JPH0565839B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a radioactively contaminated metal decontamination device for removing surface contamination of metals contaminated with radioactive substances. [Technical Background of the Invention] Generally, radioactive metal waste generated at nuclear power plants and the like is permanently stored within the nuclear power plant to prevent it from having a negative impact on the environment. However, when stored permanently in this way, the amount of radioactive metal waste stored continues to increase, making it difficult to secure storage space. In particular, piping contaminated with radioactivity generated during the recovery of equipment from nuclear power plants and the like is large in size and cannot be easily reduced in volume, making it difficult to store the waste. For this reason,
Consideration is being given to decontaminating radioactive metal waste and reducing its radioactivity level to the radioactivity level found in nature, that is, background level, so that it can be handled in the same way as general industrial waste. However, because radioactive metal waste has contamination fixed on its surface, complete decontamination cannot be achieved by simply removing the loosely accumulated contamination on the back side; instead, it is not possible to completely decontaminate radioactive metal waste by dissolving the surface layer of the metal base material with fixed contamination. It is necessary to remove contamination. Decontamination methods aimed at decontaminating down to the background level include the electrochemical decontamination method, which electrochemically dissolves and decontaminates the surface layer of the metal base material that is the object to be decontaminated, and the electrolytic decontamination method, which uses decontamination agents. A chemical decontamination method is known in which the surface layer of a metal base material is chemically dissolved and decontaminated. Although the electrolytic decontamination method has the advantage of high decontamination speed, it has the problem that it cannot be applied to objects to be decontaminated that have complex shapes because the electrode surface must face the surface to be decontaminated. Chemical decontamination methods are applicable to complex objects to be decontaminated, but on the other hand, the decontamination speed is slow, and once the decontamination agent is chemically consumed, it becomes unusable. There is a problem in that a large amount of dyestuff, ie secondary waste, is generated. [Purpose of the Invention] The present invention was made to solve the above-mentioned problems. It can be applied to objects to be decontaminated with complex shapes, has a high decontamination speed, and can be used to recycle spent electrolytes by recycling the electrolyte. The purpose of the present invention is to provide a decontamination device for radioactively contaminated metals that generates a small amount of liquid and has safe workability. [Summary of the Invention] In order to achieve the above objects, the present invention uses a nitric acid aqueous solution containing trivalent cerium ions and tetravalent cerium ions to produce cerium 3 through an electrolytic redox reaction.
Cerium tetravalent ions are generated from valent ions, and the oxidizing power when the generated cerium tetravalent ions change into cerium trivalent ions is used to dissolve and remove the surface of the object to be decontaminated, that is, the radioactively contaminated metal. In a decontamination device for radioactively contaminated metals, the electrolytic tank is dedicated to the electrolytic oxidation-reduction reaction, the contamination tank dissolves and decontaminates the surface of the object to be decontaminated, and the electrolytic tank is provided on a side wall of the electrolytic tank. an overflow pipe, a decontamination tank overflow pipe provided on the side wall of the decontamination tank, a decontamination tank drain pipe provided on the bottom of the decontamination tank, the electrolytic tank overflow pipe, the decontamination tank overflow pipe, and decontamination. A drain tank is connected to the tank drain pipe and receives the overflow electrolyte from the electrolytic tank, the overflow electrolyte from the decontamination tank, and the drain electrolyte from the decontamination tank; A liquid sending pipe for an electrolytic cell to return the liquid, a filter provided in the middle of the liquid sending pipe for an electrolytic cell to remove insoluble matter from the electrolytic solution, a shielding body surrounding this filter, and a filter for removing insoluble matter from the drain tank. A liquid supply pipe for the decontamination tank that returns the electrolyte to the dyeing tank, an exhaust gas duct connected to the upper space of the electrolytic tank, the decontamination tank, and the drain tank, a nitric acid vapor recovery device connected to the exhaust gas duct, and a mist An exhaust gas treatment device consisting of a recovery device and an exhaust fan, a washing water tank that stores washing water, a washing water pipe that sends washing water from this washing water tank to the decontamination tank, and a washing water pipe used in this decontamination tank. This decontamination apparatus for radioactively contaminated metals is characterized by comprising a washing water return pipe that returns water from the decontamination tank to the washing water tank. [Embodiments of the Invention] Hereinafter, an embodiment of a radioactively contaminated metal decontamination apparatus according to the present invention will be described with reference to the drawings. In the drawing, reference numeral 1 indicates an electrolytic cell, and this electrolytic cell 1
