JP5997579B2 - Treatment method of radioactive metal waste - Google Patents

Description

  The technology disclosed herein relates to a technology for treating radioactive metal waste.

  In nuclear power plants and facilities that handle radioactive materials, some of the metal waste generated by the abolition, dismantling, or operation of metal wastes (hereinafter referred to as radioactive metal wastes) has a radioactivity level higher than the standard value. Is included). From the point of view of disposal, it is desirable that these metal wastes be treated after decontaminating their surfaces and lowering the radioactivity level below a reference value. Examples of the method for decontaminating radioactive metal waste include electropolishing and drive last. Non-Patent Document 1 discloses a technique for decontaminating the surface of radioactive metal waste using electropolishing.

Norbert Eickelpasch and three others, "Lessons Learned by Dismantling Two, Gundremingen's two German boiling water reactors, Unit A (KRB) and KAHL (VAK) German BWR's in Gundremmingen, Unit A (KRB), and KAHL (VAK)) ", Radwaste Magazine, (USA), American Nuclear Society, January 1997, p.17- 36

  In the technique disclosed in Non-Patent Document 1, radioactive metal waste mainly containing iron is electropolished using phosphoric acid as an electrolytic solution. After the electrolytic polishing is completed, oxalic acid is added to the electrolytic solution (that is, phosphoric acid waste liquid) to precipitate iron in the electrolytic solution as iron oxalate. Thereafter, the iron oxalate is thermally decomposed into iron oxide, and the iron oxide is loaded into a drum and discarded (stored). On the other hand, the phosphoric acid waste liquid is subjected to evaporation treatment and reused as an electrolytic solution for electropolishing.

  In the technique of Non-Patent Document 1, evaporation in the phosphoric acid waste liquid is precipitated using oxalic acid, and then the regenerated phosphoric acid solution is reused as an electrolytic solution for electropolishing. By doing so, the amount of secondary waste can be reduced. However, since iron oxide produced by thermal decomposition is discarded as it is, there is a possibility that a problem of requiring a lot of space in the disposal facility may arise. Therefore, it is conceivable to reduce the volume by melting iron oxide. However, since the melting point of iron oxide is generally high, the melting treatment is difficult.

  The present specification discloses a technique that enables secondary waste to be further reduced in volume and stabilized by appropriately melting secondary waste generated by surface decontamination of radioactive metal waste.

  The processing method of the radioactive metal waste which this specification discloses has an electropolishing process and a melt-solidification process. In the electrolytic polishing step, the radioactive metal waste is electrolytically polished using an electrolytic solution containing a phosphoric acid solution. In the melt-solidifying step, the waste liquid containing phosphoric acid after electropolishing is put into a melting furnace to melt and solidify the dissolved matter in the waste liquid.

  In the above processing method, the radioactive metal waste is electropolished using an electrolytic solution containing a phosphoric acid solution. Thereby, the surface of the radioactive metal waste is dissolved and eluted in the electrolyte. That is, the radioactive metal is dissolved in the electrolytic solution after electropolishing (hereinafter also referred to as phosphoric acid waste liquid or waste liquid). The melt | dissolved substance melt | dissolved in the phosphoric acid waste liquid by electropolishing is thrown into a melting furnace with a phosphoric acid waste liquid, and is melted and solidified. By melting the dissolved material together with phosphoric acid, phosphoric acid lowers the melting point of the dissolved material. For this reason, it becomes possible to melt | dissolve and melt | dissolve a melt | dissolution at low temperature compared with the case where only a melt | dissolution melt | dissolves. Generally, the secondary waste after the surface decontamination of radioactive metal waste is concentrated in radioactivity, but the secondary waste is disposed (stored) in a stable state by melting the secondary waste. It becomes possible.

  Details of the technology disclosed in this specification and further improvements will be described in detail in the detailed description and examples.

The flowchart of the processing method of the radioactive metal waste which concerns on Example 1 is shown. The schematic diagram of the electropolishing apparatus of stainless steel is shown. The schematic diagram of the filtration apparatus of a phosphoric acid waste liquid is shown. The schematic diagram of the internal structure of a filter is shown. The schematic of the apparatus which carries out the high frequency melting of the secondary waste of a radioactive metal waste is shown. The flowchart of the processing method of the radioactive metal waste which concerns on Example 2 is shown.

  The main features of the embodiments described below are listed. The technical elements described below are independent technical elements and exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Absent.

