Method for removing contaminants and apparatus for removing contaminants used in this method

Abstract

The present invention provides a method for removing contaminants without damaging the substrate and without causing the contaminants to volatilize. [Solution] This method involves irradiating a contaminated surface with nanopulse laser light emitted from a pulse oscillator to remove contaminants. The contaminated object to which the contaminants are attached is ultrasonically cleaned in a weakly acidic organic acid cleaning solution, ultrasonically cleaned in an alkaline neutralizing solution, dried, and then irradiated with a square-shaped, flat-top nanopulse laser light 44 to peel the contaminants off the contaminated surface. The weakly acidic organic acid cleaning solution is preferably an aqueous solution of at least one selected from the group consisting of citric acid, ascorbic acid, glycolic acid, malonic acid, acetic acid, and oxalic acid.

Description

[Technical Field]

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

[0002] Radioactive contaminants such as cesium oxides and strontium oxides, as well as chemical contaminants such as polychlorinated biphenyls (PCBs) and dioxins, are generally harmful to the human body and living organisms on Earth, and therefore need to be removed. Patent Document 1 proposes irradiating radioactive contaminants present on the surface of a metal substrate with laser light to evaporate or volatilize the radioactive contaminants along with the surface layer of the metal substrate and remove them. Patent Document 2 proposes irradiating with laser light at a scanning speed sufficiently faster than the heat generated by the energy transfer of a pulsed laser to evaporate or remove the material from the irradiated area. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-016530 [Patent Document 2] Japanese Patent Publication No. 2007-315995 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, the aforementioned conventional technology had the problem that, because the radioactive contaminants evaporated or volatilized due to the ultra-high temperature caused by the laser light, they could not be captured even by current high-performance filters such as HEPA and ULPA, and the contaminants were merely dispersed into the air without being essentially removed, and that the substrate was damaged.

[0005] To solve the aforementioned conventional problems, the present invention provides a method for removing contaminants that does not damage the substrate and does not cause the contaminants to volatilize, and a contaminant removal apparatus used in this method. [Means for solving the problem]

[0006] One embodiment of the present invention relates to a method for removing contaminants by irradiating a contaminated surface with nanopulse laser light emitted from a pulse oscillator, wherein the contaminated object to which the contaminants are attached is ultrasonically cleaned in a weakly acidic organic acid cleaning solution, ultrasonically cleaned in an alkaline neutralizing solution, dried or drained, and then irradiated with a square flat-top shaped nanopulse laser light to peel the contaminants off the contaminated surface.

[0007] Another embodiment of the present invention is a contaminant removal device used in the above-mentioned contaminant removal method, A cleaning tank equipped with an ultrasonic oscillator and for storing organic acid cleaning solution, A cleaning fluid circulation circuit is provided on the outside of the aforementioned cleaning tank. The aforementioned circulation circuit is equipped with a circulation pump, a cyclone, and a fine bubble generator. The present invention relates to a contaminant removal device comprising a fine bubble generator that generates a mixed fluid of organic acid cleaning solution, bubbles containing fine bubbles, and water, and returns it to the cleaning tank.

[0008] Yet another embodiment of the present invention is a contaminant removal device used in the above-mentioned contaminant removal method, The present invention relates to a surface contaminant removal apparatus that includes means for converting nanopulse laser light in a Gaussian shape beam emitted from a pulse oscillator into a rectangular flat-top nanopulse laser light using a flat beam shaver, and means for irradiating a contaminated surface with the rectangular flat-top nanopulse laser light. [Effects of the Invention]