An electrolytic solution 101 made of a nitric acid aqueous solution containing trivalent cerium ions and tetravalent cerium ions is stored inside. An electrode 102 consisting of an anode and a cathode is immersed in the electrolytic solution 101, which serves to generate tetravalent cerium ions from trivalent cerium ions through an electrolytic redox reaction. The electrolytic cell 1 is provided with a heater 104 for heating the electrolytic solution 101 and a temperature detector 103 for detecting and controlling the temperature of the electrolytic solution 101. Further, this electrolytic cell 1 is hermetically covered with a lid 105. Furthermore, an exhaust gas pipe 106 is connected to the upper side of the electrolytic cell 1, and a drain pipe 108 having a valve 107 and a liquid supply pipe 109 are connected to the lower side. Reference numeral 2 in the drawing indicates a decontamination tank, and the decontamination tank 2 stores an electrolytic solution 203 made of a nitric acid aqueous solution containing trivalent cerium ions and tetravalent cerium ions, similar to the electrolytic tank 1. The object to be decontaminated 3 is immersed inside. In this decontamination tank 2,
Due to the oxidizing power when tetravalent cerium ions are changed to trivalent cerium ions, the surface layer of the metal base material, which is the object 3 to be decontaminated, is dissolved, and at the same time, the contamination on the surface of the object 3 to be decontaminated is removed. The decontamination tank 2 is provided with a heater 202 for heating the electrolytic solution 203 and a temperature detector 201 for detecting and controlling the temperature of the electrolytic solution 203. Further, this decontamination tank 2 is airtightly covered with a lid 204, and an exhaust gas pipe 205 is connected to the upper side surface, and a liquid supply pipe 207 having a valve 206 is further connected to the lower surface. Reference numeral 7 in the drawing indicates a drain tank, into which water flows from the drain pipe 108 and overflow pipe 4 connected to the lower end of the electrolytic tank 1, and from the overflow pipe 5 and drain pipe 6 of the decontamination tank 2. The electrolyte 701 that has arrived is stored. Further, this drain tank 7 is airtightly covered with a lid 702, and an exhaust gas pipe 709 is connected to the upper side surface. Further, on the lower surface of the drain tank 7, there is a first pipe 704 having a valve 703, a second pipe 706 having a valve 705, and a third pipe having a valve 707.
A pipe 708 is connected to the pipe 708. As described above, the electrolytic cell overflow pipe 4 is provided on the side wall of the electrolytic cell 1, and the other end of the electrolytic cell overflow pipe 4 is connected to the drain tank 7. Conversely, from this drain tank 7 to the electrolytic tank 1, an electrolytic tank liquid feed pipe 8 is connected from a second pipe 706 via a pump 801, and a liquid supply pipe 109 is connected to the electrolytic tank 1.
is connected to. Between the electrolytic tank liquid feed pipe 8 and the liquid supply pipe 109, there is a pump 801 that sends an electrolytic solution 701 from the drain tank 7 to the electrolytic tank 1 through a second pipe 706, and a flow rate that controls the flow rate of the electrolytic solution. A regulating valve 802 and a flow meter 803 for monitoring the electrolyte flow rate are provided. Further, a filter 9 for removing insoluble matter from the electrolytic solution 701 is provided in the middle of the electrolytic cell liquid sending pipe 8 . Upstream of this filter 9 is a pressure gauge 901,
A pressure gauge 902 is provided downstream of the filter 9, so that the degree of clogging of the filter 9 can be monitored. Further, this filter 9 is surrounded by a shield body 16. As described above, the decontamination tank overflow pipe 5 having the valve 501 is provided on the side wall of the decontamination tank 2, and the other end of the decontamination tank overflow pipe 5 is connected to the drain tank 7. Conversely, the decontamination tank liquid feed pipe 10 is connected from the drain tank 7 to the decontamination tank 2 via the first pipe 704 and the pump 1001.
are connected and communicated with the liquid supply pipe 207. A first pipe 704 from the drain tank 7 to the decontamination tank 2 is connected between the decontamination tank liquid feed pipe 10 and the liquid supply pipe 207.
