Pollutant removal method and pollutant removal device used in the method

Abstract

Provided is a method for removing contaminants that removes contaminants without damaging a substrate and without volatilizing the contaminants. [Solution] In this method, a contaminated surface is irradiated with nano-pulse laser light oscillated from a pulse oscillator to remove contaminants, and the contaminated object having the contaminants attached thereto is ultrasonically cleaned in a weakly acidic organic acid cleaning solution, ultrasonically cleaned in an alkaline neutralizing solution, and dried, and then the contaminated surface is irradiated with nano-pulse laser light 44 having a rectangular flat-top shape to peel off the contaminants from the contaminated surface. The weakly acidic organic acid cleaning solution is preferably an aqueous solution of at least one acid selected from the group consisting of citric acid, ascorbic acid, glycolic acid, malonic acid, acetic acid, and oxalic acid.

Description

[Technical field]

[0001] The present invention relates to a method for removing pollutants using a water-soluble resin coating film. [Background technology]

[0002] Radioactive contaminants such as cesium oxide and strontium oxide, or chemical contaminants such as polychlorinated biphenyls (PCBs) and dioxins, are generally harmful to humans and terrestrial organisms and must be removed. Patent Document 1 proposes irradiating a radioactive contaminant present on the surface of a metal substrate with a laser beam to evaporate or volatilize the radioactive contaminant together with the surface layer of the metal substrate and remove it. Patent Document 2 proposes irradiating a laser beam at a scanning speed sufficiently faster than the generation of heat, etc., caused by energy transmission from a pulsed laser, to evaporate or remove the substance from the irradiated area. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-016530 A [Patent Document 2] JP 2007-315995 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the above-mentioned conventional technology, radioactive contaminants are evaporated or volatilized by the ultra-high temperature caused by the laser light, making them impossible to capture even with current high-performance filters such as HEPA and ULPA. There were problems in that the contaminants were simply dispersed into the air, and were not essentially removed, and that the base material was damaged.

[0005] SUMMARY OF THE PRESENT DISCLOSURE In order to solve the above-mentioned problems, the present invention provides a method for removing contaminants that does not damage a substrate and does not volatilize the contaminants, and a contaminant removal apparatus used in the method. [Means for solving the problem]

[0006] One embodiment of the present invention is a method for removing contaminants from a contaminated object, comprising the steps of: The contaminated object is removed in the cleaning tank. An aqueous solution of at least one weak acid selected from the group consisting of citric acid, ascorbic acid, glycolic acid, malonic acid, acetic acid, and oxalic acid. Ultrasonic cleaning in organic acid cleaning solution By doing so, the metal substrate is not corroded, and the metal ions are captured by the chelating effect. The water-absorbing property of the deposits is utilized, and the weak acid organic acid that has entered the gaps of the thick deposits is vibrated by the propagation of ultrasonic vibrations, which causes the thick deposits to separate. after that, Ultrasonic cleaning in alkaline neutralizing solution, then drying or draining. Next, From a pulse generator The present invention relates to a method for removing contaminants, which comprises irradiating a contaminated surface with nano-pulse laser light having a square flat-top shape, and peeling off the contaminants from the contaminated surface.

[0007] Another embodiment of the present invention is a pollutant removal apparatus for use in the pollutant removal method, comprising: a cleaning tank equipped with an ultrasonic oscillator and storing an organic acid cleaning solution; a cleaning liquid circulation circuit provided outside the cleaning tank; The circulation circuit is provided with a circulation pump, a cyclone, and a fine bubble generator, The fine bubble generator generates a mixed fluid of the organic acid cleaning solution, air bubbles containing fine bubbles, and water, and the mixed fluid is returned to the cleaning tank.

[0008] Yet another embodiment of the present invention is a pollutant removal apparatus for use in the pollutant removal method, comprising: The present invention relates to a surface contaminant removal device that includes a means for converting a Gaussian-shaped beam of nano-pulse laser light emitted from a pulse oscillator into a square flat-top shaped nano-pulse laser light using a flat beam shaver, and a means for irradiating the square flat-top shaped nano-pulse laser light onto a contaminated surface. Effect of the Invention

