JP4370231B2 - Radioactive substance decontamination method and chemical decontamination apparatus - Google Patents

Description

  The present invention relates to a chemical decontamination method for a decontamination object attached to a radioactive material generated from a nuclear facility, and more particularly, a radioactive metal waste generated at a nuclear facility such as a nuclear power plant, particularly at a periodic inspection. TECHNICAL FIELD The present invention relates to a chemical decontamination method for removing radioactive substances from the apparatus and an apparatus used in the method.

  Decontamination treatment of radioactive materials from radioactive metal waste (decontamination target) generated at nuclear facilities (nuclear power plant, fuel reprocessing plant, fuel manufacturing plant, uranium conversion and concentration plant, laboratory, etc.) It is essential to reduce the degree of contamination of the radioactive metal waste to below the specified export standard, and not only the removal of radioactive material but also the amount of secondary waste generated along with this is reduced. Since it is necessary to pay attention to whether or not to do so, the selection of the processing method is extremely important.

  Conventionally, it has been known that radioactive metal waste is a combination of pollution forms such as oil pollution, paint film pollution, dust pollution, and oxide film pollution. The following method has been adopted to lower the substance below a predetermined export standard.

(1) Oil pollution:
Organic solvent decontamination, Freon decontamination, dry ice blast decontamination (2) Coating film contamination:
Organic solvent decontamination, dry ice blast decontamination, drive last decontamination,
High pressure water jet decontamination (3) Dust contamination:
Dry ice blast decontamination, drive last decontamination, high pressure water jet decontamination (4) Oxide film contamination:
Powerful chemical decontamination, electropolishing decontamination, redox decontamination, ultra-high pressure jet decontamination,
Drive last decontamination,

  Among the above decontamination methods, for example, mechanical methods such as a dry ice blast decontamination method and a drive last decontamination method can often remove various contaminations at a time, and thus are often employed. However, the blast decontamination method irradiates the blasting material violently on the surface to be decontaminated and decontaminates the radioactive substance by the impact of this, so when decontaminating an object with a complicated structure, It is necessary to disassemble and disassemble the blasting material so that the surface to be decontaminated can be irradiated. For example, even with simple pipe decontamination, it is necessary to divide the pipe into half and make it possible to irradiate the surface to be decontaminated with blasting material before decontamination. If the structure is further complicated, there is a problem that it is necessary to perform decontamination after cutting and disassembling the structure and exposing the interior in the same manner. In addition, there is a problem that a structure having a complicated shape cannot be decontaminated.

  On the other hand, decontamination methods using organic solvents and freon to remove oil and coatings are also known, but the former is severely restricted at nuclear power plants due to safety issues (fire, etc.). Moreover, since the latter freon is prohibited worldwide due to problems such as ozone holes, it is currently difficult to adopt.

  Furthermore, strong chemical decontamination, electropolishing decontamination, redox decontamination, etc., which are decontamination methods for oxide films, have problems in terms of workability and also increase the amount of secondary waste. .

  Thus, the conventional decontamination method is not satisfactory as a decontamination method for radioactive waste generated from nuclear facilities such as nuclear power plants, and development of a better decontamination method is awaited. ing.

  Therefore, the present invention is sufficient for radioactive materials adhering to a decontamination object having a complicated shape and also for radioactive wastes having different forms of contamination such as oil contamination, coating film contamination, dust contamination, and oxide film contamination. It is an object of the present invention to provide a decontamination method and a decontamination apparatus used for the decontamination method that can obtain a decontamination effect and also solve the problem of secondary waste.

  The inventors of the present invention have been diligently studying to solve the above-described problems. As a result, an alkaline decontamination agent and an acid decontamination agent that combines an organic acid and an inorganic acid are used, and vibrations are further generated in these decontamination agents. By agitation or ultrasonic agitation, it is possible to decontaminate even radioactive substances in decontamination objects with complex shapes, or radioactive substances in radioactive waste mixed with organic dirt and metal oxide dirt. In addition, the present inventors have found that the amount of secondary waste can be easily reduced, thereby completing the present invention.