[0009] The present invention provides a method for removing contaminants and a contaminant removal device used in this method, which can efficiently remove contaminants adhering to a thick film without damaging the base material and without volatilizing the contaminants. The method involves ultrasonic cleaning a thick film contaminated object with contaminants attached therein in a weakly acidic organic acid cleaning solution, then ultrasonic cleaning in an alkaline neutralizing solution, drying or draining, and then irradiating the contaminated surface with a nano-pulse laser light having a square flat top shape to peel off the contaminants from the contaminated surface, thereby maximizing the ablation effect by the nano-pulse light and minimizing the thermal influence.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a schematic explanatory diagram of a nano-pulse laser oscillator according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic explanatory diagram of a nano-pulse laser irradiation device according to an embodiment of the present invention. [Figure 3] FIG. 3 is a schematic waveform diagram (left side) of a nano-pulse laser light with a conventional Gaussian-shaped beam and a schematic waveform diagram (right side) of a nano-pulse laser light with a square flat top-shaped beam according to an embodiment of the present invention. [Figure 4] FIG. 4A is a schematic top view showing the irradiation situation by a square flat top beam shape according to an embodiment of the present invention, and FIG. 4B is a schematic side cross-sectional view thereof. [Figure 5] FIG. 5A is a schematic top view showing the irradiation situation by a conventional Gaussian beam shape, and FIG. 5B is a schematic side cross-sectional view thereof. [Figure 6] FIG. 6A is a schematic energy distribution diagram of laser light with a flat top beam shape, and FIG. 6B is a schematic energy distribution diagram of laser light with a Gaussian beam shape. [Figure 7] FIG. 7 shows the contamination situation of the primary system of BWR and PWR type nuclear reactors announced by IAEA. [Figure 8] FIG. 8 is a cross-sectional view showing an example of the contamination situation of radioactive contaminated metal structures in the Fukushima Daiichi Nuclear Power Plant site. [Figure 9]Figure 9 is a cross-sectional view showing an example of the contamination status of radioactive metal structures within the Fukushima Daiichi Nuclear Power Plant premises. [Figure 10] Figure 10 is a cross-sectional view showing an example of the contamination status of radioactive metal structures within the Fukushima Daiichi Nuclear Power Plant site. [Figure 11] Figure 11 is a cross-sectional view showing an example of the contamination status of radioactive metal structures within the Fukushima Daiichi Nuclear Power Plant site. [Figure 12] Figure 12 is a schematic diagram illustrating an ultrasonic cleaning apparatus according to one embodiment of the present invention. [Figure 13] Figure 13 is a schematic side view of the contaminated object before and after cleaning in Embodiment 1 of the present invention. [Figure 14] Figure 14 is a schematic side view of the contaminated object before and after cleaning in Embodiment 2 of the present invention. [Modes for carrying out the invention]

[0011] A 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, thick-film contaminated objects with adhering contaminants are 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, or oxalic acid. The organic acid is weakly acidic and does not corrode the metal substrate. It also has a chelating effect and a high metal ion capture effect. The processing temperature can be room temperature (30-60°C). The deposits to be removed by the weakly acidic organic acid cleaning solution readily absorb water. Taking advantage of this water absorption, the target metal product is immersed in the acidic cleaning solution, and ultrasonic energy is used to absorb water into the gaps between the deposited particles and 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 of the thick-film deposits by the propagation of ultrasonic vibrations, making it possible to separate the adhesion between the thick-film deposits. This allows for peeling and removal of thick-film deposits on metal products of nuclear facilities in a much shorter time than conventional methods that sequentially remove them from the surface only using a nanopulse laser. Furthermore, by using ultrasonic vibrations to stimulate the cleaning water, a water flow is created in the cleaning water on the contaminated metal surface, making it possible to immediately remove contaminants deposited in the cleaning water from the metal surface. As described above, rust can be removed from the surface of metal structures using a weakly acidic organic acid cleaning solution. (2) Ultrasonic cleaning process in an alkaline neutralizing solution In this process, ultrasonic cleaning is performed using an alkaline neutralizing solution such as an aqueous solution of sodium metasilicate or sodium bicarbonate, followed by neutralization. The treatment temperature can be room temperature. Preferably, the wastewater after the organic acid cleaning treatment and the wastewater after the alkaline neutralization treatment are mixed and neutralized, and this neutralized water is heated and distilled to concentrate the contaminants before being disposed of in a landfill. Preferred organic acids for this invention have boiling points of 175°C for citric acid, 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 these are higher than the boiling point of water (100°C), and using high-boiling-point organic acids facilitates the evaporation treatment. (3) Drying process Drying can be done by air drying or by blowing out the water with compressed air, but in colder weather, you can also blow hot air on it. (4) Nanopulse laser light irradiation process A square-shaped, flat-top nanopulse laser beam is irradiated onto the contaminated surface to remove the contaminants. This process will be explained in detail later. Importantly, when there are a large number of contaminated metal structures covering a large area, the efficiency of rust removal can be further improved by adding acid ultrasonic cleaning as a pretreatment step for nanopulse laser decontamination.

[0012] The ultrasonic cleaning frequency in the organic acid cleaning solution and the ultrasonic frequency in the neutralization solution are preferably 20 to 40 kHz. It is also preferable to install a cleaning solution stirring and circulation pump to ensure consistent cleaning conditions within the cleaning tank. Furthermore, it is preferable to install a cyclone-type particulate removal device to purify the cleaning solution in the cleaning tank, thereby removing coarse particles with a particle size of 10 μm or more removed from the metal surface by ultrasonic cleaning, and preventing damage to the cleaning tank due to contact resonance with the oscillator plate vibrating by the ultrasonic waves. After removing contaminants to a thin film thickness (20 μm or less) by ultrasonic cleaning in the organic acid cleaning solution, any remaining contaminants on the substrate metal surface are removed by nanopulse laser light irradiation.