[0009] The present invention provides a contaminant removal method and a contaminant removal apparatus for use in this method, which can efficiently remove contaminants adhering to a thick film by ultrasonically cleaning a contaminated object having contaminants adhering thereto in a weakly acidic organic acid cleaning solution, ultrasonically cleaning in an alkaline neutralizing solution, drying or draining, and then irradiating the contaminated surface with nano-pulse laser light having a square flat-top shape to peel off the contaminants from the contaminated surface, thereby making maximum use of the ablation effect of the nano-pulse light and minimizing the thermal effects, without damaging the substrate and without volatilizing the contaminants. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic explanatory diagram of a nano-pulse laser oscillator according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic explanatory diagram of a nanopulse laser irradiation device according to one embodiment of the present invention. [Diagram 3] FIG. 3 shows a schematic waveform diagram (left) of a conventional nano-pulse laser beam having a Gaussian shape, and a schematic waveform diagram (right) of a nano-pulse laser beam having a square flat-top shape according to one embodiment of the present invention. [Figure 4] FIG. 4A is a schematic top view showing an irradiation state with a square flat-top beam shape according to one embodiment of the present invention, and FIG. 4B is a schematic side cross-sectional view of the same. [Diagram 5] FIG. 5A is a schematic top view showing an irradiation state with a conventional Gaussian beam shape, and FIG. 5B is a schematic side cross-sectional view of the same. [Figure 6] FIG. 6A is a schematic energy distribution diagram of laser light having a flat-top beam shape, and FIG. 6B is a schematic energy distribution diagram of laser light having a Gaussian beam shape. [Figure 7] Figure 7 shows the contamination status of the primary systems of BWR and PWR reactors announced by the IAEA. [Figure 8] FIG. 8 is a cross-sectional view of an example showing the contamination state of a radioactively contaminated metal structure within the Fukushima Daiichi Nuclear Power Plant site. [Figure 9]FIG. 9 is a cross-sectional view of an example showing the contamination state of a radioactively contaminated metal structure within the Fukushima Daiichi Nuclear Power Plant site. [Figure 10] FIG. 10 is a cross-sectional view of an example showing the contamination state of a radioactively contaminated metal structure within the Fukushima Daiichi Nuclear Power Plant site. [Figure 11] FIG. 11 is a cross-sectional view of an example showing the contamination state of a radioactively contaminated metal structure within the Fukushima Daiichi Nuclear Power Plant site. [Figure 12] FIG. 12 is a schematic explanatory diagram of an ultrasonic cleaning device according to one embodiment of the present invention. [Figure 13] FIG. 13 is a schematic side view of a contaminated object before and after cleaning in Example 1 of the present invention. [Figure 14] FIG. 14 is a schematic side view of a contaminated object before and after cleaning in Example 2 of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The method for removing contaminants according to one embodiment of the present invention includes the following steps. (1) Ultrasonic cleaning process in organic acid cleaning solution In this process, the thick-film contaminated object with the contaminants attached thereto is ultrasonically cleaned in a weakly acidic organic acid cleaning solution. The weakly acidic organic acid cleaning solution is preferably an aqueous solution of citric acid, ascorbic acid, glycolic acid, malonic acid, acetic acid, oxalic acid, or the like. The organic acid is weakly acidic and does not corrode the metal substrate. It also has a chelating effect and is highly effective in capturing metal ions. The treatment temperature may be room temperature (30 to 60°C). The deposits to be removed by the weakly acidic organic acid cleaning solution easily absorb water. By utilizing this water absorption, the target metal product is immersed in the acidic cleaning solution, and the water is absorbed into the gaps between the deposited particles, and ultrasonic energy is used to promote the removal of the deposits. By adopting this method, the weakly acidic organic acid that has entered the gaps in the thick-film deposits is vibrated in the gaps in the thick-film deposits by the propagation of ultrasonic vibrations, making it possible to dissociate the adhesion between the thick-film deposits, and it is possible to peel off and remove the thick-film deposits of the metal products in nuclear power facilities in a much shorter time than the conventional method of sequentially removing the thick-film deposits of the metal products in nuclear power facilities only from the surface direction using a nano-pulse laser. In addition, ultrasonic vibration of the cleaning water creates a water flow in the cleaning water on the contaminated metal surface, making it possible to immediately remove the contaminants precipitated in the cleaning water from the metal surface. As described above, the weak acid organic acid cleaning solution makes it possible to remove rust from the surface of a metal structure. (2) Ultrasonic cleaning process in alkaline neutralizing solution In this step, ultrasonic cleaning is performed using an alkaline neutralizing solution such as an aqueous solution of sodium metasilicate or sodium bicarbonate, and neutralization is performed. The treatment temperature may be room temperature. The wastewater after the organic acid cleaning treatment and the wastewater after the alkaline neutralization treatment are mixed and neutralized, and the neutralized water is preferably heated and distilled to concentrate the contaminants and then disposed of in a landfill. The boiling points of citric acid, which is preferred as an organic acid in the present invention, are 175°C, 190°C for ascorbic acid, 112°C for glycolic acid, 140°C for malonic acid, 118°C for acetic acid, and 365°C for oxalic acid, all of which are higher than the boiling point of water (100°C). The use of organic acids with high boiling points makes evaporation easier. (3) Drying process Drying can be done by air drying or by draining with air, but in cold seasons hot air can also be blown on the surface. (4) Nanopulse laser light irradiation process The nano-pulse laser beam with a square flat top shape is irradiated onto the contaminated surface, and the contaminants are peeled off from the contaminated surface. This process will be described in detail later. The important point is that when the contaminated metal structure is large in volume and has a large area, considering the efficiency of rust removal, the decontamination efficiency can be further improved by adding acid ultrasonic cleaning as a pretreatment process of nano-pulse laser decontamination.

[0012] The ultrasonic frequency in the organic acid cleaning solution and the ultrasonic frequency in the neutralization solution are preferably 20 to 40 kHz. In addition, it is preferable to install a cleaning solution stirring and circulating pump to make the cleaning conditions in the cleaning tank uniform. It is also preferable to install a cyclone type fine particle removal device to purify the cleaning solution in the cleaning tank, remove coarse particles with a particle size of 10 μm or more removed from the metal surface by ultrasonic cleaning, and prevent the cleaning tank from being damaged by contact resonance with an oscillator plate vibrating by ultrasonic waves. After removing the contaminants to about a thin film (20 μm or less) by ultrasonic cleaning in the organic acid cleaning solution, the remaining contaminants on the base metal surface are removed by irradiating nano-pulse laser light.