(Characteristic 1) The processing method of the radioactive metal waste which this specification discloses may further have a blast process. The radioactive metal waste may include a first radioactive metal waste and a second radioactive metal waste. In the blasting process, the surface of the second radioactive metal waste may be treated by blasting. In the electrolytic polishing step, the surface of the first radioactive metal waste may be treated by electrolytic polishing. In the melting and solidifying step, the waste liquid generated in the electropolishing step and the abrasive and the second radioactive metal waste exfoliation generated in the blasting step may be melted and solidified together. In general, radioactive metal waste generated in nuclear power plants and facilities that handle radioactive materials may be separated into relatively hard metals such as stainless steel and relatively hard metals such as carbon steel. . In such a case, the first radioactive metal waste may be the above-mentioned relatively hard metal, while the second radioactive metal waste may be the above-mentioned relatively low-hardness metal. . By performing different surface decontamination methods according to the hardness of the metal, the surface of the radioactive metal waste can be appropriately decontaminated. In the blasting process, the surface of the second radioactive metal waste is decontaminated using an abrasive such as a cast iron grid. The abrasives and exfoliated material generated by the blasting process are put into a melting furnace together with phosphoric acid waste liquid generated by the electropolishing process and melted and solidified. According to this method, since phosphoric acid lowers the melting point of the abrasive or exfoliated material, the volume of the abrasive or exfoliated material can be reduced by melting at a low temperature. In addition, since the abrasive and the exfoliated material are secondary waste, the radioactivity is concentrated, but can be stably discarded (stored) by melting. That is, in the melting and solidifying process, in addition to the melted material dissolved in the phosphoric acid waste liquid generated in the electropolishing process, the abrasives and exfoliated material generated in the blasting process are collectively melted. By doing so, it is possible to reduce the volume of secondary waste generated by different surface decontamination methods (that is, dissolved matter, abrasives, and exfoliated material in the phosphoric acid waste solution) at a time. Further, since the phosphoric acid waste liquid generated by the electropolishing process is used as the substance for lowering the melting point of the secondary waste, it is not necessary to newly prepare a melting aid for lowering the melting point of the secondary waste.

(Characteristic 2) The radioactive metal waste processing method disclosed in the present specification may further include a filtration step. The filtration step may be performed after the electropolishing step and before the melt solidification step. In the filtration step, the waste liquid after electropolishing may be filtered. The filtrate filtered by the filtration process may be used as an electrolyte in the electropolishing process. On the other hand, the waste liquid concentrated by the filtration step may be melted and solidified by the melt-solidifying step. In this method, the phosphoric acid waste liquid after electropolishing is filtered by a filtering device. The filtered filtrate is reused as an electrolyte (that is, a solution containing a phosphoric acid solution) after being adjusted to a concentration suitable for electropolishing through evaporation if necessary. On the other hand, waste liquid concentrated without filtration (hereinafter also referred to as phosphoric acid concentrated waste liquid or concentrated waste liquid) is melted together with secondary waste (that is, dissolved matter, abrasive, and exfoliated material in phosphoric acid concentrated waste liquid). It is melted and solidified in the solidification step. According to this method, since a part of the phosphoric acid waste liquid is reused, it is possible to reduce the amount of the phosphoric acid waste liquid that is put into the melt-solidifying step.

(Characteristic 3) The method for treating radioactive metal waste disclosed in the present specification further includes a step of adding oxalic acid to the waste liquid after electrolytic polishing after the electrolytic polishing step and before the filtration step. May be. In the phosphoric acid waste liquid after electropolishing, a substance constituting the surface of the first radioactive metal waste is contained as a dissolved material. By adding oxalic acid to the phosphoric acid waste liquid, the dissolved product and oxalic acid react to generate a precipitate. According to this method, the dissolved substance can be easily separated from the waste liquid after electrolytic polishing in the filtration step.

  In nuclear facilities such as nuclear power plants, metal waste is generated with the abolition, dismantling and operation of facilities. Most metal waste can be treated as general waste, but some metal waste (for example, metal waste generated by dismantling contaminated water treatment facilities) has a radioactivity level above the standard value. There may be. Such radioactive metal waste (hereinafter also referred to as radioactive metal waste) is either treated with municipal waste by decontamination and reducing the radioactivity level, or it is melted and reused. It is possible to Moreover, even when the radioactivity level does not fall to the standard value level of general waste, disposal (disposal) can be simplified. In general, the radioactive level of radioactively contaminated metal can be reduced by removing radioactive material on the surface of the metal. In this embodiment, a case where stainless steel and carbon steel are treated as radioactive metal waste will be described. Stainless steel is an example of “first radioactive metal waste”, and carbon steel is an example of “second radioactive metal waste”. Below, the processing method of the secondary waste produced with the surface decontamination of these radioactive metal wastes is demonstrated.