A pump 1001 that sends the electrolytic solution 701 through the electrolytic solution, a flow rate adjustment valve 1002 that controls the electrolytic solution flow rate, and a flow meter 1003 that monitors the electrolytic solution flow rate are provided. Further, the decontamination tank liquid sending pipe 10 is provided with a filter 17 for removing insoluble matter from the electrolytic solution 701. A pressure gauge 1004 is provided upstream of this filter 17, and a pressure gauge 1005 is provided downstream of the filter 17, so that the degree of clogging of the filter 17 can be monitored. Further, this filter 17 is covered with a shield body 18. A washing water return pipe 15 having a decontamination tank drain pipe 6 and a valve 1501 is provided at the bottom of the decontamination tank 2, and the drain pipe 6 is connected to the drain tank 7 via the valve 601. 15 communicates with the washing water tank 13. The upper spaces of the decontamination tank 1, the electrolytic tank 2, and the drain tank 7 are connected to the exhaust gas duct 11 via exhaust gas pipes 106, 205, and 704, respectively. This exhaust gas duct 11 is connected to an exhaust gas treatment device 12 in which a nitric acid vapor recovery device 1201, a mist recovery device 1202, and an exhaust fan 1203 are connected in communication. A receiving tank 1204 is provided below the nitric acid vapor recovery device 1201, and this receiving tank 1204 and the drain tank 7 are connected by a recovered liquid return pipe 1206. The receiving tank 1204 is equipped with a liquid level gauge 120 for monitoring the amount of recovered liquid.
5 is provided. In addition, the recovered liquid return pipe 1
A valve 1207 is provided in the middle of the tank 206, and the liquid level gauge 1205 allows the water to flow through the receiving tank 1204.
If the recovered liquid is stored above a certain level,
This recovered liquid can be returned to the drain tank 7. The exhaust gas duct 11 is provided with a temperature detector 1101 for monitoring the exhaust gas temperature. Furthermore, a temperature detector 1209 for monitoring the temperature of the exhaust gas, a pressure gauge 1210 for monitoring the pressure of the exhaust gas, and a flow meter 1211 for monitoring the flow rate of the exhaust gas are provided at various locations in the exhaust gas treatment device 12.
Furthermore, the mist collection device 1202 includes first and second filters 1212, 1213 and a spray nozzle 1214, and exhaust gas is blown out from a pump 1215 through a pipe 1216 to the spray nozzle 1214 and is circulated. It's summery. The exhaust gas that has flown out of the mist collection device 1202 is discharged into the atmosphere from an exhaust pipe 1218 through an exhaust fan 1203 through an exhaust gas pipe. The washing water tank 13 stores washing water 1301. The washing water tank 13 and the decontamination tank 2 are connected by a washing water supply pipe 14. A valve 141 and a pump 1302 are provided in the middle of the washing water supply pipe 14 to supply washing water to the washing tank 1.
3 to the decontamination tank 2.
Further, the bottom surface of the decontamination layer and the washing water tank 13 are connected by a washing water return pipe 15, and a valve 1501 is provided in the middle of this washing water return pipe 15.