[0013] In the present invention, between the ultrasonic cleaning step (2) in the alkaline neutralizing solution and the drying step (3), a step of forming a coating film by applying a water-soluble resin solution such as polyvinyl alcohol to the contaminated surface may be added. This allows the contaminants to be peeled off from the contaminated surface together with the water-soluble resin coating film, and the contaminants can be efficiently removed without damaging the substrate and without volatilizing them.

[0014] Next, the nanopulse laser light irradiation step will be described. The surface contaminant removal device used in the present invention includes a means for converting a Gaussian-shaped nano-pulse laser beam emitted from a pulse oscillator into a square flat-top shaped nano-pulse laser beam using a flat beam shaver, and a means for irradiating the contaminated surface with the square flat-top shaped nano-pulse laser beam. This allows efficient removal of contaminants without damaging the substrate and without volatilizing them. Damage to the substrate can compromise safety in nuclear power-related equipment, and volatilizing contaminants into the atmosphere can cause damage to the surrounding area, both of which are important issues.

[0015] The square flat-top nano-pulse laser light irradiated to the contaminated surface has an energy density per unit area of ​​1 J / cm 2 More preferably, it is 1 to 10 J / cm 2 and more preferably 5 to 10 J / cm 2 and more preferably 7 to 10 J / cm 2 By irradiating the steel surface to which the contaminants are adhered with nano-pulse laser light having an energy density higher than the van der Waals force and hydrogen bond, which are the adhesion energy of the boundary surface between the contaminants and the steel surface to which the contaminants are adhered, and lower than the metallic bond and covalent bond energy of the substrate, the ablation effect of the pulse laser light is utilized, and the contaminants can be efficiently removed without damaging the substrate and without volatilizing them. To achieve the above energy density, it is preferable to adjust the average output, pulse width, repetition frequency, laser light scanning speed, and focal diameter of the square flat top shape in combination.

[0016] The focal diameter of the nano-pulse laser beam having the square flat-top shape irradiated to the contaminated surface is preferably adjustable from 1 mm to 1.8 mm, which not only reduces the focal diameter of the laser beam and increases the energy density, but also adjusts the focal depth by combining the square flat-top shape of the laser beam with the focal diameter, thereby reducing the influence of fluctuations in the energy density of the laser beam due to unevenness of the base material.

[0017] The pollutant removal device preferably includes a pulsed Nd:YAG laser oscillator that oscillates light of a single wavelength controlled at a wavelength of 1064 nm or more and 1070 nm or less. Since the occurrence of splitting in a nano-pulse laser with a pulse width in the nanosecond range hinders uniform energy deposition on the irradiated surface, it is preferable to adopt a MOPA type pulsed Nd:YAG laser oscillator that oscillates light of a single wavelength controlled with a wavelength of 1064 nm to 1070 nm with high accuracy. The wavelength is more preferably 1064 nm or more and 1068 nm or less, and even more preferably 1064 nm or more and 1066 nm or less.

[0018] The pulse width of the pulse oscillator is preferably 20 ns to 500 ns, so that when the focused shape of the pulsed light is a flat top, the energy density at the focused surface can be increased and the range of the energy density per pulsed light can be adjusted by varying the pulse width. The frequency of the pulse oscillator is preferably 5 kHz to 50 kHz, so that the energy density of the irradiated surface can be adjusted by the combination of the scanning speed (m / s) of the laser light and the oscillation frequency (kHz) of the flat-top pulse light.

[0019] The nano-pulse laser light oscillated by the pulse oscillator preferably has a minimum average output of 500 W or more and a maximum output of 2.5 Mw or more. This makes it possible to increase the energy density (J) and energy intensity (W) at the top surface of the laser light at an oscillation frequency of 20 kHz or more when the focused shape of the pulse light is flat-topped, thereby enhancing the ablation effect of the nano-pulse laser light.

[0020] It is preferable to include a means for transmitting the nano-pulse laser light generated by the pulse oscillator to the irradiation device via a fiber cable having a length of 10 m to 150 m. This makes it possible to place the oscillator at a safe location in a wide area such as a large structure, and to mount only the irradiation device on a remote-controlled robot or the like for movement.

[0021] The method for removing contaminants of the present invention is suitable for removing radioactive contaminants and / or chemical contaminants containing chlorine components. The contaminants may also contain metals and / or inorganic substances.

[0022] The pollutant removal device of the present invention includes, as an example, the following means. (1) A cleaning tank equipped with an ultrasonic oscillator and holding an organic acid cleaning solution. A plurality of ultrasonic oscillators may be arranged. The size of the cleaning tank may be any size as long as it has a capacity to contain the contaminants. (2) A cleaning solution circulation circuit outside the cleaning tank. The circulation circuit is equipped with a circulation pump, a cyclone, and a fine bubble generator. The circulation pump circulates the cleaning liquid. The cyclone, for example, removes particles with an average diameter of 10 μm or more and removes them from the bottom. (3) A means for generating a mixed fluid of the organic acid cleaning solution, air bubbles containing fine bubbles, and water in the fine bubble generator, and returning the mixed fluid to the cleaning tank. Of the fine bubbles (all with an average diameter of 100 μm or less), the fine bubble generator preferably generates both microbubbles (average diameter of 100 μm or less) and nanobubbles (fine bubbles with a diameter of 1 μm or less), and more preferably generates ultrafine bubbles (UFB) with a diameter of several tens to hundreds of nm. Although only nanobubbles with a diameter of 1 μm or less may be used, it is reasonable to supply ultrafine bubbles (UFB), which have a bubble size similar to that of microbubbles and nanobubbles, which are consumed in large quantities during cleaning, in order to supplement the bubbles consumed by ultrasonic cleaning. If ultrafine bubbles generated by ultrasonic oscillation are supplied and mixed from the outside by an ultrafine bubble (UFB) generator into the organic acid cleaning solution, the continuity of the impact energy retention when the bubbles collide and burst is increased, improving the cleaning efficiency.