  That is, the present invention combines a decontamination object generated from a nuclear facility with an alkali decontamination step using an alkali decontamination agent and an acid decontamination step using an acid decontamination agent that combines an organic acid and an inorganic acid. It is a removal method of a radioactive substance characterized by attaching | subjecting to a decontamination process.

  The present invention also provides an alkaline decontamination tank that performs decontamination using an alkaline decontamination agent, an acid decontamination tank that performs decontamination using an acid decontamination agent that combines an organic acid and an inorganic acid, and a decontamination target. From a nuclear facility characterized by comprising means for moving between the alkali decontamination tank and the acid decontamination tank, and a waste liquid treatment apparatus for treating the alkali decontamination agent and the acid decontamination agent after use. It is a chemical decontamination device for the generated decontamination object.

  The method for removing radioactive substances of the present invention can improve the decontamination effect and expand the decontamination target range by combining and using two types of decontamination agents, alkaline decontamination agent and acid decontamination agent. It can be. In addition, secondary waste generated by decontamination can be easily reduced.

  The alkaline decontaminating agent used in the method of the present invention is not particularly limited as long as it has an alkaline decontaminating property, but preferably has a pH of about 12.5 to 13. Specific examples of alkaline decontamination agents include sodium orthosilicate, sodium metasilicate, sodium sesquisilicate, sodium silicate liquids (sodium silicates 1 to 4), phosphorous such as trisodium phosphate Examples include alkaline solutions containing at least one of acid carbonates, percarbonates such as sodium percarbonate, and carbonates such as sodium sesquicarbonate, and suitable surfactants such as polyoxyethylene octyl ether An appropriate amount may be added.

  This alkaline decontamination agent is effective in removing fats and oils from radioactive metal waste and removing coating materials. In particular, by dissolving the base material of the coating agent, its strength is reduced, and ultrasonic and other machinery These can be removed from the metal surface portion by receiving general assistance. These substances desorbed in the alkaline decontamination agent can be removed from the alkaline decontamination solution by means of adsorption, filtration or the like using their respective properties. Thus, it becomes possible to obtain a further decontamination effect by keeping the alkaline decontamination solution clean.

  On the other hand, the acid decontamination agent used in the method of the present invention is a mixture of an organic acid and an inorganic acid, and examples of the organic acid include citric acid, glycolic acid, malonic acid, oxalic acid, and the like. As the inorganic acid, for example, nitric acid, hydrofluoric acid, sulfuric acid and the like are used. The concentration of the organic acid in the acid decontaminating agent is preferably about 0.1 to 0.3%, and the concentration of the inorganic acid is preferably about 0.5 to 1.5%. Furthermore, the pH of the acid decontamination solution containing an organic acid and an inorganic acid is preferably about 0.65 to 0.8.

  In the acid decontamination solution, the organic acid acts strongly on the clad and dissolves it, and the inorganic acid acts to maintain the dissolution of the metal material and the dissolved metal ions. Take advantage of the advantages. In order to average the improvement in decontamination effect, it is preferable to stir the acid decontamination solution using a means such as a vibration stirrer to improve the liquid contact fluidity to the decontamination target.

  In the method for removing a radioactive substance of the present invention, an acid decontamination process is generally performed after an alkali decontamination process. However, this process may be repeated when the radioactive substance is difficult to be removed. The alkali decontamination step in the present invention is preferably carried out for a maximum of about 1.5 hours, although it varies depending on the nature and amount of the radioactive substance attached to the decontamination target or the type of alkali decontamination agent. Also, the acid decontamination step is preferably performed for about 3 hours at maximum, although it varies depending on the nature and amount of the radioactive substance attached to the decontamination target or the type of the alkaline decontamination agent.

  Next, an apparatus for advantageously carrying out the radioactive substance removing method of the present invention will be described with reference to the drawings. FIG. 1 is a view showing an embodiment of a unit configuration of the chemical decontamination apparatus of the present invention, wherein 1 is an alkali decontamination unit, 2 is an acid decontamination unit, 3 is a water washing unit, and 4 is a waste liquid treatment unit. .

