Introduction
In many industrial activities, harmful submicron- and micron-sized aerosol particles are generally generated using effluent gases [1]. During various chemical and nuclear engineering processes, the release of these aerosols into the environment presents notable ecological and public health challenges, as they can penetrate deeply into the human pulmonary system and lead to the development of various diseases by inhalation [2]. Typically, aerosol production during the decommissioning and dismantling of nuclear reactors raises critical concerns owing to its potential risks to workers and environmental radiation exposure [3-5]. The management and removal of aerosol particles is vital because of their implications for occupational safety and environmental integrity [6, 7]. Moreover, the generation of radioactive aerosols should be further considered in the context of dismantling nuclear reactors after severe accidents. For example, the decommissioning of Fukushima Daiichi nuclear reactors, which is a global challenge for the future of nuclear energy and environmental protection, necessitates the use of advanced remote techniques, such as high-power laser irradiation, for containment cleaning and fuel debris retrieval under complex operational conditions [8, 9].
Compared with mechanical approaches, high-power fiber lasers demonstrate exceptional capabilities in achieving minimal material removal, generating a reduced mass of re-suspended aerosols, effectively cutting high-hardness materials (such as fuel debris), and offering high-precision, non-contact operation and suitability for accessing locations requiring extended working distances. [9-11]. However, laser irradiation is expected to generate smaller-sized radioactive aerosol particles, which are generally submicron in size [3, 11-15]. The dispersion and deposition of these radioactive aerosols within the primary containment vessel and surrounding structures could pose a significant radiological hazard to the public and environment [13-17]. Thus, it is crucial to manage and eliminate these radioactive aerosols within the reactor primary containment vessel to prevent the release of radioactivity into the environment, reduce harmful impacts on humans and wildlife from increased radiation exposure via inhalation, and diminish radioactive contamination of the soil.
To facilitate the safe dismantling of reactors and mitigate the environmental release of radioactive materials, containment water spray systems are utilized for aerosol removal with several notable benefits, including the cooling of high-temperature fuel debris with decay heat and reduction of fire hazards [18, 19]. As illustrated in Fig. 1(a), sprayed water droplets effectively capture aerosol particles through various mechanisms, including mechanical (e.g., Brownian diffusion, inertial impaction, and interception), phoretic (e.g., diffusio- and thermo-phoresis), and electrostatic processes. Experimental frameworks have been established to investigate aerosol dynamics such as deposition and spray removal under diverse conditions in nuclear facilities. Examples include deposition studies conducted at the National Aerosol Test Facility (NATF) facility [20], Filtered Containment Venting Systems (FCVS) [21], and our long-pipe system [22]. Additional spray removal experiments have been performed in different facilities, such as TOSQAN (TOnus Qualification ANalytique) [18, 23], SCRUPOS (SCRUbbing by Pool and Spray) [24], COSTTHES (Containment Source Term and Thermal Hydraulics Experiment System) [25], THAI (thermal-hydraulics, hydrogen, aerosols, and iodine) [19, 26], and our UTARTS (University of Tokyo, Aerosol Removal Tests using Sprays) facilities [27-33].
However, as confirmed by previous spray removal studies [13, 14, 18, 19, 24-36], it was noted that aerosol particles with greenfield-gap sizes (i.e., diameter within 0.1~1 μm) are difficult to remove by spray droplets [4, 37, 38]. Unfortunately, a larger fraction of aerosol particles is expected to be generated when applying laser techniques for nuclear reactor decommissioning [3, 13, 14, 30, 32, 39]. Enhancing the aerosol scavenging efficiency through electrical charging of the spray has shown significant potential applicability. Coulombic interactions between charged droplets and aerosols substantially improve the particle capture [30, 40]. Many investigations emphasize the need to optimize charging system configurations, including charging methods, electrode geometry, placement, and voltage characteristics [41-46]. Different charging approaches, such as induction, conduction, and corona discharge, exhibit varying efficiencies under specific operating conditions [47]. Parameters such as the voltage settings and electrode configurations significantly influence the electric fields produced by these systems [41, 44, 45, 48]. Recent experiments conducted at our UTARTS facility highlighted the effectiveness of improved induction electrode designs for enhancing the aerosol scavenging performance [48].