[0023] As an example, the contaminant removal device of the present invention includes a means for converting a Gaussian-shaped beam of nano-pulse laser light emitted from a pulse oscillator into a square flat-top shaped nano-pulse laser light using a flat beam shaver, and a means for irradiating the square flat-top shaped nano-pulse laser light onto a contaminated surface.

[0024] Next, the description will be given with reference to the drawings. In the following, the same reference numerals in the drawings indicate the same objects. FIG. 1 is a schematic diagram of a nano-pulse laser oscillator 1 according to an embodiment of the present invention. The nano-pulse laser oscillator 1 generates pulses using a pulse generator 2. The pulse generator preferably uses a Nd:YAG fiber laser. This is an infrared laser with a single wavelength of 1064 nm, and when the average output is set to 500 w or more, the fiber cable can be extended up to 150 m in the case of 2 kW. Therefore, in a wide area such as a large structure, the oscillator is placed at a safe location, and only the irradiation device is mounted on a remote-controlled robot or the like, and can be moved. The pulse oscillator 1 is preferably a MOPA type pulse oscillator capable of oscillating pulse light with a pulse width of nanoseconds at a stable maximum output and frequency. It is necessary to irradiate the adhesion interface between the surface contaminant and the metal surface with light of a single wavelength with high precision and little scattering. Conventionally, CW lasers with a large wavelength tolerance of 1070 nm ±50 nm have been used, but with nanopulse lasers, whose pulse width is in the nanosecond range, the occurrence of spectroscopic phenomena prevents uniform energy deposition on the irradiated surface. Therefore, it is preferable to employ a MOPA-type pulse oscillator of an Nd:YAG laser that emits light of a single wavelength with extremely high precision, controlled at a wavelength between 1064 nm and 1070 nm. The pulse generated by the pulse generator 2 is converted by the seed LD 3 into picosecond to subnanosecond pulse laser light.

[0025] Next, the isolator 4 in the preamplifier 8 prevents signal leakage, protects the equipment, and reduces the effects of noise. The semiconductor laser excitation light is input from the excitation LD 6 to the rare earth doped fiber cable 7 via the coupler 5 . Next, the isolator 9 in the main amplifier 13 prevents signal leakage, protects the equipment, and reduces the effects of noise, and the bandpass filter 10 extracts physical phenomena in a specific frequency band. The semiconductor laser excitation light is then input from the excitation LD 11 to the rare-earth doped fiber cable 7 via the coupler 12. The fiber cable 7 is connected to the nano-pulse laser irradiation device 20.

[0026] 2 is a schematic diagram of a nano-pulse laser irradiation device 20 according to one embodiment of the present invention. First, the laser light is collimated by a collimator 14. Next, a Gaussian-shaped beam, which is a typical pulsed laser beam shape, is converted by a flat-beam shaver 15 into a flat-top beam shape when it reaches the irradiation surface. Next, the flat-top-shaped nano-pulse laser light passes through optical system focus adjusters 16a and 16b, a scan head 17, and a laser profiler 18, and is irradiated onto an irradiation target (contaminated surface) 19.

[0027] FIG. 3 is a schematic waveform diagram (left) of a nano-pulse laser beam of a Gaussian-shaped beam 40 according to an embodiment of the present invention, and a schematic waveform diagram (right) of a nano-pulse laser beam of a square flat-top-shaped beam 44. By flattening the laser beam shape into a trapezoidal shape like the square flat-top-shaped beam 44, the thermal effect on the irradiated surface is reduced, and only the ablation effect, which is the vibration abrasion effect, can be utilized, thereby reducing the melting and oxidation of the substrate surface. However, in this case, the excess energy part of a general pulse laser beam is flattened to an irradiation intensity up to a line where the irradiation intensity reaches a slightly upper part of the ablation threshold 47, so the irradiation intensity area at the ablation threshold 47 is expanded. However, as described above, in order to obtain a separation speed that satisfies the cleaning speed of a metal structure, the van der Waals force, which is the adhesion energy, and the several J / cm, which is the irradiation intensity line of the ablation threshold that breaks hydrogen bonds at an appropriate range and speed, are required. 2It is necessary to raise the line, and for that purpose, it is preferable to use an irradiation device having an optical system that stably outputs pulsed light of Nd:YAG fiber laser light (wavelength 1064 nm) with an average output of 500 w or more, a maximum output of 1.25 Mw or more, a pulse width of 10 ns to 500 ns, an oscillation frequency of 5 to 50 kHz, and a laser light scanning speed of 1 to 10 m / s, and that makes the pulsed light into a square flat-top shape. In FIG. 3, 41 is the excess energy of the Gaussian-shaped beam 40, 42 is the irradiation diameter, and 43 is the thermal energy. Also, 45 is the irradiation diameter of the square flat-top shaped beam 44, and 45 is the thermal energy of the skirt. Its width is extremely narrow compared to the thermal energy 43 of the Gaussian-shaped beam 40.