(Electropolishing process)
FIG. 1 shows a flowchart of a method for treating radioactive metal waste according to the first embodiment. First, in step S2, the radioactive metal waste is electropolished. A decontamination method by electropolishing will be described with reference to FIG. FIG. 2 shows a schematic diagram of an electropolishing apparatus. The electrolytic bath 32 is filled with an electrolytic solution 34. The electrolytic solution 34 contains a phosphoric acid solution. A stainless steel 36 having radioactivity (hereinafter also referred to as radioactive stainless steel) is immersed in the electrolytic solution 34. When the electrolytic cell 32 is connected to the cathode and the radioactive stainless steel 36 is connected to the anode and a current is passed, the surface of the radioactive stainless steel 36 is dissolved electrochemically. That is, a metal such as iron constituting the surface of the radioactive stainless steel 36 and a radioactive substance attached to the surface are dissolved in the electrolytic solution 34. By smoothing the surface of the radioactive stainless steel 36 by electrolytic polishing, the radioactive stainless steel 36 can be decontaminated. After the decontamination, the stainless steel 36 is pulled up from the electrolytic bath 32 and can be treated with general waste or melted and reused. Moreover, even when the radioactivity level does not fall to the standard value level of general waste, disposal (disposal) can be simplified. On the other hand, the dissolved matter such as iron dissolved in the electrolytic solution 34 along with the decontamination of the radioactive stainless steel 36 contains a radioactive substance. The existence form of the dissolved material of the radioactive stainless steel 36 in the electrolytic solution 34 is not limited to ions. For example, a part may react with the electrolytic solution 34 to generate a precipitate. Note that in this example they are collectively referred to as a lysate. In the present embodiment, the electrolytic solution 34 after electropolishing is referred to as phosphoric acid waste liquid.

(Oxalic acid addition process)
Returning to FIG. 1, the description will be continued. When the electrolytic polishing of the stainless steel 36 is completed in step S2, oxalic acid is added to the phosphoric acid waste liquid (electrolytic solution 34 after electrolytic polishing) in step S4. Oxalic acid reacts with iron ions in the phosphoric acid waste liquid and precipitates as iron oxalate (step S6). In this embodiment, oxalic acid is used as the substance for precipitating iron ions in the phosphoric acid waste liquid, but the substance for precipitating various ions in the phosphoric acid waste liquid is not limited to this.

(Filtering process)
Subsequently, in step S8, the phosphoric acid waste liquid after the addition of oxalic acid is filtered. The filtration of the phosphoric acid waste liquid will be described with reference to FIG. FIG. 3 shows an example of a filtration device 50 that filters the phosphoric acid waste liquid. The filtration device 50 includes a container 38, a pump 40, and a filter 41. The container 38 stores a phosphoric acid waste liquid (specifically, a phosphoric acid waste liquid to which oxalic acid is added) generated by electropolishing. The container 38 is connected to the pump 40 by a pipe 38a. The pump 40 is connected to the filter 41 by a pipe 40a. The pump 40 sends out the phosphoric acid waste liquid in the container 38 to the filter 41. The filter 41 is connected to the container 38 by a pipe 42a. The filter 41 is a cross-flow filter, and includes a filter housing portion 42, a permeate pipe 43, and a filter 44. The filter housing portion 42 is formed in a cylindrical shape, and a housing space is provided therein. A plurality of filters 44 can be accommodated in the accommodation space. The filter housing part 42 is formed with a mechanism (not shown) for fixing the filter 44 at both ends thereof. A permeate tube 43 is connected to the outer peripheral surface of the filter housing portion 42. The filter 44 is a cylindrical rod-shaped member, and both ends thereof are fixed to the filter housing portion 42 as shown in FIG. That is, the surface of the filter 44 (side surface of the cylinder) is exposed in the space inside the filter housing portion 42.

The internal structure of the filter 44 will be described with reference to FIG. FIG. 4 shows a partial perspective view of the columnar filter 44. The filter 44 has a columnar base material 44a. The base material 44a is composed of alumina (Al ₂ O ₃ ), but the material constituting the base material 44a is not limited to this, and may be composed of, for example, titania (TiO ₂ ). A columnar cavity 45 extending in the longitudinal direction is formed in the base material 44a. The diameter of the cavity 45 is smaller than the diameter of the bottom surface of the substrate 44a. On the inner peripheral surface of the cavity 45, an intermediate layer 44b is formed so as to cover the entire inner peripheral surface. A film layer 44c is formed on the upper surface of the intermediate layer 44b so as to cover the entire inner peripheral surface of the intermediate layer 44b. The intermediate layer 44b is made of alumina (Al ₂ O ₃ ), for example. The film layer 44c is made of alumina (Al ₂ O ₃ ), for example. The materials constituting the intermediate layer 44b and the film layer 44c are not limited to alumina (Al ₂ O ₃ ), and may be composed of titania (TiO ₂ ), for example.