If necessary, the washing water used in the decontamination tank 2 can be returned from the decontamination tank 2 to the washing water tank 13. Next, the usage and function of the radioactively contaminated metal decontamination apparatus according to the present invention will be explained. Pump 801 is turned on to circulate electrolytes 701 and 101 between drain tank 7 and electrolytic tank 1. When the pump 801 is turned on, the electrolytic solution 701 in the drain tank 7 is transferred from the second pipe 706 to the electrolytic tank 1 through the electrolytic tank liquid feed pipe 8, passes through the electrolytic tank overflow pipe 4, and is transferred to the drain tank 7. come back to. As described above, since the electrolytic cell overflow pipe 4 is provided on the side wall of the electrolytic cell 1, the liquid level height of the electrolytic solution 101 is kept constant. Therefore, the electrode 102 can be constantly immersed in the electrolytic solution 101, and the entire area of the electrode 102 can be used effectively. Since a filter 9 is provided in the middle of the electrolytic cell liquid feed pipe 8, insoluble matter containing contamination that has peeled off from the surface of the object 3 to be decontaminated is captured by the filter 9. Therefore, there is no risk of contamination entering the electrolytic cell 1. Furthermore, accumulation of contamination in the electrolyte can be suppressed. Since the filter 9 where contamination accumulates is covered with a shield 16, it is possible to reduce the exposure of workers to radiation. Heater 10 while monitoring temperature with temperature detector 103
4, the electrolytic solution 101 is heated and maintained at a predetermined temperature, and a voltage is applied to the electrode 102. Then, the electrolytic solution 101 made of a nitric acid aqueous solution containing trivalent cerium ions and tetravalent cerium ions undergoes the electrolytic redox reaction shown below, and the trivalent cerium ions (Ce ³⁺ ) are converted into the tetravalent cerium ions (Ce ⁴⁺ ). converted. Anode Ce ³⁺ →Ce ⁴⁺ +e ^- ...(1) 2OH ^- →H ₂ O+1/2O ₂ (↑) +2e ^- ...(2) Cathode H ⁺ e ^- →(1/2)H ₂ (↑) ...(3) Next, put the object 3 to be decontaminated into the decontamination tank 2, and turn on the pump 10.
01 is turned on to circulate the electrolytes 701 and 203 between the drain tank 7 and the decontamination tank 2. pump 1001
When turned on, the electrolytic solution 701 in the drain tank 7 is transferred from the first pipe 704 to the decontamination tank 2 through the decontamination tank liquid supply pipe 10, and the decontamination tank overflow pipe 5
It returns to the drain tank 7 through the Since a decontamination tank overflow pipe 5 is provided on the side wall of the decontamination tank 2, the electrolyte 20 can be removed regardless of the size of the object 3 to be decontaminated.
3 is kept constant, and there is no fear that the electrolytic solution 203 will overflow from the decontamination tank 2. Since a filter 17 is provided in the middle of the decontamination tank liquid feed pipe 10, the insoluble matter containing contamination that has peeled off from the surface of the object 3 to be decontaminated is captured by the filter 17, thereby preventing contamination of the electrolyte. The accumulation of can be suppressed. Since the filter 17, where contamination accumulates, is covered with a shield 18, it is possible to reduce the exposure of workers to radiation. In the decontamination tank 2, the electrolytic solution 203 is heated with a heater 202 and maintained at a predetermined temperature while the temperature is monitored with a temperature detector 201. In this decontamination tank 2, when the cerium tetravalent ions generated in the electrolytic tank 1 are converted into cerium trivalent ions, the surface layer of the metal base material of the object to be decontaminated 3 is dissolved by the oxidizing power at that time. Contamination on the surface of the decontaminated object 3 is removed. The reaction in which tetravalent cerium ions (Ce ⁴⁺ ) is converted to trivalent cerium ions (Ce ³⁺ ) and the surface layer of the metal base material of object 3 to be decontaminated is dissolved is shown below with the chemical formula of the metal being M. That's right. M+Ce ⁴⁺ →M ⁺ +Ce ³⁺ As can be seen from the explanation so far, if the surface of the object 3 to be decontaminated is in contact with the electrolyte 203, the surface of the object 3 to be decontaminated will be dissolved, that is, decontamination will be carried out. be exposed. Therefore, compared to the electrolytic decontamination method which requires electrodes to face the surface of the object to be decontaminated, the apparatus for decontaminating radioactively contaminated metals according to the present invention has wider applicability to objects with complex shapes. In addition, since the oxidizing power of tetravalent cerium is used to dissolve the surface of the object 3 to be decontaminated, the decontamination apparatus for radioactively contaminated metals according to the present invention has a faster decontamination speed than chemical decontamination methods. is large.