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

[0014] Next, we will explain the nanopulse laser light irradiation process. The surface contaminant removal apparatus used in the present invention includes means for converting a Gaussian-shaped nanopulse laser beam emitted from a pulse oscillator into a rectangular flat-top nanopulse laser beam using a flat beam shaver, and means for irradiating the contaminated surface with the rectangular flat-top nanopulse laser beam. This allows for efficient removal without damaging the substrate or volatilizing the contaminants. Damaging the substrate could compromise safety in the case of nuclear-related equipment, and volatilizing contaminants into the atmosphere could cause damage to the surrounding area; both are important issues.

[0015] The nanopulse laser beam from the rectangular flat-top shaped laser that irradiates the contaminated surface has an energy density of 1 J / cm² per unit area. 2 Preferably, the J / cm² is greater than or equal to 1 to 10 J / cm². 2 And more preferably 5-10 J / cm² 2 The ideal J / cm² is 7-10 J / cm², and more preferably 7-10 J / cm². 2 Therefore, by irradiating the substrate with nanopulse laser light having an energy density higher than the van der Waals forces and hydrogen bonds (which are the adhesion energies at the interface between the steel surface to which the contaminants are attached) and the contaminants, and lower than the metallic and covalent bond energies of the substrate, the ablation effect of the pulsed laser light can be utilized to efficiently remove the contaminants without damaging the substrate or causing them to volatilize. To achieve the aforementioned energy density, it is preferable to adjust the average output, pulse width, repetition frequency, laser scanning speed, and the focal diameter of the square flat-top shape in combination.

[0016] Preferably, the focal diameter of the rectangular flat-top nanopulse laser beam irradiated onto the contaminated surface is adjustable between 1 mm and 1.8 mm. This not only allows for a smaller focal diameter and higher energy density, but also allows for adjustment of the depth of focus by combining the rectangular flat-top shape and focal diameter of the laser beam, thereby reducing the influence of fluctuations in the energy density of the laser beam due to irregularities in the substrate.

[0017] The contaminant removal device preferably includes a pulse oscillator of an Nd:YAG laser that emits single-wavelength light controlled to a wavelength between 1064 nm and 1070 nm. In nanopulse lasers with pulse widths in the nanosecond range, the generation of spectral dispersion hinders the uniform energy transfer to the irradiation surface. Therefore, it is preferable to employ a MOPA-type pulse oscillator of an Nd:YAG laser that emits extremely precise single-wavelength light controlled to a wavelength between 1064 nm and 1070 nm. The wavelength is more preferably between 1064 nm and 1068 nm, and even more preferably between 1064 nm and 1066 nm.

[0018] The pulse width of the pulse oscillator is preferably 20 ns or more and 500 ns or less. This allows for an increase in the energy density of the focusing surface when the focusing shape of the pulsed light is flat top, and the range of energy density per pulse can be adjusted by changing the pulse width. The frequency of the pulse oscillator is preferably between 5 kHz and 50 kHz. This allows the energy density of the irradiated surface to be adjusted by combining the scanning speed of the laser light (m / s) and the oscillation frequency of the flat-top pulse light (kHz).

[0019] Preferably, the nanopulse laser light oscillated by the pulse oscillator has a minimum average output of 500W or more and a maximum output of 2.5MW or more. This allows for an increase in the energy density (J) and energy intensity (W) at the top surface of the laser light when the pulse light is focused into a flat top shape, thereby enhancing the ablation effect of the nanopulse laser light at oscillation frequencies of 20kHz or higher.

[0020] Preferably, the system includes means for transmitting the nanopulse laser light emitted by the pulse oscillator to the irradiation device via a fiber cable with a length of 10 m to 150 m. This allows the oscillator to be placed in a safe location over a wide area such as a large structure, and the irradiation device alone can be mounted on a remotely operated robot or the like, making it mobile.

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

[0022] The contaminant removal device of the present invention includes, as an example, the following means. (1) A cleaning tank equipped with an ultrasonic oscillator and for storing organic acid cleaning solution. Multiple ultrasonic oscillators may be used. The size of the cleaning tank is arbitrary as long as it has enough volume to hold the contaminated object. (2) A cleaning fluid circulation circuit outside the cleaning tank. The aforementioned circulation circuit is equipped with a circulation pump, a cyclone, and a fine bubble generator. The circulation pump circulates the cleaning solution. The cyclone removes particles with an average diameter of 10 μm or more, for example, and extracts them from the bottom. (3) Means for generating a mixed fluid of organic acid cleaning solution, bubbles containing fine bubbles, and water in the fine bubble generator and returning it to the cleaning tank. The fine bubble generator preferably generates both microbubbles (average diameter 100 μm or less) and nanobubbles (fine bubbles with a diameter of 1 μm or less) among the fine bubbles (all with an average diameter of 100 μm or less), and it is even more preferable to generate ultrafine bubbles (UFB) of several tens to 100 nm in size among the nanobubbles. It is also acceptable to use only nanobubbles of 1 μm or less, but to supplement the bubbles consumed by ultrasonic cleaning, it is rational to supply ultrafine bubbles (UFB), which have a bubble size similar to microbubbles and nanobubbles that are consumed in large quantities during cleaning. When ultrafine bubbles (UFB) are supplied externally from an ultrafine bubble (UFB) generator and mixed with the organic acid cleaning solution in addition to ultrafine bubbles generated by ultrasonic oscillation, the continuity of retaining the impact force energy when bubbles collide and burst is increased, improving cleaning efficiency.