  The operation | movement at the time of filtering phosphoric acid waste liquid using said filtration apparatus 50 is demonstrated. The phosphoric acid waste liquid after the iron oxalate is precipitated in step S6 is sent to the container 38 through a pipe (not shown). The phosphoric acid waste liquid in the container 38 is sent to the filter 41 through the pipe 38 a and the pipe 40 a by the pump 40. The phosphoric acid waste liquid sent to the filter 41 is filtered by the filter 44. That is, the phosphoric acid waste liquid sent to the filter 41 flows into one end of the filter housing part 42. The phosphoric acid waste liquid that has flowed into one end of the filter housing portion 42 flows from one end of the filter 44 toward the other end. At this time, as shown in the partially enlarged view of FIG. 3, a part of the phosphoric acid waste liquid is filtered by the filter 44. Specifically, a part of the phosphoric acid waste liquid passes through the membrane layer 44c, the intermediate layer 44b, and the base material 44a, so that the dissolved matter is removed, and from the outer peripheral surface of the base material 44a to the storage space of the filter storage portion 42. It flows out as permeate 48. The permeated liquid 48 that has passed through the filter 44 is discharged to the outside through the permeated liquid pipe 43 from the accommodating space of the filter accommodating portion 42 (step S10). The permeate 48 corresponds to an example of “filtered filtrate”. On the other hand, the remaining phosphoric acid waste liquid flows in the axial direction through the filter 44 and flows out from the other end of the filter 44. Accordingly, the concentration of the dissolved matter in the phosphoric acid waste liquid is higher (that is, concentrated) at the other end (outflow end) than at one end (inflow end) of the filter 44. The phosphoric acid waste liquid flowing out from the other end of the filter 44 is sent to the container 38 through the pipe 42a. The phosphoric acid waste liquid (concentrated waste liquid) sent to the container 38 is sent again to the filter 41 by the pump 40 and separated into the permeate 48 and the concentrated waste liquid by cross flow filtration. By repeating this process, the concentrated waste liquid is further concentrated, and finally the phosphoric acid concentrated waste liquid 46 is stored in the container 38 (step S12). At least a part of the phosphoric acid concentrated waste liquid 46 is dried until it becomes powdered phosphoric acid concentrated powder (step S13).

  In this embodiment, since a cross-flow filter is used, deposits on the surface of the filter 44 (surface of the cavity 45) are always washed by the flow of phosphoric acid waste liquid (so-called self-cleaning function is provided). ) The deposit deposited on the surface of the cavity 45 of the filter 44 may be peeled off from the surface by periodically flowing the permeated liquid 48 backward. That is, if a deposit adheres to the surface of the cavity 45 of the filter 44 to some extent, the membrane permeation flow rate decreases, so that the deposit on the surface of the cavity 45 of the filter 44 is peeled off and cleaned by periodically flowing the permeate 48. So-called back washing may be performed. In the present embodiment, a cross flow filtration method is described, but the filtration method is not limited to this, and a direct filtration method may be used.

  Returning to FIG. 1, the description will be continued. In step S10, the permeated liquid 48 discharged from the inner space of the filter housing portion 42 through the permeate pipe 43 is evaporated in step S14. The permeated liquid 48 which has been evaporated and has a high concentration is sent to the electrolytic bath 32 and reused as the electrolytic solution 34 in the electrolytic polishing process (step S2). In step S14, the permeate 48 is evaporated so as to have a concentration suitable for the electrolytic solution 34. That is, if the concentration of the permeate 48 in step S10 is a concentration suitable for the electrolytic solution 34, step S14 may not be performed.

(Blasting process)
Next, another decontamination method in the present embodiment will be described. In step S16, drive last is performed on carbon steel having radioactivity (hereinafter also referred to as radioactive carbon steel). Since drive last is a conventionally well-known technique, detailed description is abbreviate | omitted. The drive last of the present embodiment is performed using a barrel type and / or a conveyor type drive last device. The barrel type drive last is preferably implemented on a relatively small size radioactive carbon steel, while the conveyor type drive last is preferably implemented on a relatively large size radioactive carbon steel. The drive last method is not limited to the above-described apparatus. For example, when it is difficult to perform drive last using the above apparatus, an operator operates the nozzle in the room, and the abrasive is directly injected into the radioactive carbon steel from the tip of the nozzle. You can also. In any drive last, a cast iron grid can be used as an abrasive. Further, since the surface of the radioactive carbon steel is oxidized, the peeled material is mainly iron oxide. In the present embodiment, the surface of the radioactive carbon steel is peeled off by performing the above-described two types of drive last, so that the radioactive carbon steel can be decontaminated. The carbon steel that has been decontaminated can be treated together with general waste or melted and reused. Moreover, even when the radioactivity level does not fall to the standard value level of general waste, disposal (disposal) can be simplified. On the other hand, when the decontamination of the radioactive carbon steel is completed by the drive last, a cast iron grid and iron oxide peeled from the radioactive carbon steel by the cast iron grid are generated (steps S18 and S20). The cast iron grid and the iron oxide contain a radioactive material.

