio on surfaces in comparison with an inner and outer in a system for decontaminating a radioactivity of a metallic waste according to aneleventh embodiment of the present invention; FIG. 22A is a schematic cross sectional view showing a system for decontaminating a radioactivity of a metallic waste according to a thirteenth embodiment, and FIG. 22B is a schematic longitudinally sectional view showing the system shown in FIG. 22A; FIG. 23 is a bar graph showing a dissolution ratio under the comparison of a presence and absence of a disc in a system for decontaminating a radioactivity of a metallic waste according to a fourteenth embodiment of the present invention; FIG. 24 is a system diagram schematically showing a system for decontaminating a radioactivity of a metallic waste according to a fifteenth embodiment of the present invention; and FIG. 25A is a solubility distribution characteristic diagram for explaining a first embodiment in a sixteenth embodiment of the present invention, and FIG. 25B is a solubility distribution characteristic diagram for explaining a second example of the sixteenth embodiment of the present invention. There will be described in detail an apparatus and method for decontaminating a radioactivity of a metallic waste according to preferred embodiments of the present invention in reference with the attached drawings. There is described a system for decontaminating a radioactivity of a metallic waste according to first and fourth embodiments of the present invention in reference with FIGS. 1-3. FIG. 1 is a system diagram showing an example with respect to the first and fourth embodiments. In FIG. 1, numeral 1 denotes an insulating shield plate, and 2 denotes an electrolysis bath which includes a lid 2a, an electrolyte 3, and an electrolyte heater 4. The electrolysis bath 2 is divided into an anode chamber 13 and a cathode chamber 14. The anode chamber 13 has an anode 5 comprised of a deactivate metal, the cathode chamber 14 has a cathode 6 and a metallic waste 7, and the anode 5 and the cathode 6 are connected with a direct current power source 8, respectively. An exhaust gas treating system 9 is connected to an upper portion of the electrolysis bath 2 to treat a steam and gas generated from the electrolyte 3. The electrolyte 3 circulates through the electrolysis bath 2, a filter 11 and an electrolyte circulation line 12 by a circulating pump 10. Next, there is described an electrolysis reaction in the method for decontaminating the radioactivity of the metallic waste according to the first embodiment of the present invention by commonly using FIG. 2 showing a plan view of the electrolysis bath 2 and FIG. 3 showing a longitudinally sectional view of the electrolysis bath 2. The insulating shield plate 1 is formed in a shape of a character "U" as shown in FIG. 2, the cathode 6 is arranged on an inner surface of the insulating plate 1, the anode 5 is arranged on an outer surface of the insulating plate 1, and the anode 5 faces to the cathode 6 with the insulating plate 1 between. On the other hand, a radioactive metallic waste 7 is grounded in the opposite direction to the anode 5 in the manner of facing to the cathode 6. An ion in the electrolyte 3 moves only in a gap between the insulating shield plate 1 and a side wall of the electrolysis bath 2, and an upper end 1a of the insulating shield plate 1 is provided higher than a liquid surface 3a of the electrolyte 3 and a lower end 1b of the insulating shield plate 1 is connected to a bottom portion of the electrolysis bath 2, in order to prevent the ion to move through upper and lower portions of the electrolysis bath 2. Furthermore, the quality of the material is an insulating material or a metal lined with an insulating material. In this condition, the circulating pump 10 circulates the electrolyte 3 and the electrolyte heater 4 heats the electrolyte 3 to a predetermined temperature. Then, when the DC power source 8 supplies a DC voltage having a predetermined current density to a portion between the anode 5 and the cathode 6, a reaction represented by the following equations (1)-(3) occurs with respect to the anode 5, cathode 6 and the metallic waste 7 so as to cause a surface (M) of the metallic waste 7 to be charged of a positive electrode by a dielectric function so as to be resolved: (anode) H 2 O → 2H + + 1/2•O 2 ↑ + 2e - (cathode) H + + 2e - → H 2 ↑ (metallic waste) M - M n+ + ne - A radioactivity fixed to the metallic waste 7 or permeated in the base metal is eliminated from the metallic waste 7 to move into the electrolyte 3 by dissolving the base metal, thereby decontaminating a radioactivity or decreasing a radioactivity level of the metallic waste 7. When the metallic waste 7 is contaminated over the entire surface, a polarity of the DC power source 8 is inverted to charge an opposite surface to be positive so as to dissolve the base metal. On the other hand, the exhaust gas treating system 9 treats a mist, steam, gas and the like occurring from the electrolyte 3. There is described a system according to a second embodiment of the present invention in accordance with FIG. 4, which shows a relative dissolution ratio (an experimental value/theoretical value) in comparison with stainless steels in any cases when the insulating shield plate is a simple plate-shape shield plate 23 and when the insulating plate is the U-shape insulating plate 1. In the second embodiment, a sulfuric acid is selected as an acid electrolyte, which has a concentration of 0.5 mol/L and an electrolyte temperature of 80 °C, and an electrolysis is performed by supplying a DC voltage having a current density of 0.6 A/cm 2 to a portion between the anode and cathode which are comprised of titanium coated by platinum. As understood from FIG. 4, even though the stainless steel is hardly dissolved when the insulating shield plate is the simple plate 23, it is possible to obtain a relative dissolution ratio of 0.2 when the plate is the U-shape insulating shield plate 1. A cause of this is that an electrolysis between the anode and cathode is prior to everything because the anode is near the cathode even though the simple plate of the insulating shield plate is provided between the anode and cathode when the insulating shield plate is the simple plate 23. On the other hand, when the U-shape insulating shield plate 1 is used, an electric field between the cathode and anode is shut out by plate-shape stainless steel. Therefore, a current decreases for leaking between the anode and cathode. Since the distance between the anode and cathode becomes far away by the U-shape insulating shield plate 1, a supplied voltage increases. When a bipolar electrolytic with non-contact dissolves a metal, it is necessary to supply an overpotential larger than an equilibrium potential of a metal dissolving reaction. Accordingly, when a voltage supplied between the anode and cathode becomes large, it is possible to efficiently dissolve the base metal by supplying an overvoltage larger than an equilibrium potential for dissolving a stainless steel. Furthermore, for causing the distance between