[0023] The contaminant removal apparatus of the present invention includes, as an example, means for converting Gaussian-shaped nanopulse laser light emitted from a pulse oscillator into rectangular flat-top nanopulse laser light using a flat beam shaver, and means for irradiating the contaminated surface with the rectangular flat-top nanopulse laser light.

[0024] Next, the following will be explained using the drawings. In the following, the same reference numerals in the drawings indicate the same object. Figure 1 is a schematic diagram of a nanopulse laser oscillator 1 according to one embodiment of the present invention. The nanopulse laser oscillator 1 generates pulses with a pulse generator 2. Preferably, an Nd:YAG fiber laser is used as the pulse generator. This is an infrared laser with a single wavelength of 1064 nm, and when the average output is 500 watts or more, the fiber cable can be extended up to 150 m in the case of 2 kW. Therefore, in large structures and other areas with a wide surface area, the oscillator can be placed in a safe location, and only the irradiation device can be mounted on a remotely operated robot or the like, making it movable. Preferably, the pulse oscillator 1 uses a MOPA type pulse oscillator that can oscillate pulsed light with a pulse width of nanoseconds at a stable maximum output and frequency. It is necessary to irradiate the interface between the surface contaminant and the metal surface with single-wavelength light with high precision and minimal spectral dispersion. Conventionally, CW lasers with a large wavelength tolerance of 1070 nm ± 50 nm have been used. However, in nanopulse lasers, where the pulse width is in the nanosecond range, the generation of spectroscopy hinders the uniform energy delivery to the irradiated surface. Therefore, it is preferable to employ a MOPA-type pulse oscillator of an Nd:YAG laser that emits extremely precise single-wavelength light controlled between 1064 nm and 1070 nm. The pulses generated by pulse generator 2 are converted into picosecond or sub-nanosecond pulsed laser light by seed LD3.

[0025] Next, the isolator 4 in the preamplifier 8 prevents signal interference, protects the equipment, and reduces the effects of noise, and the semiconductor laser excitation light from the excitation LD6 is input to the rare-earth doped fiber cable 7 via the coupler 5. Next, an isolator 9 in the main amplifier 13 prevents signal interference, protects equipment, and reduces the effects of noise. A bandbus filter 10 extracts physical phenomena in a specific frequency band, and semiconductor laser excitation light from the excitation LD 11 is input to the rare-earth doped fiber cable 7 via the coupler 12. The fiber cable 7 is connected to a nanopulse laser irradiation device 20.

[0026] Figure 2 is a schematic diagram of a nanopulse laser irradiation device 20 according to one embodiment of the present invention. First, the laser beam is made parallel by the collimator 14. Next, the flat beam shaver 15 converts the Gaussian beam, which is the beam shape of a typical pulsed laser, into a flat-top beam shape when it reaches the irradiation surface. Then, the flat-top nanopulse laser light is passed through the optical focus adjusters 16a, 16b, scan head 17, and laser profiler 18 and irradiated onto the target object (contaminated surface) 19.