(Melt solidification process)
Phosphoric acid concentrated waste liquid 46 (step S12) and phosphoric acid concentrated powder (step S13) generated by electrolytic polishing of radioactive stainless steel 36, cast iron grid (step S18) and iron oxide (step S20) generated by radioactive carbon steel drive last Is melted and solidified by a high-frequency melting apparatus in step S22. As described above, the phosphoric acid concentrated waste liquid 46 contains a dissolved material that has not been precipitated by oxalic acid and a precipitate such as iron oxalate. Below, with reference to FIG. 5, the melt-solidification process of these secondary wastes (namely, phosphoric acid concentration waste liquid 46, phosphoric acid concentration powder, cast iron grid, and iron oxide) is demonstrated.

  FIG. 5 is a schematic diagram of the high-frequency melting apparatus 2. The high-frequency melting apparatus 2 includes a furnace body 11, a lid 15 attached to the upper end of the furnace body 11, and a lifting device 20 disposed below the furnace body 11. The furnace body 11 includes a furnace body 12, a cooling nozzle 13, and an induction heating coil 14 disposed along the outer peripheral surface of the furnace body 12. The furnace body 12 is formed in a cylindrical shape with an upper end and a lower end opened, and an accommodation space 12a is provided therein. The melting container 10 can be stored in the storage space 12a. A charging port 12 b is provided at the upper end of the furnace body 12, and an opening 12 c is provided at the lower end of the furnace body 12. The inlet 12b and the opening 12c communicate with the accommodation space 12a. The opening 12c is formed in a size that allows the melting container 10 to pass therethrough. The charging port 12b is formed smaller than the opening 12c, and is formed in a size that the melting container 10 cannot pass through. The cooling nozzle 13 is arranged in the lower part of the furnace body 12 so as to penetrate the outer wall of the furnace body 11 and the furnace body 12. A space is formed between the outer wall of the furnace body 11 and the furnace body 12. The induction heating coil 14 is held in the cavity by a holding arm (not shown). The induction heating coil 14 is disposed so as to cover the side surface of the melting vessel 10 through the furnace body 12. The induction heating coil 14 is connected to a high-frequency power source (50 to 3000 Hz) (not shown).

  The lid 15 is attached to the upper end of the furnace body 11. When the lid 15 is attached to the furnace body 11, the charging port 12b at the upper end of the furnace body 12 is closed. A charging machine 16 is installed on the lid 15. The charging machine 16 is disposed above the charging port 12b (that is, the accommodation space 12a) of the furnace body 12. The charging machine 16 is loaded with solid waste (i.e., phosphoric acid concentrated powder, cast iron grid, iron oxide, etc.) among secondary wastes to be melted. The input device 16 inputs the secondary waste into the melting container 10 accommodated in the accommodation space 12a. The lid 15 is provided with a nozzle 17. The nozzle 17 communicates with the accommodation space 12 a of the furnace body 12. The nozzle 17 puts liquid waste (that is, phosphoric acid concentrated waste liquid 46) into the melting container 10 among the secondary waste to be melted. The lid 15 is provided with an exhaust gas outlet 18. The exhaust gas outlet 18 communicates with the accommodation space 12 a of the furnace body 12. The exhaust gas outlet 18 discharges gas generated during the melting process. An exhaust gas treatment facility (not shown) is connected to the exhaust gas outlet 18. The exhaust gas discharged from the exhaust gas outlet 18 is rendered harmless in the exhaust gas treatment facility and discharged to the atmosphere.

  The lifting device 20 includes a flat table 20a and a lifting mechanism (not shown) that lifts and lowers the flat table 20a. The abutment 24 is placed on the flat table 20 a, and the melting container 10 is placed on the abutment 24. When the flat table 20a is raised to the upper end position (position indicated by the solid line in FIG. 1) by the lifting mechanism, the lower end of the accommodation space 12a is closed. When the flat table 20a is lowered to the lower end position (position indicated by the two-dot chain line in FIG. 1) by the lifting mechanism, the lower end of the accommodation space 12a is opened.

  The melting container 10 is a bottomed container and is made of a conductive ceramic containing carbon. When a high frequency power source is applied to the induction heating coil 14, an eddy current flows in the melting container 10, and the melting container 10 can be suitably heated. The temperature of the melting container 10 can be controlled by adjusting the electrical resistivity and thickness of the melting container 10. For example, the melting container 10 can be heated to 1550 [° C.].