the anode 5 and the cathode 6 to be distant and making the distance between the cathode 6 and the metallic waste 7 be near, it is possible to enlarge a size of the electrolysis bath 2. However, in this case, an increase of the liquid amount of the electrolyte 3 causes an occurring amount of a secondary waste (a decontaminated waste fluid) to be also increased. However, since there is provided the U-shape insulating shield plate 1, it is possible to leave a space between the anode 5 and the cathode 6 without the increase of the capacity of the electrolysis bath 2. As described above, since the second embodiment of the present invention uses the U-shape shield plate 1, it is possible to efficiently dissolve the metallic waste, to decontaminate a radioactivity of the metallic waste, and to decrease the radioactivity level. Next, there is described a system for decontaminating a radioactivity of a metallic waste according to a third embodiment of the present invention in accordance with FIG. 5 showing a relative dissolution ratio (an experimental/theoretical value) when both surfaces of a plate-shape metal (a stainless steel) are dissolved by inverting a polarity of a direct current power source. In the third embodiment, an electrolysis is performed by supplying a DC voltage to a portion between the anode 5 and the cathode 6 of titanium coated by platinum under the condition that an acid electrolyte is comprised of sulfuric acid having a density of 0.5 mol/L and 80 °C of a temperature. Even though the stainless steel is not dissolved when the insulating shield plate is formed from the simple plate 23 as shown in FIG. 4, it is possible to obtain a relative dissolution ratio 0.2 when the shield plate is the U-shape shield plate 1. The reason resides in that the electrolysis between the anode and the cathode is rapid because the cathode is near the anode even through the insulating shield plate when the plate is the simple plate 23. On the other hand, since an ion must move along the U-shape shield plate and the side wall of the electrolyte bath to reach to the inner cathode when the plate is the U-shape shield plate 1, the distance between the cathode and anode is nearer than the distance between the cathode and the stainless steel, thereby resulting a little current for leaking the portion between the cathode and anode. Furthermore, a.supplied voltage increases because of becoming a large distance between the anode 5 and the cathode 6 through the U-shape shield plate 1. When a metal is dissolved by a bipolar electrolytic with non-contact, the metal surface must be supplied an overpotential larger than an equilibrium potential of the metal dissolving reaction. Accordingly, when the large voltage is supplied to a portion between the anode and the cathode, it is possible to efficiently dissolve the base metal by supplying a potential larger than the equilibrium potential for dissolving the stainless steel. Even though it is possible to lengthen the distance between the anode 5 and the cathode 6 and to shorten the distance between the cathode 6 and the metallic waste 7 by enlarging the size of the electrolyte bath 2, this case results an increase of the generated amount of the secondary waste (a decontamination waste liquid) with the decontamination because the liquid amount of the electrolyte 3 increases. However, the U-shape shield plate 1 can enlarge the distance between the anode 5 and the cathode 6 without increasing a capacity of the electrolysis bath 2. As described above, since the second embodiment of the present invention uses the U-shape insulating shield plate 1, it is possible to effectively dissolve the metallic waste to decontaminate the radioactivity of the metallic waste, thereby decreasing the radioactivity level. Next, there is described a third embodiment with reference to FIG. 5 showing a relative dissolution ratio (an experiment/theory value) when both surfaces of the plate-shaped metal (a stainless steel) are dissolved by inverting a polarity of a DC power source. In the same way as the second embodiment shown in FIG. 4, the third embodiment uses a sulfuric acid selected as an acid electrolyte, which has a concentration of 0.5 mol/L and an electrolyte temperature of 80 °C, and an electrolysis is performed by supplying a DC voltage having a current density of 0.6 A/cm 2 to a portion between the anode and cathode which are comprised of titanium coated by platinum. As clearly understood from the third embodiment, it has been possible to efficiently dissolve both surface (a dissolved surface (A) and a dissolved surface (B)) of the plate-shape stainless steel by inverting a polarity of the DC power source. A dissolving reaction of the dissolved surface (B) causes one surface of the metallic waste 7 facing the anode 5 to be a negative polar with an electrostatic charge, and the other surface of the metallic waste 7 to be a positive polar with an electrostatic charge, thereby dissolving the dissolved surface (B). As described above, the provision of the U-shape insulating shield plate 1 can efficiently dissolve an entire surface of the metallic waste 7 only by inverting a polarity of the DC power source, thereby decontaminating the radioactivity or reducing the radioactivity level of the metallic waste 7. If the electrolyte of the present invention is comprised of phosphoric acid, nitric acid, sodium sulphate or sodium nitrate without sulfuric acid according to the second embodiment, the same effect can be obtained. Accordingly, the inversion of a polarity of the DC power source can change from the anode chamber to the cathode chamber and from the cathode chamber to the anode chamber. Any embodiment can be provided to the present invention as far as the insulating shield plate decontaminates the radioactivity of the metallic waste. Next, there is described a decontamination system according to a fourth embodiment of the present invention with reference to FIGS. 1 and 2. In the fourth embodiment, sulfuric acid solution is selected as the electrolyte 3, the anode 5 is arranged in the anode chamber 13 which is divided by the insulating shield plate 1, the cathode 6 and metallic waste 7 are arranged in the cathode chamber 14, the electrolyte 3 circulates by the circulation pump 10 to increase a temperature to a predetermined value by the electrolyte heater 4, and the DC voltage is supplied from the DC power source 8 to a portion between the anode 5 and the cathode 6 during a predetermined time interval. The supply of the DC voltage results a reaction shown by the equation (1) around the anode 5 to generate oxygen gas, and results a reaction shown by the equation (2) around cathode 6 to generate hydrogen gas. On the other hand, one surface of the metallic waste 7 facing to the cathode 6 has an electrostatic charge of the positive polar, and the other surface of the waste 7 has a charge of the negative polar. Here, even though the waste 7 is easy to be dissolved by sulfuric acid and nitric acid when the metallic waste 7 is a carbon steel, it is difficult to dissolve when an oxide layer and rust are attached on the