  FIG. 2 is a drawing showing the configuration of the alkali decontamination unit 1, in which 5 is a crane, 6 is a hook, 7 is a rail for the crane, 10 is an alkaline decontamination tank, 11 is an electric heater, and 12 is a super An ultrasonic vibrator, 13 is a super vibration stirrer, 14 is an oil adsorption tower, 15 is a backwash filter, 16 is a buffer tank for backwash, and 17 is a transfer pump.

  In the alkali decontamination unit 1 of FIG. 2, an ultrasonic vibrator 12 and a super vibration agitator 13 are installed in the alkali decontamination tank 10, and an alkali decontamination agent A is placed therein. The alkali decontamination liquid A is communicated and circulated from the alkali decontamination tank 10 to the oil adsorption tower 14, the backwashing filter 15, and the electric heater 11 through the piping by the transfer pump 17. Further, a back dyeing buffer tank 16 is attached to the backwashing filter 15.

  In the alkali decontamination unit 1, for example, a basket-shaped decontamination container (not shown) through which the treatment liquid can be circulated is attached to the hook 6 of the crane 5 provided in the upper part, and the decontamination target is placed in this container. After putting an object, it is immersed in the alkali decontamination tank 10 by the movement of the crane 5 and the up and down of the hook 6 to perform alkali decontamination.

  3 is a drawing showing the configuration of the acid decontamination unit 2. In the figure, 5 to 7 are the same as those in FIG. 2, 20 is an acid decontamination tank, 21 is an electric heater, and 22 is a super-vibration agitation. , 23 is a cationic resin tower, 24 is an acid decontaminant decomposition tower, 25 is a replaceable filter, 26 and 27 delivery pumps, and 28 is a recycled waste liquid treatment tank.

  In the acid decontamination unit 2, a super vibration agitator 22 is attached to the acid decontamination tank 20, and the acid decontamination liquid B is placed therein. The acid decontamination liquid B is communicated and circulated from the acid decontamination tank 20 to the exchangeable filter 25, the cation resin tower 23, and the electric heater 21 through a pipe by a transfer pump 26. Further, an acid decontaminating agent decomposing tower 24 is also attached to this pipe, and the organic acid in the acid decontaminating agent can be decomposed as necessary.

In this acid decontamination unit 2, as in the alkali decontamination unit 1, the decontamination target that has been washed with water as necessary after alkaline decontamination, for example, by moving the crane 5 provided in the upper part and moving the hook 6 up and down is provided. In the acid decontamination tank 20, acid decontamination is performed.

  Further, FIG. 4 is a drawing showing the structure of the water washing unit 3. In the figure, 5 to 7 denote the above, 30 is a water washing tank, 31 is a mixed bed resin tower, 32 is a replaceable filter, 33 is Indicates a transfer pump.

  The water washing unit 3 includes a water washing tank 30 containing washing water C, a mixed bed resin tower 31 and an exchangeable filter 32 communicated with the water washing tank 30 through a pipe. The washing water C circulates from the washing tank 30 through the mixed bed resin tower 31 and the exchangeable filter 32, and various ionic components are removed from the mixed bed resin tower 31 and solid impurities are removed from the exchangeable filter 32.

  The water washing unit 3 in FIG. 4 is produced by immersing the decontamination object after alkaline decontamination and the decontamination object after acid decontamination, and resulting from the decontamination of alkali and acid components from each decontamination solution. The solid impurities and the like are removed. Since the washing water C is circulated and purified as described above, the washing function can be maintained for a long time. In addition, it is the same as the alkali decontamination unit 1 and the acid decontamination unit 2 that the decontamination object is moved by moving the crane 5 provided on the top and moving the hook 6 up and down and immersed in the water washing tank 30 in the water washing. It is.