[0027] Figure 3 shows schematic waveform diagrams of nanopulse laser light from a Gaussian-shaped beam 40 (left) and a rectangular flat-top shaped beam 44 (right) according to one embodiment of the present invention. By flattening the laser beam shape into a trapezoidal shape, as in the rectangular flat-top shaped beam 44, the thermal effects on the irradiation surface are reduced, and only the ablation effect, which is a vibrational abrasion effect, can be utilized, thereby reducing melting oxidation of the substrate surface. However, in this case, the excess energy portion of a typical pulsed laser beam is flattened to an irradiation intensity that reaches a line where the irradiation intensity reaches just above the ablation threshold 47, thus expanding the irradiation intensity area at the ablation threshold 47. However, as mentioned above, in order to achieve a decontamination rate that satisfies the cleaning rate of metal structures by removing contaminants attached to the surface of a metal substrate, the irradiation intensity line of the ablation threshold, which is several J / cm², is required to break the van der Waals forces and hydrogen bonds, which are the adhesion energy, within an appropriate range and at an appropriate speed. 2It is necessary to raise it to the line. For this purpose, a pulsed light of a Nd:YAG fiber laser beam (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 beam scanning speed of 1 to 10 m / s is stably output. An irradiation device having an optical system such that the pulsed light has a beam shape of a rectangular flat top shape is preferable. 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 rectangular flat top-shaped beam 44, and 45 is the thermal energy at 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 an irradiation situation by a rectangular flat top beam shape of an embodiment of the present invention, and FIG. 4B is a schematic side cross-sectional view thereof. FIG. 5A is a schematic top view showing an irradiation situation by a conventional Gaussian beam shape, and FIG. 5B is a schematic side cross-sectional view thereof. In FIGS. 4A-B and FIGS. 5A-B, the contaminant is red rust (Fe2O3) generated on the surface of the steel material, and it is assumed that the red rust layer is about 500 μm. To remove this red rust, the adhesion energy at the interface between the steel material surface to which the contaminant is fixed and the contaminant is the van der Waals force, and its magnitude is 1 cm 2 Since it is 2 to 4 kj / mol per hit, it is estimated that it is fixed at the following energy density. A Adhesion energy (J / cm 2 ) Calculation example 1 Red rust (Fe2O3) (1) Determine the molecular weight per mole of the adhering red rust: 160 g / mol (2) Thickness of the adhered red rust: 500 μm (3) Weight of red rust per 1 cm of the adhesion surface area 2 : 5.24 g / cm 2 (Density of Fe2O3) × 1 / 20 = 0.262 g (4) Number of moles of red rust adhered per 1 cm 2 : 0.262 g ÷ 160 g / mol = 0.0016375 mol (5) Van der Waals bond strength of 0.0016375 mol of red rust: 3.275 J / cm 2 ~6.55 J / cm 2

[0029] From the above, to remove red rust with a thickness of 500 μm, 1 cm 2 7 J / cm² 2 Theoretically, it can be interpreted that pollutants can be separated by irradiating them with pulsed laser light that has energy density. However, the energy density of a nanopulse laser is 7 J / cm². 2 Of this, the energy density converted into the ablation effect is 7 J / cm². 2 Within this, a certain proportion is present, and the ablation effect is reduced as a certain amount of energy is absorbed by the free electrons of the base metal. Furthermore, the actual interface between the rust-generating surface and the steel is not flat, but rather has irregularities due to corrosion, increasing the interface area. In addition, the distance from the laser beam emission lens to the interface surface changes subtly due to the irregularities of the interface surface. Moreover, the actual thickness of the rust layer is not constant. Therefore, the following points must be considered when removing rust from steel using pulsed laser beam irradiation. (1) The pulsed light to be irradiated is 1 cm 2 7 J / cm² 2 A laser with an energy density equivalent to the above is preferred. With this energy density, a theoretical thickness of 500 μm of red rust cannot be removed in a single irradiation, but by irradiating back and forth several times, the contaminants can be separated solely by the ablation effect of the nanopulse laser without vaporizing or volatilizing the contaminants, and without damaging the substrate. During this back and forth irradiation, care must be taken to avoid overheating of the contaminants and the substrate metal caused by the ablation effect. Furthermore, if the amount of contaminants attached is sufficiently small, it is possible to separate the contaminants with a single irradiation at an energy density close to the theoretical value. In other words, the ablation effect of nanopulse lasers utilizes the repeated generation and extinction of plasma, which is produced by the pulse oscillation of nanopulse laser light, at microsecond intervals. The number of repetitions can be controlled by the repetition frequency of the pulse light. The resulting ablation effect can break the van der Waals forces and hydrogen bond energy that are the adhesive forces of contaminants. However, if the contaminants are attached as a thick layer, the radiation level may be 7 J / cm². 2 Even when pulsed energy with a certain energy density is applied, as mentioned above, not all of the energy is converted into mechanical energy in the form of ablation; some of the energy is consumed as thermal energy. This cannot be determined because it varies depending on the characteristics of the contaminant, such as whether it is a metal oxide or an organic substance, and the electrical resistance of the base metal. Therefore, in many cases, complete removal is difficult by irradiating with a nanopulse laser at an energy density calculated by theoretical calculations. Therefore, 7 J / cm² is used to avoid damaging the substrate surface. 2 By using a nanopulse laser with a high energy density, contaminants can be gradually separated and removed from the surface without vaporizing or volatilizing them, and without damaging the substrate. (2) To satisfy this requirement, the pulsed light irradiation intensity threshold must be 7 J / cm 2 The advantage lies in the irradiation of a flat-top shaped beam. With the flat-top light in Figure 4A, irradiation can be performed without gaps, even if the pulsed light oscillation frequency and scanning speed are the same, compared to the Gaussian beam light in Figure 5. (3) It is preferable that the irradiation pitch is appropriate based on the conditions in (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 are well-balanced, resulting in no gaps on the irradiation surface and ensuring reliable removal of contaminants. However, as shown on the right side of Figure 4, if the scanning speed is too fast compared to the repetition frequency, gaps will form on the irradiation surface, resulting in remaining contaminants in those areas. Also, the unit area (cm²) of contaminants... 2If the number of fixed moles per unit area is not constant but fluctuates, the contaminants can be removed without leaving any residue by changing the combination of the laser scanning speed and repetition frequency to change the irradiation overlap rate of the pulsed light.