[0028] Fig. 4A is a schematic top view showing the irradiation state with the square flat-top beam shape of one embodiment of the present invention, and Fig. 4B is a schematic side cross-sectional view of the same. Fig. 5A is a schematic top view showing the irradiation state with a conventional Gaussian beam shape, and Fig. 5B is a schematic side cross-sectional view of the same. In Figs. 4A-B and 5A-B, it is assumed that the contaminant is red rust (Fe2O3) that has occurred on the surface of a steel material, and that the red rust layer is about 500 μm thick. In order to remove this red rust, the adhesion energy of the boundary surface between the steel material surface to which the contaminant is adhered and the contaminant itself is the van der Waals force, and its magnitude is 1 cm 2 Since the energy density is 2 to 4 kJ / mol per mole, it is estimated that the atoms are fixed at the following energy density: A Adhesion energy (J / cm 2 ) Calculation example 1. Red rust (Fe2O3) (1) Calculate the molecular weight per mole of attached red rust: 160g / mol (2) Thickness of attached red rust: 500 μm (3) Adhesion surface area: 1 cm 2 Weight of rust per unit: 5.24g / cm 2 (Density of Fe2O3) x 1 / 20 = 0.262g (4) 1 cm 2 Number of moles of red rust per mole: 0.262g ÷ 160g / mol = 0.0016375mol (5) Van der Waals bond strength of 0.0016375 mol of red rust: 3.275 J / cm 2 ~6.55J / cm 2

[0029] From the above, it takes 1 cm to remove red rust with a thickness of 500 μm. 2 7J / cm 2 It is theoretically possible to dissociate pollutants by irradiating them with pulsed laser light with a certain energy density. However, the energy density of the nano-pulse laser is 7 J / cm 2 The energy density converted into ablation effect is 7J / cm 2 The ablation effect is reduced by a certain percentage of the total surface area of ​​the red rust, and a certain amount of energy is absorbed by the free electrons in the base metal. The boundary surface between the surface where the red rust actually occurs and the steel material is not flat, but has irregularities caused by corrosion, increasing the area of ​​the boundary surface. The distance from the laser light emitting lens to the boundary surface changes slightly due to the irregularities from the boundary surface. In addition, the thickness of the actual red rust layer is not constant. Therefore, the following points must be kept in mind when removing red rust from steel materials by irradiating them with pulsed laser light. (1) The pulsed light is 1cm 2 7J / cm 2 It is preferable to use a laser with an energy density equivalent to 1000μm. With this energy density, red rust with a theoretical thickness of 500μm cannot be removed with a single irradiation, but with several round-trip irradiations, the contaminants can be separated without evaporating or vaporizing them, and without damaging the base material, by the ablation effect of the nano-pulse laser alone. When irradiating back and forth in this way, care must be taken not to overheat the contaminants and the base metal due to the ablation effect. When the amount of attached contaminants is sufficiently small, it is possible to separate the contaminants by a single irradiation with an energy density close to the theoretical value. In other words, the ablation effect of nano-pulse lasers utilizes the repetition of the generation and extinction of plasma generated by the pulse oscillation of nano-pulse laser light at microsecond intervals. The number of times this repetition occurs can be controlled by the repetition frequency of the pulse light. The ablation effect that occurs as a result can destroy the van der Waals forces and hydrogen bond energy that hold contaminants together. However, if the contaminant is deposited in a thick layer, it may be as low as 7 J / cm 2 As mentioned above, even if pulse energy with an energy density of 1000 nm is applied, not all of the energy is converted into mechanical energy, which is called the ablation effect, and some of the energy is consumed as thermal energy. This cannot be specified because it depends on the characteristics of the contaminant, whether it is a metal oxide or an organic substance, and the electrical resistance of the base metal. Therefore, in many cases, it is difficult to completely remove the contaminant by irradiating a nano-pulse laser with an energy density calculated by theoretical calculation. Therefore, 7 J / cm2 is used to prevent damage to the substrate surface. 2 By using a nano-pulse laser with an energy density of 1000 nm to gradually remove contaminants from the surface, it is possible to remove the contaminants without evaporating or vaporizing them and without damaging the substrate. (2) To meet this requirement, the threshold of pulsed light irradiation intensity is 7 J / cm 2 The flat-top beam in Fig. 4A is more effective than the Gaussian beam in Fig. 5 in irradiating the target with less gaps if the oscillation frequency and scanning speed of the pulsed light are the same. (3) It is preferable that the irradiation pitch is appropriate under the condition (1) above. The appropriate irradiation pitch is determined by the combination of the scanning speed and repetition frequency of the laser light. In the irradiation condition setting on the left side of Figure 4, the scanning speed and repetition frequency settings are well-balanced, so there are no gaps on the irradiated surface, making it possible to remove the contaminants reliably. However, if the scanning speed is too fast compared to the repetition frequency conditions, as in the right side of Figure 4, gaps will appear on the irradiated surface, and these will remain unremoved. Also, the unit area (cm2) of the contaminant 2If the number of adhering moles per particle is not constant but increases or decreases, the irradiation pitch can be changed in combination with the scanning speed and repetition frequency of the laser light to change the overlap rate of the pulsed light irradiation, thereby making it possible to remove the contaminants without leaving any residue.