  Furthermore, FIG. 5 is a drawing showing the configuration of the waste liquid treatment unit 4, in which 40 denotes a waste liquid treatment tank and 41 denotes a transfer pump. In this unit, the alkali waste solution and the acid waste solution which are discharged from the alkali decontamination unit 1, the acid decontamination unit 2 and the water washing treatment unit 3 are received and neutralized.

  By the way, in the chemical decontamination method, it is a conventional technique to raise the temperature of both the alkali decontamination liquid and the acid decontamination liquid. In the apparatus of the present invention, the heaters 11 and 21 for heating each liquid are alkali decontamination. It is provided in the tank 10 and the acid decontamination tank 20. The heating by these is preferably 50 to 75 ° C. As the heater, an electric heater is generally used and has good operability and is easy to handle. However, a heater using steam or any other heater may be used as long as it can maintain a predetermined temperature. When steam is used, the indirect heating method is preferable to direct heating from the viewpoint of reducing the amount of waste liquid generated.

  In addition, in order to remove radioactive substances adhering to the metal material to be decontaminated, oils and fats or coating materials adhering thereto, it is effective to give mechanical vibration in addition to the treatment with the decontamination solution. Therefore, in the apparatus of the present invention, the ultrasonic vibrator 12 and the super vibration agitator 13 are provided in the alkali decontamination tank 10, and the super vibration agitator 22 is provided in the acid decontamination tank 20.

  Among these, as the super-vibration stirrers 13 to 22, for example, an α-type vibration stirrer (manufactured by Nippon Techno Co., Ltd .; commercial product) can be used, and the liquid contact flow of the decontamination agent is applied to the surface to be decontaminated. The decontamination effect can be improved by increasing. This α-type vibration agitation is effective because it enters the back of the object and the gaps and spreads throughout the structure, greatly improving the decontamination effect and creating fluid flow. Is.

  Moreover, since the ultrasonic irradiation by the ultrasonic vibrator 12 is accompanied by its strong peeling action, the deposits from the surface portion are destroyed and removed, and further the penetration of the decontamination solution is promoted to improve the decontamination efficiency. The effect of this ultrasonic irradiation attenuates as the distance from the irradiation point increases due to its nature. The effect is also very small for the back of the irradiated surface. When this apparatus is used in a fixed manner as in the prior art, spots are generated in the decontamination effect, and a portion that can be well decontaminated and a portion that is not so often occur. In order to eliminate this drawback, it is preferable to be able to move within the apparatus without fixing the ultrasonic transducer, and in this way, the action in the tank can be made uniform, A sufficient effect can be obtained with 1 or 2 units that required about 4 or 5 units.

  In the apparatus of the present invention, the radioactive substance removed by the decontamination process, the oil and fat, or the coating material is removed by an apparatus communicated with the alkali decontamination tank 10 or the acid decontamination tank 20 via a pipe. That is, the oil and fat content and the paint content removed from the metal material to be decontaminated are in the oil adsorption tower 13 in the alkali cleaning unit 1, and solid matters such as dust containing radioactive substances are backwashed in the alkali cleaning unit 1. It is removed by the mold filter 14 or the exchange filter 25 in the acid cleaning unit 2. In addition, radioactive metal ions chemically dissolved mainly by acid decontamination are removed by the cation resin tower 23 in the acid cleaning unit 2. And by these devices, each decontamination solution can be kept clean and secondary contamination can be prevented, so that the life of the decontamination agent can be extended and the amount of secondary waste generated can be reduced. can do.

  Moreover, each decontamination agent used in the apparatus of the present invention is disposed of as follows. First, as a final treatment, the acid decontaminant B oxidizes the organic acid contained therein in the decontaminant decomposition tower 24 of the acid cleaning unit 2 (oxygen oxidizers such as hydrogen peroxide, ozone, oxygen gas, etc.). ) Decomposes into water and carbon dioxide. The remaining inorganic acid is stored in the recycled waste liquid treatment tank 28 via the transfer pump 27, and finally, alone or after neutralization with the alkali decontamination agent described later, for example, in the nuclear power plant. It is processed at the existing radioactive waste treatment facility. On the other hand, the final treatment of the alkaline decontaminant A is sent to the waste liquid treatment tank 40 of the waste liquid treatment unit 4 and, for example, neutralized alone with the acid decontamination liquid after removing the organic acid, Like the decontaminating agent, it is processed at the existing radioactive waste treatment facility in the nuclear power plant.