[0030] Figure 6A shows a schematic energy distribution diagram of a flat-top beam laser beam, and Figure 6B shows a schematic energy distribution diagram of a Gaussian beam laser beam. Compared to the Gaussian beam shown in Figure 6A, the flat-top beam shown in Figure 6B has a wider optimal energy density zone in both the focal length direction and the focal diameter. Within this optimal zone, the energy necessary for separation can be appropriately supplied. Furthermore, within this optimal zone, the energy supplied is greater than that that can break the van der Waals forces and hydrogen bonds that adhere to the contaminants, but not enough energy to break the metallic bonding energy of the base metal, so the steel portion will not be destroyed. Based on the above, it can be reliably removed from the surface of metal structures and other materials without damaging the substrate by irradiating the interface between the contaminant and the substrate with a flat-top beam-shaped nanopulse laser beam at an appropriate repetition frequency, scanning speed, and 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 systems of boiling water reactors (BWRs) and pressurized water reactors (PWRs), as previously published by the International Atomic Energy Agency (IAEA). On a base metal 21 such as stainless steel, there are layers: a lower layer 22 made of a base alloy of Fe, Cr, Ni, etc., an intermediate layer 23 with deposited chromium-rich contaminants such as FeCr2O4 and Cr2O3, and an outer layer 24 with deposited ion-rich contaminants such as Fe2O3. As can be seen from Figure 7, the base metal 21 is stainless steel containing a large amount of Cr, Ni, etc. for corrosion prevention, and its radioactive contamination layer can be broadly classified into three layers. The outer layer 24 is mainly composed of Fe oxides, the intermediate layer 23 is a deposit layer of contaminants (Cs, Sr oxides, etc.) mainly composed of Cr oxides, and the lower layer 22 is the SUS alloy layer of the base metal. Furthermore, excluding the activated base metal portion in the lower layer, the contamination layer consists of a maximum thickness of 15 μm of base metal oxides, radioactive nuclide oxides, and, in some cases, their respective chlorides, deposited on the base metal surface.

[0032] Figures 8 to 11 are cross-sectional views showing an example of the contamination status of radioactively contaminated metal structures within the Fukushima Daiichi Nuclear Power Plant site. Figure 8 shows a base metal 25 with minor radioactive contamination, on which a metal oxide layer 26 (radioactive contamination layer) containing Cr etc., several micrometers thick, and a layer 27 (radioactive contamination layer) containing Fe oxide and chloride, several micrometers thick, are deposited. Figure 9 shows a base metal 28 with minor radioactive contamination, a protective coating layer 29 made of acrylic resin, urethane resin, etc., several tens of micrometers thick, a radioactive contamination film 30 several tens of micrometers thick on top of the protective coating layer 29 containing deteriorated coating particles, sludge, etc., and a resin deterioration section 31 located in the thickness direction of the protective coating layer 29. The radioactive contamination film 30 and the resin deterioration section 31 contain Fe oxide, Fe chloride, sea salt, Ce oxide, Sr oxide, Ce chloride, Sr chloride, etc. Figure 10 shows a base steel material 32 with minor radioactive contamination, on top of which is a layer 33 containing a mixture of Ce oxide, Sr oxide, Ce chloride, Sr chloride, sludge, etc., with a thickness of several tens to several hundreds of micrometers, and on top of that is a contaminated layer 34 containing Fe oxide, Fe chloride, sea salt, etc. Figure 11 shows a base steel material 35 with minor radioactive contamination, on which is an epoxy resin layer 36 with a thickness of several hundred μm to several mm, and on top of that is a layer of Ce, Sr chloride particulate matter 37 with a thickness of several μm. The epoxy resin layer 36 contains tritium on its surface, is water-absorbing, and is radioactively contaminated.

[0033] As shown in Figures 8 to 11, the contamination levels of metal structures within the Fukushima Daiichi Nuclear Power Plant vary depending on the base metal, the treatment of the metal surface, and whether the structure existed before or after the accident. In addition to these differences in contamination levels, 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 and stored according to their level of contamination. Figure 9 shows large structures with low levels of radioactive contamination that are installed or stored outdoors. Figure 10 shows tools, equipment, etc., used in dismantling work, which are stored separately. Figure 11 shows items that have not yet been dismantled or removed and are installed in high-radiation areas. The pollutant removal method of the present invention can be applied to any form of contamination.