entire surface of the waste. Since the stainless steel has a passive state layer on its surface, the stainless steel has an excellent anti-corrosion. However, if the surface of the stainless steel or carbon steel is charged to the negative polar, a reaction shown by the following equations (4) and (5) happens to the surface so as to reduce and eliminate the passive state layer, oxide layer and rust on the surface. Metallic waste (negative charged surface): Passive layer, oxide layer: Fe 3 O 4 + 8H + + 2e - → 3Fe 2+ + 4H 2 O Reduction of rust: Fe 2 O 3 + 6H + + 2e - → 2Fe 2+ + 3H 2 O In the above manner, after the passive state layer, oxide layer or rust is reduced and eliminated from the surface of the metallic waste 7, the base metal exposes to be activated. Under the condition, a stop of the DC voltage supply from the DC power source makes the metallic waste be dissolved by oxidizing force of sulfuric acid. If the anode 5 in the anode chamber 13 changes a cathode and the cathode 6 in the cathode chamber 14 changes an anode by converting the polarity of the DC power source 8 so as to cause the surface of the metallic waste 7 facing to the converted anode to be a negative polar by an electrostatic charge, the decontamination is performed in the same manner. Accordingly, a radioactivity,"which is attached on the metallic waste with the oxide layer or soaks into the base metal, moves into the electrolyte with the oxide layer which is removed from the metallic waste by reducing the oxide layer and dissolving the base metal, thereby decontaminating the radioactivity and decreasing the radioactivity level of the metallic waste. Next, there is described a decontamination system according to a fifth embodiment for recognizing an effect of the system according to the fourth embodiment, with reference to FIG. 6. In the fifth embodiment, a dissolving experimentation is performed with a stainless steel (SUS 304) by supplying the DC voltage of 5 V for five minutes to a portion between the anode and cathode made of titanium coated by platinum in sulfuric acid having a density of 1 mol/L and 2 mol/L. In FIG. 6, a longitudinal axis denotes a relative dissolution ratio (a dissolution ratio at each temperature against a dissolution ratio at 60 °C), and a horizontal axis denotes an inverse of an absolute temperature of the electrolyte. A dissolution ratio of a stainless steel has a linear relationship with the inverse of the absolute temperature, and increases by an exponent function with the temperature of the electrolyte. As described above, the decontamination system according to the fifth embodiment can reduce a radioactivity level or decontaminate a radioactivity of the metallic waste because an electrostatic charge of a negative polar makes a surface of the metallic waste be easily dissolved by oxidizing force of sulfuric acid. Accordingly, the system of the present invention can apply to an electrolysis decontamination which has conventionally been difficult to decontaminate with respect to a complex shaped object such as a curved pipe or curved valve. The electrolyte using the electrolyte of the fifth embodiment can change from sulfuric acid to nitric acid or chloric acid so as to obtain the same effect. Next, there is described a decontamination system according to a sixth embodiment of the present invention with reference to FIG. 7 showing a longitudinal cross section of the electrolysis bath 2. In FIG. 7, numeral 15 denotes a shielded vessel having an insulation and an opening portion at an upper end, the anode 5 is arranged at a bottom of the shielded vessel 15, a supporting member 16 having an insulation and a mesh-shape is arranged at an upper end of the anode 5, and a cathode 6 is arranged at a bottom portion of the electrolysis bath 2 in the manner of putting a bottom portion of the shielded vessel 15 therebetween. The metallic waste 7 is stored in the shielded vessel 15 and supported by the insulation supporting member 16 to which a plurality of through holes are opened in a mesh-shape for passing therethrough the electrolyte and oxygen gas so as not to contact the metallic waste 7 to the anode 5. Under the condition, when a DC voltage is supplied to a portion between the anode 5 and cathode 6, a surface opposite to the anode 5 of the metallic waste 7 is charged in electrostatic to a positive polar because an ion in the electrolyte moves and passes through the holes of the supporting member 16. This charge causes the electrolyte to generate a dissolving reaction shown in FIG. 3, thereby decontaminating the radioactivity and decreasing the radioactive level of the metallic waste. Accordingly, the shielded vessel 15 having the opening at the upper portion-according to the sixth embodiment can increase a decontamination treated amount of the metallic waste per 1 batch. Next, there is described a decontamination system according to a seventh embodiment with reference to FIG. 8 showing a longitudinal cross section of the electrolysis bath 2. In FIG. 8, numeral 17 is an insulating basket having an opening at an upper portion, in which a metallic waste 7 is stored. The basket 17 is arranged in the insulating shielded vessel having the opening at its upper portion. The anode 5 is arranged at a bottom of the shielded vessel 15, and the cathode 6 is arranged at a bottom of the electrolysis bath 2 in the manner of putting the bottom of the shielded vessel 15 therebetween. Under this condition, when a DC voltage is supplied to a portion between the anode 5 and cathode 6, a surface opposite to the anode 5 of the metallic waste 7 is charged in electrostatic to a positive polar because an ion in the electrolyte moves and passes through the holes of the supporting member 16. This charge causes the electrolyte to generate a dissolving reaction shown in FIG. 3, thereby decontaminating the radioactivity and decreasing the radioactive level of the metallic waste. Accordingly, the insulating basket 17 having the opening at the upper portion according to the seventh embodiment also can increase a decontamination treated amount of the metallic waste 7 per 1 batch in the same manner of the sixth embodiment. Furthermore, since the insulating basket 17 can be stored and taken out by using the driving mechanism in and from the electrolysis bath 2, it becomes easy to automatically perform a mass processing. When the systems according to the first through third embodiments are combined with the systems according to the fourth through sixth embodiments or the system according to the fourth embodiment, it is possible to alternatively repeat oxidization and reduction on the surface of the metallic waste 7 by inverting a polarity of a DC power source at each predetermined time. By this, it is possible to selectively eliminating an oxide layer and rust by reducing them as a contamination source of the metallic waste 7. After that, since it is possible to decontaminate and decrease the radioactivity and its level by little dissolving amount of the base metal when an oxidization dissolves the base metal, it is possible to decrease an occurring amount of