The trivalent cerium ions generated in the decontamination tank 2 are returned to the cerium 4 value in the electrolytic tank 1. That is,
The electrolyte is regenerated and used repeatedly. Therefore, there is no generation of used electrolyte. When the decontamination of the object 3 is completed, the valve 601 is closed, and the electrolytic solution 203 in the decontamination tank 2 is drained into the drain tank 7 through the decontamination tank drain pipe 6. Decontamination tank 2
Once all the electrolyte 203 inside has been drained into the drain tank 7, close the valve 601 and turn off the pump 1302.
is turned ON, the washing water 1301 in the washing water tank 13 is transferred to the decontamination tank 2 through the washing water supply pipe 14, and the object 3 to be decontaminated after decontamination is washed with water to wash away the electrolyte that has adhered to it. The electrolyte adhering to the inner wall of the dye tank 2 is washed away. The valve 1501 is left open, and the washing water used in the decontamination tank 2 is passed through the washing water return pipe 1.
5 and return to the washing tank 13. After washing with water, the decontamination tank lid 204 is opened and the object 3 to be decontaminated is taken out. Electrolyte layer lid 105 and decontamination tank lid 20 during the above operation
4 and the drain tank lid 702 are closed, and the exhaust gas treatment device 12 is kept in operation at all times. This results in
The electrolytic tank 1, decontamination tank 2, and drain tank 7 are always maintained at a weak negative pressure. Nitric acid vapor and mist generated in the electrolytic tank 1, decontamination tank 2, and drain tank 7, as well as hydrogen gas and oxygen gas generated by electrolysis in the electrolytic tank 1, are collected in the nitric acid vapor recovery device 1 of the exhaust gas treatment device 12.
201 and a mist recovery device 1202, and then exhausted by an exhaust blower 1203. In this way, during operation, the electrolytic tank lid 105, decontamination tank lid 204, and drain tank lid 702 are closed, and the exhaust gas treatment device 12 is constantly operated, so that electrolyte mist containing nitric acid vapor and radioactivity escapes from the system. In addition, it is possible to prevent the hydrogen gas generated by electrolysis from locally accumulating and causing combustion or explosion, thereby maintaining the safety of workers. Also, the only lid that can be opened during work is the decontamination tank lid 204, and the decontamination tank lid 204 is opened after the object to be decontaminated 3 and the inner wall of the decontamination tank 2 have been washed with water, as described above.
It is time to take out the object 3 to be decontaminated. Therefore, there is no risk that the worker will come into contact with the electrolyte containing nitric acid and radioactivity, and work safety is maintained. An example of a case where an object to be decontaminated is treated with the decontamination apparatus according to the present invention will be described. Inside the decontamination tank 2, the object to be decontaminated 3 is made of SUS304 (2B
sch40) piping was immersed. in the electrolyte 203
The concentration of Ce ⁴⁺ is 0.6 mol/. After 1 hour of decontamination work, the results shown in the table were obtained. The table shows the decontamination results as a result of temperature changes. Although no decontamination effect was seen at an electrolyte temperature of 30°C, it was confirmed that decontamination down to the background could be achieved at an electrolyte temperature of 80°C. Note that the polishing thickness indicates the amount of decontamination calculated from the weight loss, the alpha ray intensity indicates the amount of surface contamination of the object to be decontaminated, and the decontamination speed indicates the amount of surface contamination of the object to be decontaminated removed per second.

[Table] [Effects of the Invention] According to the present invention, there are the following effects. (1) Decontamination can be performed as long as the surface of the object to be decontaminated is in contact with the electrolyte, so it can be applied to objects with complex shapes. (2) The decontamination speed is high because the oxidizing power when cerium tetravalent ions change to cerium trivalent ions is used to dissolve the radioactively contaminated metal surface that is the object to be decontaminated. (3) Since an electrolytic cell overflow pipe is provided on the side wall of the electrolytic cell, the electrolyte level is kept constant and the entire electrode area is used effectively. (4) Since a decontamination tank overflow pipe is installed on the side wall of the decontamination tank, the electrolyte level remains constant regardless of the size of the object to be decontaminated, and there is no risk of electrolyte spilling from the decontamination tank. do not have. (5) A filter is installed in the middle of the electrolytic cell liquid pipe from the drain tank to the electrolytic cell, and since this filter is covered with a thin material, there is no risk of contamination entering the electrolytic cell. It suppresses the accumulation of contamination in the liquid and reduces the exposure of workers to radiation. (6) During operation, the electrolytic tank lid, decontamination tank lid, and drain tank lid are closed, and the exhaust gas treatment equipment is always in operation, so that nitric acid vapor and electrolyte mist containing radioactivity do not escape from the system and are safe for workers. safety is maintained. (7) After the decontamination target objects and the inner walls of the decontamination tank have been washed with water, the decontamination tank lid can be opened and the decontaminated objects can be taken out, so there is no risk of workers coming into contact with the electrolyte containing nitric acid and radioactivity. , worker safety is maintained.