[0030] Figure 6A is a schematic energy distribution diagram of a laser beam with a flat-top beam shape, and Figure 6B is a schematic energy distribution diagram of a laser beam with a Gaussian beam shape. Compared to the Gaussian beam shown in Figure 6A, the optimum energy density of the flat-top beam shown in Figure 6B can be set in a wider optimum energy density zone in both the focal length direction and focal diameter. Within this optimum zone, the energy required for separation can be appropriately applied. Also, within the optimum zone, energy is applied that is greater than the energy required to break the van der Waals force and hydrogen bonds that are the adhesive force of the contaminants, but energy that breaks the metal bond energy of the base metal is not applied, so the steel part will not be destroyed. From the above, in order to reliably remove contaminants such as radioactive contaminants from the surface of a metal structure, etc., without damaging the substrate, it is possible to achieve this by irradiating the interface between the contaminant and the substrate with a flat-top beam-shaped nano-pulse laser light at an appropriate repetition frequency, appropriate scanning speed, and appropriate focal diameter, thereby providing an energy density that resists van der Waals forces and hydrogen bonds.

[0031] Figure 7 shows the contamination status of the primary system of a boiling water reactor (BWR) and a pressurized water reactor (PWR) previously announced by the International Atomic Energy Agency (IAEA). On top of a base metal 21 such as stainless steel, a lower layer 22 made of a base alloy such as Fe, Cr, Ni, etc., an intermediate layer 23 in which contaminants from the chromium-rich layer such as FeCr2O4 and Cr2O3 have accumulated, and an outer layer 24 in which contaminants from the ion-rich layer such as Fe2O3 have accumulated are layered. As can be seen from Figure 7, the base metal 21 is stainless steel that contains a large amount of Cr, Ni, etc. to prevent corrosion, and its radioactively contaminated layer is roughly divided into three layers. The outer layer 24 is a layer mainly composed of Fe oxides, the intermediate layer 23 is a layer of contaminants (Cs, Sr oxides, etc.) mainly composed of Cr oxides, and the lower layer 22 is a SUS alloy layer of the base metal. The thickness of the contamination layer, excluding the activated portion of the base metal in the lower layer, is a maximum of 15 μm, consisting of base metal oxides, radioactive nuclide oxides, and in some cases chlorides of each, deposited on the surface of the base metal.

[0032] 8 to 11 are cross-sectional views of an example showing the contamination state of a radioactively contaminated metal structure on the premises of the Fukushima Daiichi Nuclear Power Plant. In Fig. 8, a metal oxide layer 26 (radioactively contaminated layer) containing Cr and the like and a layer 27 (radioactively contaminated layer) containing Fe oxide and chloride and having a thickness of several µm are deposited on a base metal 25 that is lightly radioactively contaminated. In Figure 9, on top of a base metal 28 with only slight radioactive contamination, there is a protective coating layer 29 made of acrylic resin, urethane resin, etc. with a thickness of several tens of microns, a radioactively contaminated film 30 on top of that which is several tens of microns thick and contains a mixture of deteriorated coating particles, sludge, etc., and a resin deteriorated part 31 existing in the thickness direction of the protective coating layer 29. The radioactively contaminated film 30 and the resin deteriorated part 31 contain Fe oxides, Fe chlorides, sea salt, Ce oxides, Sr oxides, Ce chlorides, Sr chlorides, etc. In FIG. 10, on top of a base steel material 32 with only slight radioactive contamination is a layer 33 several tens to several hundreds of μm thick containing a mixture of Ce oxides, Sr oxides, Ce chlorides, Sr chlorides, sludge, etc., and on top of that is a contaminated layer 34 containing Fe oxides, Fe chlorides, sea salt, etc. In Fig. 11, an epoxy resin layer 36 having a thickness of several hundred µm to several mm is placed on a base steel material 35 with only slight radioactive contamination, and a Ce, Sr chloride fine particle layer 37 having a thickness of several µm is placed on the epoxy resin layer 36. The epoxy resin layer 36 contains tritium in the surface layer, is water absorbent, and is radioactively contaminated.

[0033] As shown in Figures 8 to 11, the contamination status of metal structures within the Fukushima Daiichi Nuclear Power Plant varies depending on the base metal, the treatment of the metal surface, and the differences between structures before and after the accident. In addition to these differences in contamination status, there are also differences in the shape, dimensions, installation, and storage conditions of the metal structures. Figure 8 shows items that have already been dismantled and removed and stored in casks, separated by level of contamination. Figure 9 shows large structures with a low level of radioactive contamination that are installed or stored outdoors. Figure 10 shows items such as tools and equipment used in dismantling work that are stored separately. Figure 11 shows items that have not yet been dismantled or removed and are still installed in high radiation areas. The method for removing pollutants of the present invention can be applied to any form of contamination.