the secondary waste in accordance with the decontamination. Accordingly, since the seventh embodiment has an effect for the metallic waste made of the carbon steel which has thick oxide layer and rust including a radioactivity and strongly fixed on its surface, a repeated. processing of oxidization and reduction can remove the radioactivity in a short time, thereby decreasing the radioactive level. There is described a decontamination system according to an eighth embodiment of the present invention with reference to FIGS. 9 and 10. FIG. 9 is a system diagram showing an example of a system for explaining the eight embodiment, in which numeral 1 denotes the insulating shield plate, 2 is the electrolysis bath including the electrolyte 3 and storing the electrolyte heater 4. The electrolysis bath 2 is divided into the anode chamber 13 and cathode chamber 14 by the insulating shield plate 1. The anode 5 made of inert metal is stored in the anode chamber 13, and the metallic waste 7 and the cathode 18 are stored in the cathode chamber 14. The cathode 18 has a bar shape or a rectangular pipe shape which is made of the inert metal, and the anode 5 and the cathode 18 are respectively connected to the DC power source 8. Furthermore, an exhaust gas processing system 9 is connected to the upper portion of the electrolysis bath 2 for processing a steam and gas occurring from the electrolyte 3. The electrolyte 3 circulates in the electrolysis bath 2, filter 11 and electrolyte circulation line 12 by the circulation pump 10. There is described operation for decontaminating a radioactivity of the metallic waste according to the eighth embodiment of the present invention with reference to FIG. 10 showing a plan view of the electrolysis bath 2 shown in FIG. 9. The insulating shield plate 1 has the U-shape having an inner surface to which the bar-shaped or rectangular-shaped cathode 18 is arranged and faced, and an outer surface to which the anode 5 is arranged and faced. The cathode 18 and anode 5 are arranged to face each other in the manner of putting the insulating waste 1 therebetween. On the other hand, the metallic waste 7 is grounded in the direction opposite to the insulating shield plate 1 for facing the cathode 18. An ion in the electrolyte 3 moves in only a gap between the insulating shield plate 1 and the side wall of the electrolysis bath 2, and the upper portion of the shield plate 1 is higher than a liquid surface 3a of the electrolyte 3 and the lower portion of the shield plate 1 is connected to the bottom portion of the electrolysis bath 2 in order to prevent an ion from moving through the upper and lower portions of the electrolysis bath 2. Furthermore, a material of the electrolysis bath 2 is the insulating material or the metal coated by an insulating material. In this condition, the circulation pump 10 circulates the electrolyte 3 to increase the temperature to a predetermined value by the electrolyte heater 4 so as to supply a DC voltage having a predetermined current density from the DC power source 8 to a portion between the anode 5 and cathode 18. When the driving mechanism 19 moves the cathode 18 with keeping a predetermined interval against the surface of the metallic waste 7, the surface of the metallic waste (M) against the cathode 18 is dissolved by a reaction shown in the equation (3) on the basis of the dielectric function. When the decontamination is performed with respect to the curved plate, if the plate-shaped cathode is used, since the distance between the cathode and the surface of the metallic waste becomes partially different from each other, it is possible to leave a partial contamination. In the eighth embodiment, since the bar-shaped or pipe-shaped cathode 18 is used to decontaminate the radioactivity by moving with keeping the predetermined gap against the metallic waste surface by the driving mechanism 19, it is possible to uniformly dissolve the metal surface, thereby equally decontaminating the radioactivity after preventing the partial contamination remaining. When the metallic waste before decontaminated has a partial contamination, the dissolution of the entire contaminated surface increases an occurring amount of the secondary waste. However, since the system according to the eighth embodiment can decontaminate by moving the bar-shaped or pipe-shaped cathode near to the contaminated portion of the metallic waste, it is possible to largely decrease the occurring amount of the secondary waste in comparison to the dissolution of the entire metal surface. Accordingly, when the decontamination is performed by moving the bar-shaped or pipe-shaped cathode by the driving mechanism, the metallic waste having the curved plate can be even decontaminated for its surface. Furthermore, since the partial contamination can be contaminated within this region, it is possible to improve an application for the shape of the metallic waste, thereby largely decreasing the occurring amount of the secondary waste with the decontamination. Next, there is described a modified example of the eighth embodiment with respect to the method and system for decontaminating the radioactivity with reference to FIGS. 11 through 13. FIG. 11 shows of the blind-shaped cathode 21 in which a plurality of bar-shaped cathodes 18 are arranged in a blind shape by means of a connection by a flexible cable 20, thereby freely bending a portion of the flexible cable 20. For a better understanding, but not contributing to the scope of the invention, FIG. 12 shows a case in which the blind-shape cathode 21 shown in FIG. 11 is used for the curved metallic waste 7. Since the blind shape cathode 21 can be bent at a portion of the flexible cable 20, the cathode 21 changes to the curved shape along the shape of the metallic waste 7, thereby uniformly decontaminating the radioactivity on the metal surface with keeping a predetermined interval against the surface of the metallic waste. FIG. 13 shows the modified example of the eighth embodiment in which an insulating elastic body 22 allowing a water passing through is attached with the blind shape cathode 21. The insulating elastic body 22 can prevent the blind shape cathode 21 and metallic waste 7 from a contact and can keep the distance between the metallic waste 7 and blind shape cathode 21 to a predetermined degree, thereby uniformly decontaminating the radioactivity on the surface of the metallic waste 7. The insulating elastic body 22 can be utilized by a material such as a rubber having a plurality of holes or a sponge. In the above-mentioned system or method for decontaminating a radioactivity of the metallic waste according to the embodiments of the present invention, since the electrolysis occurs on the wall surface of the electrolysis bath by a dielectric function when the material of the vessel is metal, it is impossible to efficiently charge with electrostatic to a positive or negative polar on the surface of the metallic waste. Accordingly, the electrolysis bath, insulating shield plate, shield vessel, supporting member in the shield vessel and basket are made of the simple of the