[0034] Figure 12 is a schematic diagram of an ultrasonic cleaning apparatus 50 according to one embodiment of the present invention. In this ultrasonic cleaning apparatus 50, ultrasonic oscillators 52a-52d are directly connected to a cleaning tank 51 and emit ultrasonic waves with a frequency of 20-40 kHz. Contaminated objects 53 are 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 to circulate the cleaning liquid 57. The cyclone 55 removes particles 58 with an average diameter of 10 μm or more, for example, and removes them from the bottom of the cyclone. The fine bubble generator 56 generates a mixed fluid of organic acid cleaning liquid, 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 coating. [Examples]

[0035] The present invention will be described below using examples. However, the present invention is not limited to these examples. (Example 1) (1) Contaminated object After spraying saltwater onto the steel plate, the surface was cleaned using the aforementioned contaminant removal device to remove the red rust (Fe2O3) that had formed due to natural exposure. The reasons for verifying the removal of red rust formed by saltwater spraying were, firstly, to answer the question of whether red rust formed on a steel plate containing saltwater can be removed by laser. Secondly, to answer the question of whether impurities other than red rust, such as NaCl (salt), can be removed. Since Na belongs to the same alkali metals as the radioactive nuclide Cs, it is presumed that if Na can be removed, Cs can also be removed. (2) Protective coating on the inner surface of the ultrasonic cleaning tank For the protective coating on the inner surface of the ultrasonic cleaning tank, a two-component curing acrylic urethane paint with uniformly dispersed 7nm-sized silica particles was used, and the coating was applied to a dry film thickness of 20μm. This resulted in a coating with high acid resistance, alkali resistance, heat resistance, and adhesion resistance, preventing damage to the stainless steel (SUS) forming the cleaning tank. The performance of this coating is shown in Table 1.

[0036] [Table 1]

[0037] (2) Ultrasonic cleaning process in organic acid cleaning solution Using the ultrasonic cleaning apparatus 50 shown in Figure 12 (however, without a fine bubble generator 56), a 5% by mass aqueous solution of ascorbic acid was prepared. A contaminated object containing rust and radioactive contaminants was placed in this aqueous solution, and the ultrasonic frequency was set to 20kHz to 40kHz. The processing temperature was room temperature (40°C). A cleaning solution stirring and circulation pump was installed to ensure uniform cleaning conditions within the cleaning tank. In addition, a cyclone-type particulate removal device was attached to purify the cleaning solution in the cleaning tank, removing coarse particles with a particle size of 10μm or more that were removed from the metal surface by ultrasonic cleaning, and preventing damage to the cleaning tank due to contact resonance with the vibrating oscillator plate, etc., caused by the ultrasound. As a result, the contaminants were reduced to a thin film (20μm or less).

[0038] (3) Ultrasonic cleaning process in an alkaline neutralizing solution A 10% by mass aqueous solution of sodium bicarbonate was added, and the contaminated object was placed in this solution, ultrasonically cleaned, and neutralized. The treatment temperature was room temperature (25°C). The wastewater after organic acid washing and the wastewater after alkaline neutralization were mixed and neutralized, and this neutralized water was heated and distilled at 100°C or below to concentrate the contaminants before being disposed of in a landfill.

[0039] (4)Drying process The drying method was air-drying.

[0040] (5) Contaminant removal device The apparatus shown in Figures 1 and 2 was used. Nanopulse laser oscillator 1 is a Narran product, product name "ROD 1000W", Nd:YAG fiber laser. Average power: 500W, optical wavelength: 1064-1070nm, frequency: 10kHz, pulse width: 20ns, size: 400%, focus: 172, speed: 6000mm / s, energy density per unit area: 1.5J / cm² 2 The specifications are: focal diameter: 1 mm, transmission fiber length: 15 m, irradiator weight: 2.5 kg, ambient temperature: 5~45°C, weight: 245 kg, power consumption: 5 kW. I also used a Narran flatbeam shaver 15. The waveform of the square-shaped, flat-top nanopulse laser beam irradiated onto the contaminated surface was as shown on the right side of Figure 3.

[0041] (6) Results of contaminant removal Figure 13 is a schematic side view showing photographs of the contaminated object (pipe) 60 before and after cleaning. While contaminants have accumulated in the untreated central section 62, the contaminants have been almost completely removed from the section 61 treated with ultrasonic waves in organic acid on the left. The section 63 treated with a nanopulse laser on the right shows some remaining contaminants, but they were largely removed.