  Next, an example will be described in which the secondary waste that is the object of the melting process is melted and solidified by the high-frequency melting apparatus 2 described above. First, the elevating device 20 is driven to position the flat table 20a at the lower end position (position indicated by the two-dot chain line in FIG. 1). Next, the abutment 24 is placed on the flat table 20 a, and the melting container 10 is placed on the abutment 24. The melting container 10 is preliminarily charged with secondary waste such as phosphoric acid concentrated powder, cast iron grid, and iron oxide. Next, the elevating device 20 is raised until the flat table 20 a comes into contact with the lower surface of the furnace body 12. As a result, the melting container 10 disposed on the abutment 24 is accommodated in the accommodating space 12 a of the furnace body 12.

  Next, a high frequency power source is applied to the induction heating coil 14. As a result, an eddy current is generated in the melting container 10 and the melting container 10 generates heat. When the temperature of the melting container 10 rises, the secondary waste charged in the melting container 10 is melted. That is, phosphoric acid concentrated powder, cast iron grid, and iron oxide melt. When these secondary wastes are melted and reduced in volume, a space is formed above the melting container 10, so the input machine 16 is driven to input secondary waste into the space. The input device 16 inputs solid secondary waste (that is, phosphoric acid concentrated powder, cast iron grid, iron oxide, etc.), but the type of secondary waste is not limited to this. For example, in step S13, the phosphoric acid concentrated waste liquid 46 is dried until it is pulverized. However, the degree of drying of the phosphoric acid concentrated waste liquid 46 is not limited to this, and is dried when the phosphoric acid concentrated waste liquid 46 becomes sludge. (Hereinafter, the sludge-like phosphoric acid concentrated waste liquid 46 is also referred to as phosphoric acid concentrated sludge). When the phosphoric acid concentrated waste liquid 46 is in the form of sludge, the phosphoric acid concentrated sludge may be packed in a bag and charged from the charging device 16 or may be melted in advance before the melting container 10 is stored in the storage space 12a. The container 10 may be loaded with phosphoric acid concentrated sludge. When the secondary waste charged from the charging device 16 is melted and reduced in volume, a space is formed above the melting container 10, and the phosphoric acid concentrated waste liquid 46 is charged into the space from the nozzle 17. In addition, the order which drives the injection machine 16 and the nozzle 17 is not restricted to this. For example, the nozzle 17 may be driven prior to the charging device 16 and the phosphoric acid concentrated waste liquid 46 may be charged first. Further, it is not necessary to drive the charging machine 16 and the nozzle 17 alternately. For example, after the secondary waste is continuously charged in several times from the charging machine 16, the nozzle 17 may be driven. . Moreover, the secondary waste thrown from the thrower 16 does not always need to be charged with phosphoric acid concentrated powder, cast iron grid, and iron oxide (including phosphoric acid concentrated sludge) all at once. It may be charged separately. That is, as long as the secondary waste sequentially fed from the feeder 16 and the nozzle 17 is appropriately melted and reduced in volume, the order in which the secondary waste is thrown in, the type of secondary waste to be thrown in at a time, etc. May be anything. By repeating the above process, the secondary waste is melted. At this time, a cooling gas is supplied from the cooling nozzle 13 to the melting container 10 to cool the melting container 10. By controlling the cooling nozzle 13, the temperature of the melting container 10 is maintained at a temperature suitable for the melting process. The melting temperature of the secondary waste is, for example, 1200 [° C.], but is not limited thereto. The gas generated during the melting process is discharged from the exhaust gas outlet 18 and processed in an exhaust gas processing facility (not shown). When the melting process is completed, the melting container 10 is unloaded from the high-frequency melting apparatus 2. The procedure is the reverse of the procedure described above for housing the melting vessel 10 in the furnace body 12. Next, the melting container 10 that has completed the melting process is moved to the cooling chamber 22 and the molten metal inside thereof is cooled and solidified. The melting container 10 obtained by cooling and solidifying the molten metal is discarded in a predetermined storage facility. In the present embodiment, the entire melting container 10 is discarded, but it may be discarded after the melting container 10 is loaded in a predetermined container.