insulating materials having a chemical-proof material such as a fluorocarbon polymer, fiber reinforced plastic (FRP) and the like, or metal lined by the insulating member. Furthermore, the shape of the electrolysis bath, shield vessel and basket are not limited in the rectangular shape and may be applicable to cylindrical shape. Furthermore, it is possible to utilize electrodes which are made of cooper coated by titanium further coated by platinum, simple platinum electrode, metal except titanium coated by platinum, and lead compound electrode, in addition to titanium coated by platinum as a material of the electrode used in the above embodiment. For a better understanding, but not contributing to the scope of the invention, there is described a system for decontaminating a radioactivity of the metallic waste with reference to FIGS. 14-16. FIG. 14 is a longitudinal sectional view showing an example of an electrolysis bath for explaining the system. In the figure, numeral 31 denotes an electrolysis bath, in which an electrolyte 32 is filled up. There are provided in the electrolyte 32 a cylindrical anode 33, a cylindrical metal 34 as a radioactive metallic waste for an object and enclosed by the cylindrical anode 33, and a bar-shape cathode 5 in the cylindrical metal 34. The cylindrical metal 34 is fixed on a platform 36, and the cylindrical anode 33 and the bar-shape cathode 35 are connected to a direct current power source 37. A handling mechanism 38 is arranged at the upper portion of the electrolysis bath 31 for storing and taking the cylindrical metal 34 in and out the vessel 31. There is described in detail a positional relationship between the cylindrical anode 33, cylindrical metal 34 and bar-shape cathode 35 with reference to FIGS. 15A and 15B, in which FIG. 15A is a plan view of the system and FIG. 15B is a longitudinal sectional view of the system. The cylindrical metal 34 is arranged at the center of the cylindrical anode 33, the bar-shape cathode 35 is arranged at the center of the cylindrical metal 34, and the DC power source 37 is connected to the cylindrical anode 33 and the bar-shape cathode 35, respectively. Under this condition, when a DC voltage is supplied from the DC power the DC power source 37 to a portion between the cylindrical anode 33 and the bar-shape cathode 35, a reaction shown in FIG. 16 occurs on the surface of the cylindrical anode 33 and between an inner surface of the cylindrical metal 34 and an outer surface of the bar-shape cathode 35. The followings are principle of the above reaction: (anode) H 2 O → 2H + + 1/2•O 2 ↑ + 2e - (cathode) H + + 2e - → H 2 ↑ (outer of metal) H + + 2e - → H 2 ↑ (inner of metal) M → M n+ + ne - Since the outer surface of the cylindrical metal 34 faces with the cylindrical anode 33, the outer of the metal 34 is charged in a negative polar by dielectric function. Since the inner surface of the cylindrical metal 34 faces with the bar-shape cathode 35, the inner surface is charged in a positive polar, thereby dissolving the inner surface of the base metal. By this, a dissolution of the base metal decontaminates a radioactivity which is fixed on the inner surface of the cylindrical metal 34 or soaked into the base metal from the metal 34 to move into the electrolyte 32, thereby decontaminating the radioactivity or decreasing the radioactivity level of the cylindrical metal 34. There is described a decontamination system for a radioactive metallic waste according to a ninth embodiment of the present invention with reference to FIGS. 17-19. FIG. 17 is a longitudinal sectional view of an example of an electrolysis bath in the system for explaining the ninth embodiment, in which a numeral 31 denotes an electrolysis bath into which an electrolyte 32 is filled. There are provided in the electrolyte 32 a cylindrical electrode clamper 39 and the cylindrical metal 34 in the clamper 39. The metal 34 is fixed to the platform 36, and the cylindrical electrode clamper 39 is connected to the DC power source 37. Furthermore, the handling mechanism 38 is set over the electrolysis bath 31 in order to insert and take the cylindrical metal 34 into and out of the bath 31. There is described in detail the cylindrical electrode clamper 39 with reference to FIGS. 18A and 18B, in which FIG. 18A is a plan view and FIG. 18B is a longitudinal section view. The cylindrical electrode clamper 39 is provided for holding the cylindrical anode 33 to the inner wall of a cylindrical insulating shield body 40 and the cylindrical cathode 41 to the outer wall of the cylindrical insulating shield body 41. The cylindrical metal 34 is set in the cylindrical anode 33, and the DC power source 37 is connected to the cylindrical anode 33 and the cylindrical cathode 41, respectively. Under this condition, when the DC voltage having a predetermined current density is supplied to the cylindrical anode 33 and the cylindrical cathode 41, respectively, a reaction shown in FIG. 19 occurs at the surfaces of the cylindrical anode 33 and cylindrical cathode 41 and at the inner and outer surfaces of the cylindrical metal 34. The reaction has a principle represented by the equations (1'), (2'), (6) and (3'). The bar-shape cathode 41 represented by the equation (2') can be replaced by the cylindrical cathode. Since the outer surface of the cylindrical metal 34 faces to the cylindrical anode 33, the outer surface is charged by dielectric function to the negative polar, and the inner surface of the cylindrical metal 34 is divided in a polar to be charged in the positive polar, thereby dissolving the inner surface of the base metal. By this, a dissolution of the base metal decontaminates a radioactivity which is fixed on the inner surface of the cylindrical metal 34 or soaked into the base metal from the metal 34 to move into the electrolyte 32, thereby decontaminating the radioactivity or decreasing the radioactivity level of the cylindrical metal 34. Next, there is described a decontamination system according to a tenth embodiment of the present invention with reference to FIG. 20. FIG. 20 shows dissolution results of the inner surface of the cylindrical metal 34 formed of the stainless steel by the conventional contact electrolysis (an insert cathode), an insert cathode according to Figs. 14-16, and an insert cathode of the tenth embodiment, by using a relative dissolution ratio (experiment/theory values). The theory value can be obtained by Faraday's law. In the tenth embodiment, phosphoric acid is selected as the electrolyte, in which a density of phosphoric acid is 40%, a temperature of the electrolyte is 60 °C, and a DC voltage having a current density of 0.6 A/cm 2 is supplied to a portion between the anode and cathode formed of titanium coated by platinum so as to perform an electrolysis. As clearly understood from figures, even though a relative dissolution ratio of this embodiment is slower than that of the conventional contact electrolysis, since the inner surface of the cylindrical metal can be dissolved in the electrolysis method disclosed herein, it is possible to decontaminate a radioactivity or decrease a