[0034] FIG. 12 is a schematic explanatory diagram of an ultrasonic cleaning device 50 according to an embodiment of the present invention. In this ultrasonic cleaning device 50, ultrasonic oscillators 52a-52d are directly connected to a cleaning tank 51, and ultrasonic waves having a frequency of 20 to 40 kHz are generated. A contaminated object 53 is immersed inside the cleaning tank 51. A cleaning liquid circulation circuit 59 is provided outside the cleaning tank 51, and this circuit is equipped with a circulation pump 54, a cyclone 55, and a fine bubble generator 56, and circulates a cleaning liquid 57. As an example, the cyclone 55 removes particles 58 having an average diameter of 10 μm or more, and takes them out from the bottom of the cyclone. The fine bubble generator 56 generates a mixed fluid of an organic acid cleaning liquid, air bubbles containing fine bubbles, and water, and returns it to the cleaning tank 51. This cleaning tank 51 can also be used as an ultrasonic cleaning tank in an alkaline neutralizing solution. The material of the cleaning tank 51 may be SUS, but it is preferable to paint it with a protective paint. EXAMPLES

[0035] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. Example 1 (1) Contaminated objects After salt spraying on a steel plate, the red rust (Fe2O3) that had developed through natural exposure was cleaned using the above-mentioned contaminant removal device. The reasons for verifying the removal of red rust that had developed through salt spraying were, firstly, to resolve the question of whether red rust that had developed on steel plates containing salt water can be removed by laser. Secondly, to resolve the question of whether NaCl (salt), an impurity other than red rust, can be removed. Since Na belongs to the same alkali metal group as Cs, a radioactive nuclide, it is presumed that if Na can be removed, Cs can also be removed. (2) Protective coating on the inside of the ultrasonic cleaning tank A two-component curing acrylic urethane paint with 7 nm-sized silica uniformly dispersed was used as a protective coating for the inner surface of the ultrasonic cleaning tank, and the coating was applied so that the dry film thickness was 20 μm. This formed a coating film with high acid resistance, alkali resistance, heat resistance, and adhesion resistance, preventing damage to the stainless steel (SUS) that constitutes the cleaning tank. The performance of this coating film is shown in Table 1.

[0036] [Table 1]

[0037] (2) Ultrasonic cleaning process in organic acid cleaning solution The ultrasonic cleaning device 50 shown in FIG. 12 (however, not equipped with a fine bubble generator 56) was used, and a 5% by mass aqueous solution of ascorbic acid was prepared. A contaminated object having rust and radioactive contaminants on the surface of a steel plate was placed in this aqueous solution, and the ultrasonic frequency was set to 20 kHz to 40 kHz. The treatment temperature was room temperature (40°C). A cleaning solution stirring and circulating pump was installed to make the cleaning conditions in the cleaning tank uniform. In addition, a cyclone-type fine particle removal device was installed to purify the cleaning solution in the cleaning tank, and coarse particles with a particle size of 10 μm or more that were removed from the metal surface by ultrasonic cleaning were removed, and damage to the cleaning tank due to contact resonance with the oscillator plate vibrating by ultrasonic waves was prevented. As a result, the contaminants became about a thin film (20 μm or less).

[0038] (3) Ultrasonic cleaning process in alkaline neutralizing solution The contaminated object was placed in an aqueous solution containing 10% by mass of sodium bicarbonate, and was subjected to ultrasonic cleaning and neutralization. The treatment temperature was room temperature (25°C). The wastewater from the organic acid cleaning treatment and the wastewater from the alkaline neutralization treatment were mixed and neutralized, and the neutralized water was heated and distilled at 100°C or less to concentrate the contaminants, and then disposed of in a landfill.

[0039] (4) Drying process The drying was carried out by air drying.

[0040] (5) Pollutant removal equipment The apparatus shown in Figures 1 and 2 was used. The nano-pulse laser oscillator 1 was a Nd:YAG fiber laser manufactured by Narran, product name "ROD 1000W". Average output: 500 W, optical wavelength: 1064-1070 nm, frequency: 10 kHz, pulse width: 20 ns, size: 400%, focus: 172, speed: 6000 mm / s, energy density per unit area: 1.5 J / cm 2 , focal diameter: 1mm, transmission fiber length: 15m, irradiator weight: 2.5kg, ambient temperature: 5~45℃, weight: 245kg, power consumption: 5kW. The Flat Beam Shaver 15 was also made by Narran. The waveform of the square flat-top nano-pulse laser light irradiated onto the contaminated surface was as shown on the right side of Figure 3.

[0041] (6) Pollutant removal results Figure 13 is a schematic side view of a contaminated object (pipe) 60 before and after cleaning. Contaminants have accumulated in the untreated part 62 in the center, but the part 61 on the left that was ultrasonically treated in organic acid has had most of the contaminants removed. The part 63 on the right that was treated with a nano-pulse laser still had some contaminants, but they had been completely removed.

[0042] Example 2 The same procedure as in Example 1 was carried out, except that an ultrasonic cleaning device 50 incorporating a fine bubble generator 56 was used. The fine bubble generator used was an "OK Nozzle" manufactured by OK Engineering Co., Ltd. This fine bubble generator is a device that generates microbubbles of several tens to several hundred μm and ultrafine bubbles (UFB) of several tens to several hundred nm. Figure 14 is a schematic side view of a contaminated object (pipe) 60 before and after cleaning. The untreated portion 62 and the portion 61 that was ultrasonically treated in organic acid are essentially the same as those in Figure 13, but the portion 64 on the right that was treated with the nano-pulse laser had not only the contaminants but also the black rust completely removed, revealing the bare metal. In addition, because there was no oil or grease, there was no smoke generated during pulse cleaning, and the dust collection filter was not clogged. From the above, it was confirmed that the contaminant removal device of this embodiment can remove contaminants without damaging the substrate and without volatilizing the contaminants. [Industrial Applicability]

[0043] The contaminant removal device and the contaminant removal method of the present invention are useful for removing harmful substances including radioactive contaminants such as cesium oxide, strontium oxide, etc., and / or chemical contaminants such as polychlorinated biphenyls (PCBs), dioxins, etc. Furthermore, they are also useful in nuclear power-related facilities, waste incineration facilities, etc. [Explanation of symbols]