  Advantages of the radioactive metal waste processing method according to the first embodiment will be described. Conventionally, secondary waste such as iron oxide generated after surface decontamination of radioactive metal waste has been difficult to melt. Specifically, since the melting point of iron oxide is high, radioactive materials such as cesium are volatilized and the processing speed is slow. In addition, there is a problem that only small amounts can be processed when cementing powders and granular materials such as iron oxide. Therefore, these secondary wastes are processed by cement solidification little by little or stored as they are, and there is a problem in that a large space is required for the disposal facility. However, in the radioactive metal waste processing method of the present embodiment, the phosphoric acid concentrated waste liquid 46 is added to the secondary waste such as iron oxide and melted. Since phosphoric acid contained in the phosphoric acid concentrated waste liquid 46 lowers the melting point of iron oxide, it becomes possible to melt iron oxide at a lower temperature than before. Specifically, normal iron oxide melts at about 1500 [° C.], whereas it can be melted at about 1200 [° C.] by adding phosphoric acid. Therefore, when the secondary waste such as iron oxide is melted, the occurrence of the above-described event can be suppressed, and the secondary waste such as iron oxide can be melted relatively easily compared to the conventional case. In addition, since the volume of secondary waste can be reduced by melting and solidifying treatment, the problem of space in the disposal facility is also improved. Moreover, since the number of melting containers 10 used for disposal decreases with volume reduction, the cost accompanying the melting container 10 can also be reduced. Furthermore, since secondary radioactive wastes such as iron oxide generated after the surface decontamination of radioactive metal wastes are concentrated in radioactivity, they can be disposed of in a more stable form by melting and solidifying.

  Conventionally, the phosphoric acid waste liquid after electropolishing is partially reused as the electrolytic liquid for electropolishing, but the remaining phosphoric acid waste liquid (that is, the waste liquid in which radioactive metal waste is dissolved or precipitated) For example, the precipitates were stored as they were after neutralization and coagulation precipitation treatment. However, in the method for treating radioactive metal waste according to the present embodiment, the remaining phosphoric acid waste liquid (that is, phosphoric acid concentrated waste liquid 46) is added to the secondary waste such as iron oxide generated by the blasting process and melted. Thereby, the phosphoric acid concentration waste liquid 46 in which the precipitate is stored as it is after neutralization and coagulation precipitation treatment can be utilized as a melting aid for lowering the melting point of iron oxide. At the same time, the phosphoric acid concentrated waste liquid 46 itself can be reduced in volume. That is, by using the phosphoric acid concentrated waste liquid 46 that is secondary waste generated by the electrolytic polishing process, the melting point of the iron oxide or cast iron grid that is secondary waste generated by the drive last process is lowered, and these secondary wastes are removed. Collectively melt and solidify. Since the secondary wastes generated by different decontamination methods are melted together, the secondary wastes can be disposed at a time, and the processing efficiency of the radioactive metal wastes can be improved. Moreover, since the phosphoric acid concentration waste liquid 46 which is a secondary waste is used as phosphoric acid which functions as a melting auxiliary agent, it is not necessary to provide a new facility for producing the melting auxiliary agent. Can be implemented.

  Further, in the oxalic acid addition step, various ions such as iron ions present in the phosphoric acid waste liquid after electropolishing are precipitated as oxalic acid compounds. Prior to the filtration step, the dissolved matter dissolved in the phosphoric acid waste liquid is precipitated to some extent, thereby reducing the burden on the subsequent filtration treatment. In the filtration step, the permeated liquid 48 and the phosphoric acid concentrated waste liquid 46 can be separated with higher accuracy. Moreover, the time required for the filtration step can be shortened, and workability is improved.

  Moreover, the phosphoric acid waste liquid after electropolishing is separated into a permeate 48 and a phosphoric acid concentrated waste liquid 46 by a filtration process. In the filtration step, cross flow filtration is performed using a ceramic membrane having excellent corrosion resistance and chemical resistance. By adopting the cross-flow filtration method, the phosphoric acid waste liquid flows parallel to the surface of the ceramic membrane, so it is possible to suppress deposits such as dissolved substances and iron oxalate in the phosphoric acid waste liquid from depositing on the film surface. . That is, the film surface can be constantly cleaned (self-cleaning). Even when the above substances are deposited on the film surface, the deposits can be removed by backwashing, so that the filter 44 can be used for a long period of time while maintaining high accuracy. In addition, by providing a filtration step, it is possible to reliably remove the dissolved matter and precipitate in the phosphoric acid waste liquid, so that the number of times that the permeate 48 can be reused as an electrolyte increases, and as a result, the amount of secondary waste is reduced. Can be reduced.

  A processing method according to the second embodiment will be described with reference to FIG. Note that the description of the same steps as those in Embodiment 1 is omitted.

(Electropolishing process, filtration process)
First, in step S32, the surface of the radioactive stainless steel 36 is decontaminated by electropolishing. Next, the phosphoric acid waste liquid after electropolishing is filtered using the filtration device 50 in step S38. As a result, a permeate 48 is generated (step S40) and a phosphoric acid concentrated waste liquid 46 is generated (step S42). At least a part of the phosphoric acid concentrated waste liquid 46 is dried until it becomes powdered phosphoric acid concentrated powder (step S43). Thereafter, the permeate 48 is reused as the electrolyte 34 in the electropolishing process.