  In the present invention described above, the contamination taken into the oil and fat or the painted surface is removed by the alkali decontamination treatment, and the radioactive cladding taken into the oxide film is removed by dissolution with the acid decontamination agent. This combination enables effective decontamination treatment of objects that could not be achieved independently. In particular, the method of the present invention exhibits a great effect when decontaminating a radioactive substance from a pipe-shaped object to be decontaminated, which has conventionally been difficult to remove. Of course, when any one of them is sufficient, each method can be applied.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
(1) Using a test apparatus provided with a vibration stirrer (400 W, 40 Hz) and an ultrasonic vibrator (900 W, 26 KHz) in a decontamination tank (60 liters) as outlined in FIG. It was investigated whether the attached radioactive material could be removed by the method of the present invention. As a test sample, carbon steel, stainless steel, aluminum, copper plate, etc. whose surface was contaminated to about 10,000 cpm with radioactive substances were placed in a decontamination tank having a size of L300 × W300 × H100 (mm). What was cut into the size to enter was used. Moreover, the alkaline decontamination liquid and the acid decontamination liquid used in this test are as follows.

Alkaline decontamination solution:
Sodium silicate 0.3%
Polyoxyethylene octyl ether 0.01%
Acid decontamination solution:
Oxalic acid 0.3%
Sulfuric acid 1%

(2) The alkali decontamination test uses the above-mentioned test apparatus, and decontaminates the test sample to which the radioactive material has adhered for 90 minutes at a decontamination liquid temperature of 50 ° C. in a decontamination tank containing 60 liters of the above alkaline decontamination liquid. After dyeing, the surface contamination density was measured with a GM tube survey meter.

  As a result, it was possible to decontaminate 1,090 kg (about 75%) out of 1,468 kg of all test objects until the standard value for carrying out from the radiation control area was satisfied by alkali decontamination.

(3) Furthermore, an acid decontamination test was performed on a test sample from which radioactive material was not removed by alkali decontamination. In the acid decontamination test, 60 liters of the acid decontamination solution was placed in the decontamination tank of the test apparatus, and the test piece to which the radioactive material adhered was decontaminated at a decontamination solution temperature of 75 ° C. for 180 minutes. Thereafter, the surface contamination density was measured by a GM tube survey meter.

  Of the 378kg test pieces that could not be decontaminated by alkali decontamination, 268kg of acid decontamination results in 190kg (about 70%) that could be decontaminated until the standard value for carrying out from the radiation control area was satisfied. there were.

  Based on the above results, 75% can be decontaminated by alkali decontamination, and the remaining 25% that could not be decontaminated by alkali decontamination can be decontaminated until approximately 70% of acid decontamination satisfies the standard value for carrying out from the radiation control area. It was. That is, in this test, by combining the alkali decontamination method and the acid decontamination method, it was possible to decontaminate until about 90% of the contaminated metal material due to the radioactive material to be decontaminated satisfied the unloading standard value.

  According to the method of the present invention, it is possible to easily decontaminate the radioactive metal waste of the object to be decontaminated in a complicated shape and the radioactive waste combined with organic dirt and metal oxide dirt, Chemical decontamination of radioactive metal waste generated at the time of periodic inspection of a nuclear power plant can be advantageously performed.

The figure which showed the unit structure of this invention chemical decontamination apparatus. The figure which showed the structure of the alkali decontamination unit of this invention chemical decontamination apparatus. The figure which showed the structure of the acid decontamination unit of this invention chemical decontamination apparatus. The figure which showed the structure of the washing unit of this invention chemical decontamination apparatus. The figure which showed the structure of the waste liquid processing unit of this invention chemical decontamination apparatus. The figure which showed typically the testing apparatus used in the Example.