[0042] (Example 2) The procedure was carried out in the same manner as in Example 1, except that an ultrasonic cleaning device 50 incorporating a fine bubble generator 56 was used. The fine bubble generator used was manufactured by OK Engineering Co., Ltd., and its product name was "OK Nozzle". This fine bubble generator is a device that generates microbubbles of several tens to 100 μm and ultrafine bubbles (UFB) of several tens to 100 nm. Figure 14 is a schematic side view showing photographs of the contaminated object (pipe) 60 before and after cleaning. The untreated portion 62 and the portion 61 treated with ultrasonic waves in organic acid are substantially the same as in Figure 13, but the portion 64 on the right treated with a nanopulse laser had all contaminants and even black rust completely removed, revealing the bare metal. Furthermore, because there was no oil or grease, there was no smoke during pulse cleaning, and the dust collection filter did not become 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 them. [Industrial applicability]

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

[0044] 1. Nanopulse laser oscillator 2. Pulse Generator 3rd Seed LD3 4.9 Isolator 5,12 Coupler 6,11 Excited LD 7 Fiber optic cables 8 Preamplifier 10-bandbus filter 13 Main amplifier 14 Collimator 15 Flat Beam Shaver 16a,16b Optical system focus adjuster 17 Scanhead 18 Laser Profilers 19. Object to be irradiated (contaminated surface) 20 Nanopulse 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 contamination membrane 31. Degraded resin section 32,35 Base steel 33. Mixed layer containing oxides, chlorides, sludge, etc. 34. Contamination layers containing oxides, chlorides, sea salt, etc. 36 Epoxy resin layer 37. Chloride particulate layer of Ce,Sr 40 Gaussian-shaped beam 41. Excess energy of Gaussian-shaped beams 42. Irradiation diameter of a Gaussian-shaped beam 43 Thermal energy of a Gaussian-shaped beam 44 square flat-top beams 45-square flat-top beam irradiation diameter Thermal energy of a 46-sided flat-top beam 47 Ablation threshold 50 Ultrasonic cleaning equipment 51 Washing Tank 52a-52d Ultrasonic Oscillator 53 Contaminated objects 54 Circulation pump 55 Cyclone 56 Fine bubble generator 57 Cleaning solution 58 particles 59 Circulation circuit 60. Contaminated object (pipe) 61. Parts treated with ultrasound in organic acid 62 Untreated portion 63,64 Nanopulse laser treated areas

Claims

1. A method for removing contaminants from contaminated objects, In the cleaning tank, the contaminated object is ultrasonically cleaned in a weakly acidic organic acid cleaning solution, ultrasonically cleaned in an alkaline neutralizing solution, and then dried or drained. A method for removing contaminants, characterized by irradiating the contaminated surface with nanopulse laser light from a rectangular flat-top shape to peel the contaminant off the contaminated surface.

2. The method for removing contaminants according to claim 1, wherein the weakly acidic organic acid cleaning solution is an aqueous solution of at least one selected from the group consisting of citric acid, ascorbic acid, glycolic acid, malonic acid, acetic acid, and oxalic acid.

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

4. The method for removing contaminants according to claim 1, wherein the organic acid ultrasonic cleaning solution is a mixed fluid of bubbles containing fine bubbles and water that can be circulated and replenished from an external ultrafine bubble generating device.

5. The nanopulse laser beam from the rectangular flat-top shaped laser that irradiates the contaminated surface has an energy density of 1 to 10 J / cm² per unit area. 2 The method for removing pollutants according to claim 1.

6. The method for removing contaminants according to claim 1, wherein the focal diameter of the rectangular flat-top shaped nanopulse laser beam irradiated onto the contaminated surface can be adjusted to be between 1 mm and 1.8 mm.

7. The method for removing contaminants according to claim 1, wherein the contaminant removal device includes a pulse oscillator of an Nd:YAG laser that emits light of a single wavelength controlled to have a wavelength of 1064 nm or more and 1070 nm or less.

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

9. The method for removing contaminants according to claim 1, wherein the frequency of the pulse oscillator is 5 kHz or more and 50 kHz or less.

10. The method for removing contaminants according to claim 1, wherein the nanopulse laser light emitted by the pulse oscillator has a minimum average output of 500 watts or more and a maximum output of 2.5 MW or more.

11. The method for removing contaminants according to claim 1, comprising means for transmitting nanopulse laser light oscillated by the pulse oscillator to an irradiation device via a fiber cable with a length of 10 m to 150 m.

12. A contaminant removal device used in the contaminant removal method according to any one of claims 1 to 11, A cleaning tank equipped with an ultrasonic oscillator and for storing organic acid cleaning solution, A cleaning fluid circulation circuit is provided on the outside of the aforementioned cleaning tank. The aforementioned circulation circuit is equipped with a circulation pump, a cyclone, and a fine bubble generator. A contaminant removal device characterized by comprising a means for generating a mixed fluid of organic acid cleaning solution, bubbles containing fine bubbles, and water in the fine bubble generator, and returning it to the cleaning tank.

13. A contaminant removal device used in the contaminant removal method according to any one of claims 1 to 11, A surface contaminant removal apparatus characterized by comprising means for converting nanopulse laser light in a Gaussian shape beam emitted from a pulse oscillator into a rectangular flat-top nanopulse laser light using a flat beam shaver, and means for irradiating a contaminated surface with the rectangular flat-top nanopulse laser light.

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