(Blasting process, melt solidification process)
On the other hand, in step S46, the surface of the radioactive carbon steel is decontaminated by drive last. When the drive last is completed, a cast iron grid (ground material) and iron oxide (exfoliation material on the surface of the radioactive carbon steel) are generated as secondary waste (steps S48 and S50). Phosphoric acid concentrated waste liquid 46 (step S32) and phosphoric acid concentrated powder (step S43) generated by electrolytic polishing of radioactive stainless steel 36, cast iron grid (step S48) and iron oxide (step S50) generated by radioactive carbon steel drive last Is melted and solidified in a high-frequency melting apparatus in step S52.

  The processing method of the radioactive metal waste which concerns on Example 2 does not include an oxalic acid addition process. When the dissolved matter in the phosphoric acid waste liquid can be separated from the permeate 48 by cross-flow filtration using a ceramic membrane, the oxalic acid addition step does not have to be performed as in the treatment method of this embodiment. Prior to the filtration step, the phosphoric acid waste liquid may be centrifuged using a centrifuge. By separating the lysate from the phosphoric acid waste liquid by centrifugation and then filtering the phosphoric acid waste liquid, the phosphoric acid waste liquid can be reliably separated into the permeate 48 and the phosphoric acid concentrated waste liquid 46. The method of separating the lysate from the phosphoric acid waste liquid is not limited to centrifugation, and may be separated using other methods. Further, the processing method of this embodiment does not include a process of evaporating the permeate 48. If the concentration of the permeate 48 after the filtration step is within the concentration range of the electrolytic solution 34, the evaporation process may not be performed as in the processing method of the present embodiment. By not performing these treatments, the time required for the melting and solidification treatment of the secondary waste can be shortened, and the working efficiency can be improved.

  As mentioned above, although the Example of the technique which this specification discloses was described in detail, these are only illustrations, and the processing method of the radioactive metal waste which this specification discloses variously changes the above-mentioned example, Includes changes. For example, although the electrolytic cell 32 is directly connected to the cathode in the electropolishing process, the cathode plate may be immersed in the electrolytic solution 34. Further, the surface decontamination method is not limited to electrolytic polishing and drive last. For example, secondary waste such as iron oxide generated as a result of surface decontamination of radioactive metal waste using other methods may be melted together with the phosphoric acid concentrated waste liquid 46. Moreover, you may implement a centrifugation process also when implementing an oxalic acid addition process. In addition, when all of the concentrated phosphoric acid waste liquid 46 generated in step S12 (or step S42) is dried and pulverized in step S13 (or step S43), the phosphoric acid concentrated powder is charged from the feeder 16. Therefore, the nozzle 17 need not be used. That is, the technology disclosed in this specification can be implemented even in a high-frequency melting apparatus that does not include a nozzle.

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

2: High frequency melting apparatus 10: Melting vessel 12: Furnace body 14: Induction heating coil 16: Dosing machine 17: Nozzle 18: Exhaust gas outlet 20: Lifting apparatus 20a: Plane table 22: Cooling chamber 24: Abutment 32: Electrolyzer 34: Electrolyte 36: Radioactive stainless steel 38: Containers 38a, 40a, 42a: Piping 40: Pump 41: Filter 42: Filter housing part 43: Permeate pipe 44: Filter 44a: Base material 44b: Intermediate layer 44c: Membrane Layer 45: Cavity 46: Phosphate concentrated waste liquid 48: Permeate 52: Filtration surface

Claims (3)

A method for treating a first radioactive metal waste made of stainless steel and a second radioactive metal waste made of carbon steel ,
An electropolishing step of electropolishing the first radioactive metal waste using an electrolytic solution containing a phosphoric acid solution;
A blasting step of blasting the surface of the second radioactive metal waste;
A melt-solidifying step in which the waste liquid containing phosphoric acid after electropolishing, the abrasive material produced by the blasting step and the exfoliated material of the second radioactive metal waste are put together in a melting furnace, and melted and solidified;
A method for treating radioactive metal waste comprising: After the electropolishing step and before the melting and solidifying step, further comprising a filtration step of filtering the waste liquid after electropolishing,
The filtrate filtered by the filtration step is used as an electrolyte in the electropolishing step,
The method for treating radioactive metal waste according to claim 1, wherein the waste liquid concentrated by the filtration step is melted and solidified by the melt-solidification step. The radioactivity according to claim 2 , further comprising an oxalic acid addition step of adding oxalic acid to the waste liquid after electropolishing after the electropolishing step and before the filtering step. Metal waste disposal method.

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