radioactive level of the metallic waste. As described above, since the system according to Figs. 14-16 can dissolve the inner surface of the curved metal without a connection between the cylindrical metal and the anode, it is possible to improve work efficiency and to decrease an exposure amount for an operator. Furthermore, since the ninth embodiment of the present invention can dissolve the inner surface of the curved metal without an insertion of the cathode into the cylindrical metal, it is possible to easily insert and take the metal into and out of the electrolysis bath 31, to easily automate the system using the handling mechanism, and to further decrease the exposure amount for an operator. Next, there is a system for decontaminating a radioactivity according to an eleventh embodiment of the present invention with reference to FIG. 21. FIG. 21 shows a relative dissolution ratio (experiment/theory values) as a dissolved result of the inner and outer surfaces of the cylindrical metal 34 formed of a stainless steel in the eleventh embodiment in which a polarity of DC current power source in the tenth embodiment is inverted. In the eleventh embodiment, a density of phosphoric acid is 40%, a temperature of the electrolyte is 60°C, and a current density is 0.6A/cm 2 , thereby performing an electrolysis by supplying a DC voltage to a portion between the anode and the cathode which are formed of titanium coated by platinum. As understood from the eleventh embodiment, it is possible to dissolve the inner and outer surfaces of the cylindrical metal at substantially the same dissolution ratio by inverting a polarity of the DC power source. As described above, since only inversion of a polarity of the DC power source can dissolve the inner and outer surfaces of the cylindrical metal, it is possible to decontaminate a radioactivity and decrease a radioactive level of the cylindrical metal. Next, there is described a decontamination system according to a twelfth embodiment of the present invention with reference to FIGS. 14-15B. When the cylindrical metal 34 is a carbon steel, an oxide layer and rust are thickly and firmly formed on the surface of the base metal, and the oxide layer is hard to be dissolved by a simple anode electrolysis. Since a radioactivity is almost included in the oxide layer, it is a long time to decontaminate the radioactivity from the cylindrical metal 34. It is effective that there is a method of alternatively inverting a polarity of the DC power source 37 for decontaminating in a short time the radioactivity of the cylindrical metal 34 of the carbon steel. A charge in the negative polar to the inner surface (contaminated surface) of the cylindrical metal 34 of the carbon steel causes a reaction on the inner surface to occur so as to reduce the oxide layer and rust. (Charged surface of negative polar) Reduction of oxide layer and rust : Fe 3 O 4 + 8H + + 2e - → 3Fe 2+ + 4H 2 O Fe 2 O 3 + 6H + + 2e - → 2Fe 2+ + 3H 2 O In this manner, the oxide layer and rust are reduced and eliminated from the contaminated surface of the cylindrical metal 34, and at the same time the radioactivity in the oxide layer is decontaminated. Furthermore, since the polarity of the DC power source 37 is alternatively converted, the contaminated surface is also charged to the positive polar so as to dissolve the base metal exposing after the oxide layer is removed, and to remove a contamination soaked into the base metal. Accordingly, a radioactivity attached on or soaked into the base metal with the oxide layer of the cylindrical metal 34, can be decontaminated with little dissolution amount from the cylindrical metal 34 by reducing the oxide layer and dissolving the base metal to move into the electrolyte 32, thereby decontaminating the radioactivity and decreasing the radioactive level of the cylindrical metal 34, so as to decrease occurring amount of a secondary waste with the decontamination. Next, there is described a decontamination system for a radioactive metallic waste according to a thirteenth embodiment with reference to FIGS. 22A and 22B. FIG. 22A is a plan view showing a condition in which an insulating discs 42 each having openings are attached at upper and lower portions of the cylindrical insulating shield body 40, and FIG. 22B is a longitudinal section view showing the above condition. A cylindrical anode 33 is arranged on the inner wall of the cylindrical insulating shield body 40, and a cylindrical cathode 41 is arranged on the outer wall of the cylindrical insulating shield body 40. The cylindrical metal 34 is set in the cylindrical anode 33, and a DC power source 37 is connected to the cylindrical anode 33 and the cylindrical cathode 41, respectively. Under the condition, when a voltage having a predetermined current density is supplied from the DC power source 37 to a portion between the cylindrical anode 33 and the cylindrical cathode 41, the reaction explained by a theory of the equations (1'), (2'), (6) and (3') occurs to charge in a positive polar with the inner surface of the cylindrical metal 34, thereby dissolving the base metal arranged on the inner surface. Next, there is described a decontamination system according to fourteenth embodiment of the present invention in reference with FIG. 23 showing each dissolution ratio (experiment/theory values) of results of dissolution the inner surface of the cylindrical metal 34 of the stainless steel according to the ninth and thirteenth embodiments, in which the ninth embodiment uses the cylindrical insulating shield body 40 and the thirteenth embodiment uses the shield body 40 and the insulating discs 42 each attached to the upper and lower ends of the body 40 and having opening. In the fourteenth embodiment, phosphoric acid is selected as the electrolyte 32, and the electrolysis is performed under the condition that a density of phosphoric acid is 40%, a temperature is 60°C, a current density is 0.6A/cm 2 , thereby supplying the DC voltage to a portion between the cylindrical anode 33 and the cylindrical cathode 41 which is titanium coated by platinum. As understood from FIG. 23, the relative dissolution ratio improves about 1.3 times by attaching the insulating discs 42 to the upper and lower ends of the cylindrical insulating shield body 40, respectively. There is a reason that the current leaking into a portion between the anode 33 and the cathode 41 is broken and the electrolysis is suppressed between the anode and cathode by means of an attachment of the insulating discs 42 respective to the upper and lower ends of the cylindrical insulating shield body 40. As described above, since the insulating discs 42 are attached at the upper and lower ends of the cylindrical insulating shield body 40, respectively, it is possible to decontaminate in a short time a radioactivity of the cylindrical metal 34 and to decrease the radioactive level of the metal. There is described a decontamination system according to a fifteenth embodiment with reference to FIG. FIG. 24 is a longitudinal section view of the electrolysis bath 31 in the system according to the