[0044] 1. Nano-pulse laser oscillator 2 Pulse generator 3 Seed LD3 4,9 Isolator 5,12 Coupler 6,11 Excitation LD 7 Fiber optic cable 8. Preamplifier 10 Bandpass Filters 13 Main amplifier 14 Collimator 15 Flat beam shaver 16a,16b Optical system focus adjuster 17 Scan Head 18 Laser Profiler 19 Irradiation target (contaminated surface) 20 Nano-pulse laser irradiation device 21 Base metal 22 Lower layer 23 Middle Class 24 Outer layer 25,28 Base metal 26,27 Radioactive Contamination Layer 29 Protective Coating Layer 30 Radioactive Contaminated Membrane 31 Resin deterioration part 32,35 Base steel 33 Mixed layer of oxides, chlorides, sludge, etc. 34 Contamination layers such as oxides, chlorides, sea salt, etc. 36 Epoxy resin layer 37Ce, Sr chloride fine particle layer 40 Gaussian Shaped Beam 41 Excess energy in Gaussian-shaped beams 42 Irradiation diameter of Gaussian beam 43 Thermal Energy of Gaussian Shaped Beam 44 Square flat top beam 45 Square flat top beam diameter 46 Thermal energy of a rectangular flat-top beam 47 Ablation Threshold 50 Ultrasonic cleaning equipment 51 Cleaning tank 52a-52d Ultrasonic oscillator 53 Contaminants 54 Circulation Pump 55 Cyclone 56 Fine bubble generator 57 Cleaning Fluid 58 particles 59 Circulation circuit 60 Contaminated object (pipe) 61 Part sonicated in organic acid 62 Untreated area 63,64 Nanopulse laser treated area

Claims

1. 1. A method for removing contaminants from a contaminated object, comprising: In a cleaning tank, the contaminated object is ultrasonically cleaned in an organic acid cleaning solution, which is at least one weakly acidic aqueous solution selected from the group consisting of citric acid, ascorbic acid, glycolic acid, malonic acid, acetic acid, and oxalic acid, thereby capturing metal ions by a chelating effect without corroding the metal substrate, and detaching the thick-film deposits by vibrating the weakly acidic organic acid that has entered the gaps in the thick-film deposits using the water-absorbing properties of the deposited deposits through the propagation of ultrasonic vibrations, Then, ultrasonically clean the product in an alkaline neutralizing solution, and then dry or drain the product. Next, the contaminated surface is irradiated with nano-pulse laser light having a square flat-top shape from a pulse oscillator, thereby peeling off the contaminants from the contaminated surface.

2. 2. The method for removing contaminants according to claim 1, wherein the frequency of ultrasonic cleaning in the organic acid cleaning solution and the frequency of ultrasonic cleaning in the neutralization solution are 20 to 40 kHz.

3. 2. The method for removing pollutants according to claim 1, wherein the organic acid ultrasonic cleaning solution can be circulated and replenished with a mixed fluid of air bubbles containing fine bubbles and water from an external ultra-fine bubble generator.

4. The square flat-top nano-pulse laser light irradiated to the contaminated surface has an energy density per unit area of ​​1 to 10 J / cm 2 2. The method for removing pollutants according to claim 1 ,

5. 2. The method for removing contaminants according to claim 1, wherein a focal diameter of the nano-pulse laser light having a square flat-top shape that is irradiated onto the contaminated surface is adjustable within a range of 1 mm or more and 1.8 mm or less.

6. 2. The method for removing pollutants according to claim 1, wherein the pulse oscillator includes a pulse oscillator of a Nd:YAG laser that oscillates light of a single wavelength controlled to be not less than 1064 nm and not more than 1070 nm.

7. 2. The method for removing pollutants according to claim 1, wherein the pulse width of the pulse oscillator is 20 ns or more and 500 ns or less.

8. 2. The method for removing pollutants according to claim 1, wherein the frequency of the pulse oscillator is between 5 kHz and 50 kHz.

9. 2. The method for removing pollutants according to claim 1, wherein the nano-pulse laser light oscillated by the pulse oscillator has a minimum average output of 500 W or more and a maximum output of 2.5 Mw or more.

10. 2. The method for removing pollutants according to claim 1, further comprising a means for transmitting the nano-pulse laser light oscillated by the pulse oscillator to an irradiation device via a fiber cable having a length of 10 m or more and 150 m or less.

11. A contaminant removal apparatus for use in the contaminant removal method according to any one of claims 1 to 10, a cleaning tank equipped with an ultrasonic oscillator and storing an organic acid cleaning solution; a cleaning liquid circulation circuit provided outside the cleaning tank; The circulation circuit is provided with a circulation pump, a cyclone, and a fine bubble generator, A pollutant removal device characterized in that the fine bubble generator is provided with a means for generating a mixed fluid of organic acid cleaning solution, air bubbles containing fine bubbles, and water, and returning the mixed fluid to the cleaning tank.

12. A contaminant removal apparatus for use in the contaminant removal method according to any one of claims 1 to 10, A surface contaminant removal device comprising: a means for converting a Gaussian-shaped beam of nano-pulse laser light emitted from a pulse oscillator into a square-shaped flat-top shaped nano-pulse laser light using a flat beam shaver; and a means for irradiating the square-shaped flat-top shaped nano-pulse laser light onto a contaminated surface.

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