Explanation of symbols

1 …… Alkaline decontamination unit 2 …… Acid decontamination unit 3 …… Washing unit 4 …… Waste liquid treatment unit 5 …… Crane 6 …… Hook 7 …… Crane rail 10 …… Alkaline decontamination tank 11 …… Electricity Heater 12 …… Ultrasonic vibrator 13 …… Super vibration stirrer 14 …… Backwash filter 15 …… Backwash buffer tank 16 …… Transfer pump 20 …… Acid decontamination tank 21 …… Electric heater 22 …… Super Vibrating Stirrer 23 …… Cation Resin Tower 24 …… Acid Decontaminant Decomposition Tower 25 …… Exchange Filter 26, 27 …… Transfer Pump 28 …… Recycled Waste Liquid Treatment Tank 30 …… Washing Tank 31 …… Mixed Bed Resin Tower 32 …… Exchangeable filter 33 …… Transfer pump 40 …… Waste liquid treatment tank 41 …… Transfer pump 51 …… Decontamination tank 52 …… Decontamination basket 53 …… Electric heater 54 …… Replacement heater 55 …… I -Exchange resin tower 56 ...... ultrasonic transducer 57 ...... vibration agitator 58 ...... circulation pump 59 ...... way valve A ...... alkaline decontaminant B ...... acid decontaminant C ...... washing water
more than

Claims (8)

Radioactive metal waste generated from nuclear facilities contains compounds selected from the group consisting of sodium orthosilicate, sodium metasilicate, sodium sesquisilicate, sodium silicate solution, trisodium phosphate, sodium percarbonate, sodium sesquicarbonate Selected from the group consisting of an alkaline decontamination step using an alkaline decontamination agent having a pH of 12.5 to 13 and a concentration of 0.1 to 0.3% citric acid, glycolic acid, malonic acid and oxalic acid. Acid removal using an acid decontamination agent having a pH of 0.65 to 0.8 in combination with an organic acid and an inorganic acid selected from the group consisting of nitric acid, hydrofluoric acid and sulfuric acid having a concentration of 0.5 to 1.5% A method for removing a radioactive substance, which is subjected to a decontamination process combined with a dyeing process. An alkali decontamination tank that performs decontamination using an alkali decontamination agent, an acid decontamination tank that performs decontamination using an acid decontamination agent that combines an organic acid and an inorganic acid, And a means for moving between the acid decontamination tank and a waste liquid treatment apparatus for treating the alkali decontamination agent and the acid decontamination agent after use, wherein the alkali decontamination tank comprises an oil adsorption tower , a solid content A chemical decontamination apparatus for radioactive metal waste generated from a nuclear facility, comprising a filter for removing water and a heater for heating a decontamination solution , wherein the acid decontamination tank has a cation exchange resin tower. Furthermore, the chemical decontamination apparatus according to claim 2 , wherein the alkali decontamination tank has a vibration agitator and / or an ultrasonic vibrator. The chemical decontamination apparatus according to claim 3 , wherein the ultrasonic transducer in the alkali decontamination tank has a moving function for enabling uniform ultrasonic irradiation. The chemical decontamination according to claim 2 , wherein the acid decontamination tank further comprises a filter for removing solids, a catalytic oxidative decomposition tower for decomposing the organic acid in the acid decontaminant, and a heater for heating the decontamination liquid. Dyeing equipment. Furthermore, the chemical decontamination apparatus according to claim 2 or 5 , wherein the acid decontamination tank has a vibration stirrer. The acid decontamination agent is a combination of an organic acid selected from the group consisting of citric acid, glycolic acid, malonic acid and oxalic acid and an inorganic acid selected from the group consisting of nitric acid, hydrofluoric acid and sulfuric acid. The chemical decontamination apparatus according to any one of 2 to 6 . The alkali decontaminant is selected from the group consisting of sodium orthosilicate, sodium metasilicate, sodium sesquisilicate, sodium silicate solution, trisodium phosphate, sodium percarbonate, and sodium sesquicarbonate. The chemical decontamination device according to any one of the above .

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