fifteenth embodiment, in which numeral 43 denotes a supporting vessel having an opening at its upper portion, and the supporting vessel 43 is hung down in the electrolysis bath 31 by a handling mechanism 38 which is set to the upper portion of the bath 31. The supporting vessel 43 has a side surface formed of an insulating material and having a plurality of holes in a mesh-shape for the electrolyte flowing through and a bottom portion formed of a metal material 48, and stores a plate-shape metal 44 as the radioactive metallic waste therein. The supporting vessel 43 is set in the insulating shield vessel 45 having an opening at its upper portion, the plate-shape anode 46 is set to the bottom portion of the shield vessel 45, and the plate-shape cathode 47 is set to the bottom portion of the electrolysis bath 31 through the bottom portion of the shield vessel 45. Under the condition, when the DC voltage is supplied between the plate-shape anode 46 and the plate-shape cathode 47, since the potential difference occurs at both sides of the insulating shield vessel 45, the difference causes a surface of the metal material 48 provided at the bottom portion of the supporting vessel 43 facing to the plate-shape anode 46 to be charged to the negative polar. On the other hand, since the metal 44 contacts with the metal material 48 at the bottom portion of the supporting vessel 43, a polar of the metal 44 is divided to charge in a positive polar with the upper surface of the metal 44, thereby dissolving the base metal of the metal 44. Next, there is described a contamination system according to a sixteenth embodiment with reference to FIGS. 25A and 25B. FIG. 25A shows a relative dissolution distribution of the plate-shape stainless steel of a second example when the bottom portion is formed of a passive metal of titanium coated by platinum by a relative dissolution ratio (dissolution ratio at each position against a mean dissolution ratio), and FIG. 25B shows a dissolution distribution of the plate-shape stainless steel of a first example when the bottom portion is formed of the insulating material having a plurality of holes in a mesh-shape for passing the electrolysis therethrough by a relative dissolution ratio (dissolution ratio at each position against a mean dissolution ratio). In the sixteenth embodiment, phosphoric acid is selected as the an acid electrolyte and an electrolysis is performed by supplying a DC - voltage between the anode and cathode formed of titanium coated by platinum under the condition that a density of phosphoric acid is 40%, a temperature of the electrolyte is a room temperature, and a current density is 0.6A/cm 2 . As understood from FIG. 25B, the surface of the stainless steel can be uniformly dissolved by constructing the bottom of the supporting vessel by a metal material. However, as understood from FIG. 25A, the end is hard to be dissolved, when the bottom of the supporting vessel is made of the insulating material having the plurality of holes in the mesh-shape. The reason is, even though it has been necessary to supply the potential larger than the equilibrium potential of the metal dissolution reaction to the surface of the metal when the metal is dissolved by a bipolar electrolytic with non-contact, that the potential at the end portion is low and the potential at the center of the metal surface is high by an influence of the leaking current between the anode and cathode. As described above, when the bottom of the supporting vessel 43 is made of the metal material 48, since the metal surface can be uniformly dissolved, it is possible to uniformly decontaminate a metallic waste having a curved shape, a chip after cutting the metallic waste, and sundries such as tools, thereby removing a radioactivity and reducing a radioactive level of the metallic waste. Furthermore, since the use of the supporting vessel having an insulation and an opening at its upper portion can increase a decontamination amount of the metallic waste per one batch, and can be inserted into and taken out of the electrolysis bath by using the driving mechanism, it is possible to easily utilize an automation for a mass processing. In several embodiments mentioned above, even though phosphoric acid is used as the electrolyte, sulfuric acid, nitric acid, sodium sulfate and sodium nitrate can result the same effect. Furthermore, in a decontamination system and method according to the present invention, since the electrolysis occurs at the wall surface of the electrolysis bath by a dielectric function when the material of the electrolysis bath is a metal, it is impossible to efficiently charge the surface of the metallic waste to a positive or negative polar. Accordingly, the systems disclosed herein use an insulating material simple body having a drug resistance and heat resistance such as fluorocarbon polymers and FRP, or a metal lined by an insulating material for manufacturing the electrolysis bath, the insulating shield body and insulating discs for the cylindrical metallic waste, and the insulating shield vessel for the plate-shape metallic waste. Even though the openings of the insulating discs may have holes for allowing the cylindrical metal for an insertion, since the insertion is done through the upper disc, the lower disc is fixed and various types of the upper discs may be attached in the manner of matching a diameter of the opening with the outer diameter of the cylindrical metal. The electrode can be made of copper lined by titanium, further coated by platinum, a simple platinum, a metal without titanium coated by platinum, and lead compound in addition to titanium coated by platinum utilized in the above-mentioned embodiments. Furthermore, the supporting vessel for storing the metallic waste may be made of the above-mentioned electrode material, and may be lined its side surface by the insulating material having a drug resistance and heat resistance such as fluorocarbon polymers and fiber reinforced plastics (FRP).

Claims: A system for decontaminating radioactive metallic waste (7) by performing bipolar electrolysis without contact between said metallic waste and either of a pair of electrodes in an electrolyte in an electrolysis bath (2), and by dissolving a base metal from said waste by an electrochemical reaction so as to eliminate radioactivity therefrom, said system comprising: an electrolysis bath (2) for containing said electrolyte; an anode (5) in said electrolysis bath arranged to be charged in a positive polarity by a direct current (DC) voltage supplied from a DC power source (8); and a cathode (6) in said electrolysis bath (2) arranged to be charged in a negative polarity by said DC voltage supplied from said DC power source (8); characterized by : an insulating shield (1, 15, 40, 45) for dividing said electrolysis bath (2) into a chamber (13) containing said anode and a chamber (14) containing said cathode, said insulating shield having a plurality of faces for preventing ions in said electrolyte from flowing therethrough. A system according to Claim 1, wherein: said insulating shield is an insulating