[Brief explanation of the drawing]

The drawing is a system diagram showing an embodiment of the radioactively contaminated metal decontamination apparatus according to the present invention. 1... Electrolytic cell, 101... Electrolyte, 102...
Electrode, 103... Temperature detector, 104... Heater, 2... Decontamination tank, 201... Temperature detector, 20
2... Heater, 3... Decontamination object, 4... Electrolytic tank overflow pipe, 5... Decontamination tank overflow pipe, 6... Decontamination tank drain pipe, 601... Valve,
7... Drain tank, 701... Electrolyte, 8... Liquid feed pipe for electrolytic tank, 801... Pump, 802... Flow rate adjustment valve, 803... Flow meter, 9... Filter, 901... Pressure gauge , 902...pressure gauge, 10
...Liquid pipe for decontamination tank, 1001 ...Pump, 10
02...Flow rate adjustment valve, 1003...Flow meter,
1004...Pressure gauge, 1005...Pressure gauge, 11
...Exhaust gas duct, 1101 ...Temperature detector, 1
2... Exhaust gas treatment device, 1201... Nitric acid vapor recovery device, 1202... Mist recovery device, 1203
...Exhaust fan, 1204...Receiving tank, 1205...Liquid level gauge, 1206...Collected liquid return pipe, 1207...
... Valve, 1208 ... Pressure gauge, 1209 ... Temperature detector, 1210 ... Pressure gauge, 1211 ... Flow meter, 13 ... Washing water tank, 1302 ... Pump,
14... Rinsing water supply pipe, 15... Rinsing water return pipe, 1501... Valve 16... Shyahei body, 1
7... Filter, 18... Shiyahei body.

Claims (1)

[Claims] 1 Using a nitric acid aqueous solution containing trivalent cerium ions and tetravalent cerium ions, trivalent cerium ions are generated into tetravalent cerium ions through an electrolytic redox reaction, and the generated tetravalent cerium ions are converted into trivalent cerium ions. In a radioactively contaminated metal decontamination device that uses the oxidizing power of the decontaminating object to melt and decontaminate the surface of the decontaminated object, the electrolytic cell dedicated to the electrolytic redox reaction and the electrolytic cell for dissolving the surface of the decontaminated object are used. a decontamination tank for decontamination, an electrolytic tank overflow pipe installed on the side wall of the electrolytic tank, a decontamination tank overflow pipe installed on the side wall of the decontamination tank, and a decontamination tank installed on the bottom of the decontamination tank. The dye tank drain pipe is connected to the electrolytic tank overflow pipe, the decontamination tank overflow pipe, and the decontamination tank drain pipe, and the overflow electrolyte from the electrolytic tank, the overflow electrolyte from the decontamination tank, and the decontamination tank are connected to each other. A drain tank that receives drain electrolyte from the tank, an electrolytic tank liquid feed pipe that returns the electrolyte from this drain tank to the electrolytic tank, and a liquid feed pipe for the electrolytic cell that is installed in the middle of the electrolytic cell liquid feed pipe to remove insolubles in the electrolyte. a filter surrounded by a shield body for removing electrolyte, a decontamination tank liquid feed pipe for returning electrolyte from the drain tank to the decontamination tank, and spaces above the electrolytic tank, decontamination tank, and drain tank. an exhaust gas treatment device consisting of an exhaust gas duct connected to the exhaust gas duct, a nitric acid vapor recovery device, a mist recovery device, and an exhaust fan connected to the exhaust gas duct, a washing water tank that stores washing water, and the decontamination from the washing water tank. A flushing water supply pipe that sends flushing water to the tank;
A decontamination device for radioactively contaminated metals, comprising: a wash water return pipe for returning wash water used in the decontamination tank from the decontamination tank to the wash tank.