shield plate (1) and is formed in the shape of a U as viewed from above the electrolysis bath and set with its three walls parallel to respective side walls of said electrolysis bath. A system according to Claim 1, wherein: said insulating shield is a vessel (15) having an opening at an upper portion thereof; said anode (5) is arranged at a bottom portion of said electrolysis bath; said cathode (6) is arranged at a bottom portion of said vessel; said metallic waste is supported by a supporting member (16); and said supporting member (16) is arranged at the bottom of said vessel (15) having said opening at the upper portion and has a plurality of holes each of which opens in a mesh-shape for passing through said electrolyte. A system according to Claim 3, wherein: said supporting member comprises a basket (17) having an opening at an upper portion thereof and for supporting the metallic waste (7) therein. A system according to Claim 1, wherein: said cathode comprises a rectangular pipe or a bar-shaped body (18), and is arranged to move to keep a constant interval from said metallic waste (7) by a driving mechanism (19). A system according to Claim 5, wherein: said cathode (21) is formed by connecting a plurality of rectangular pipes or bar-shaped bodies (18) by a flexible cable (20), said cathode having an insulating elastic body (22) for allowing water to pass therethrough. A system according to Claim 1, wherein: said insulating shield is a cylindrical insulating shield body (40) which is arranged in said electrolyte in said electrolysis bath; said anode is a cylindrical anode (33) arranged on an inner wall of said cylindrical insulating shield body; said cathode is a cylindrical cathode (41) arranged on an outer wall of said cylindrical insulating shield body; said waste is a cylindrical metallic waste (34) arranged in said cylindrical anode; and said direct current power source (37) being connected to said cylindrical anode and said cylindrical cathode. A system according to Claim 7, wherein: insulating discs (42) each having an opening are arranged at upper and lower ends of said cylindrical insulating shield body (40). A system according to Claim 1, wherein: said insulating shield is an insulating shield vessel (45) arranged in said electrolysis bath (31) and having an opening at the upper portion thereof; said cathode (47) is arranged at the bottom of said electrolysis bath; said anode (46) is arranged at the bottom of said insulating shield vessel; a supporting vessel (43) is arranged in said insulating shield vessel to keep a metallic waste (44) and has an opening at the upper portion thereof, wherein: said supporting vessel (43) has a side surface formed of insulating material, and a bottom portion formed of metal material; and said side surface of said supporting vessel has a plurality of holes for allowing the electrolyte therethrough. A system according to Claim 9, wherein: an electric circuit (37) is configured in the manner that the DC voltage is supplied between said anode (46) and said cathode (47) for charging the surface of the metallic waste (44) facing said anode (46) negatively, and for charging the opposite surface of said metallic waste (44) positively. A method for decontaminating radioactive metallic waste, comprising the steps of: providing an electrolysis bath (2) containing an electrolyte (3) and a pair of electrodes (5, 6) present in chambers (13, 14) and separated by an insulating shield (1) having a plurality of faces for preventing ions in said electrolyte from flowing therethrough; placing metallic waste (7) in one of the chambers such that it does not contact either of the electrodes; performing bipolar electrolysis in said bath by supplying a direct current from a DC power source (10) to said pair of electrodes to charge one of them positively as an anode (5) and the other negatively as a cathode (6), to either positively or negatively charge a decontamination surface of the metallic waste respectively facing the cathode or anode, so as to dissolve base metal from the metallic waste by an electrochemical reaction, thereby eliminating radioactivity from said waste. A method according to Claim 11, wherein: when an entire surface of said metallic waste is contaminated by the radioactivity, a polarity of said DC power source is converted to change said anode to a cathode and said cathode to an anode so as to dissolve the other surface of said metallic waste. A method according to Claim 11, further comprising: a step of using inorganic acid as said electrolyte; a step of reducing and destroying a passive or oxide layer on said decontamination surface of said metallic waste facing said cathode by charging in a negative polarity said decontamination surface; and a step of stopping a supply of said DC voltage and dissolving said base metal of said metallic waste by using acid force of said inorganic acid. A method according to Claim 13, wherein: a dissolution of said base metal and a reduction and destruction of said passive or oxide layer are repeated by alternatively inverting the polarity of said DC power source. A method according to Claim 11, wherein said anode is a cylindrical anode (33), said cathode is a cylindrical cathode (41), said insulating shield is a cylindrical shield body (40) and said metallic waste is a cylindrical metallic waste (34), further comprising: a step of setting said cylindrical insulating shield body (40) in said electrolyte; a step of setting said cylindrical anode (33) on an inner wall of said cylindrical insulating shield plate; a step of setting said cylindrical cathode (41) on an outer wall of said cylindrical insulating shield plate; a step of setting cylindrical metallic waste (34) in said cylindrical anode; a step of connecting said DC power source to said cylindrical anode and said cylindrical cathode; a step of supplying said DC voltage from said DC power source to said cylindrical anode and cylindrical cathode; and a step of charging in a negative polarity an outer surface of said cylindrical metallic waste and in positive polarity an inner surface of said cylindrical metallic waste, thereby dissolving the base metal at an inner surface of said cylindrical metallic waste. A method according to Claim 15, wherein: when both of said inner surface and said outer surface of said cylindrical metallic waste are contaminated, a cathode of said DC power source is inverted to an anode, and an anode of said DC power source is inverted to a cathode, thereby dissolving said outer surface of said cylindrical metallic waste. A method according to Claim 15, wherein: a dissolution of said base metal of said inner surface of said cylindrical metallic waste and a reduction and destruction of said oxide layer formed on said inner surface of said cylindrical metallic waste are repeated by alternatively inverting the polarity of said DC power source.

Inventors: ENDA MASAMI, HEKI HIDEAKI, HOSAKA KATSUMI, SAKAI HITOSHI, ENDA, MASAMI, HEKI, HIDEAKI, HOSAKA, KATSUMI, SAKAI, HITOSHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1995
Main classification: G21F9
IPC codes: G21F9