However, the removal efficiency of aerosols can be improved by agglomeration methods such as chemical [49], acoustic [50, 51], and electrostatic methods [52]. Agglomerated particles with larger sizes can be more easily captured by droplets through enhanced mechanical effects. However, the implementation of these methods in the context of damaged reactor dismantling is restricted by harsh operational conditions including intense radiation exposure. For instance, chemical and acoustic agglomerations face limitations owing to their associated environmental pollution and high energy requirements. In contrast, electrostatic agglomeration using AC voltage, as suggested by Watanabe et al. [52], appears to have broader potential for industrial applications. Despite this, integrating electrostatic precipitators with surface cleaning and debris-cutting technologies (e.g., mechanical or laser systems) into high-radiation environments characterized by complex thermal-hydraulic interactions and multiscale phenomena remains a formidable challenge. To address these limitations, in recent years, we introduced an innovative agglomeration strategy that involves the pre-injection of submicron water mist particles prior to spray activation. As shown in Fig. 2(b), this method promotes aerosol-mist agglomeration, thereby increasing the particle size and enhancing the particle wettability. Water mist interacts with aerosols through various mechanisms, including Brownian motion, gravitational settling, laminar and turbulent shear, turbulent inertial coagulation, and shear-induced coagulation [53-57]. Experimental investigations conducted at UTARTS demonstrated that pre-injecting water mist substantially enhances aerosol scavenging efficiency [27-32, 48, 58-60]. However, the studies also revealed that the effectiveness of this approach is influenced by the operational characteristics of the spray, particularly the droplet velocity [31]. Recently, we developed an electrostatic charging system for water mist during their injection [59]. The charged water mist is supposed to enhance the aerosol-mist agglomeration. Using zirconia aerosol particles as simulant aerosols generated from laser irradiation, we validated the effectiveness of charging mist in accelerating aerosol removal.
Effective aerosol management and mitigation are of great importance for reducing radiation exposure and minimizing contaminated water production in damaged reactor dismantling. Many previous investigations have provided valuable insights into the characterization of laser-induced aerosols and improved aerosol removal by spray scavenging. However, the techniques for improving aerosol removal have mostly been verified using simulant aerosols. Our study aimed to develop efficient methods and strategies for controlling aerosol dispersion during laser cutting and decontamination processes and to mitigate radiation risks. Therefore, the applicability of these developing techniques must be validated in a more realistic scenario, which is of great significance for the technical strategies of reactor dismantling and decommissioning.
To explore more realistic conditions for Fukushima Daiichi decommissioning, in this study, innovative experimental studies are performed in our UTARTS facility for the removal of aerosols generated from laser irradiation by water sprays by improving the electrical charging mist system. The primary objective was to adapt our techniques for remote dismantling activities at the Fukushima Daiichi nuclear reactor and to address radiation hotspots. This study offers important knowledge and potential applications for the decontamination and prevention of fine particle dispersion, ensuring environmental safety, and prolonging the operational lifespan of electronic equipment. Furthermore, insights gained from this study can be useful for surface cleaning and the destruction of structures or buildings contaminated by hazardous substances, ensuring compliance with safety standards set by relevant industries.
Experimental conditions
Basic experimental setup
Figure 2 illustrates a schematic of the UTARTS facility for our laser-induced aerosol generation and spray scavenging experiments with pre-injection of charged water mist. The UTARTS facility was designed to replicate various phenomena, including laser decontamination and concurrent mist and spray operations. Its main containment vessel, constructed from stainless steel, has a height of 2.5 m, diameter of 1.5 m, and total volume of 3.92 m3. The vessel was equipped with 13 optical windows positioned along the sidewalls for internal observation and measurement. A single access manhole was integrated into the wall to facilitate post-experimental cleaning. Several pipes were installed within the vessel for aerosol and mist injections, aerosol sampling, and data collection. The aerosol sampling lines were connected to aerosol analyzers Palas Welas 3000, which are capable of measuring submicron aerosol particles. A 1.5-kW continuous-wave (CW) fiber laser with a Galvano scanning system was employed for laser irradiation. Test surfaces made of materials such as carbon steel (CS), stainless steel (SS), and concrete were subjected to infrared (IR) laser irradiation at varying power levels. Gas and water discharge pipes equipped with valves at the top and bottom of the vessel allowed for the removal of aerosol particles and water after the experiments. Before the release, the gas was filtered through a High-Efficiency Particulate Air (HEPA) filter. An air compressor was used to inject dry air into the vessel, ensuring a uniform distribution of aerosol particles (generally, a 5-minutes dry air injection was adopted, as confirmed by our previous experiments measuring aerosol concentration at varying locations in the facility [31]). A spray system installed at the top center of the vessel features a nozzle for injecting water droplets to capture aerosol particles. Additionally, the mist injection and charging systems were mounted on the side wall to introduce water mist into the vessel, further facilitating aerosol particle management.
Laser light source and Galvano scanning system
A Class-4 continuous oscillation fiber laser (Raycus, RFL-C1500) with a maximum output power of 1.5 kW and central wavelength of 1080 nm served as the laser light source for processing. The system was managed by a digital controller, NCL-151 (NISHIHARA), which also supported a maximum output of 1.5 kW. A Galvano scanning system was integrated into the laser head to enable precise scanning of the designated areas on the irradiated specimen surface. The Galvano scanning system comprises a laser, galvanometers (a set of mirrors), and lenses that focus the laser beam onto the target location. This setup provides a highly effective solution for laser-scanning applications, offering precision and versatility for tasks such as laser cleaning, cutting, and welding. The galvanometers were mounted on high-speed motors to ensure rapid and accurate movement, with their positions controlled electronically. The Galvano scanning system featured a focal length of 900 mm and an effective focal length of 998 mm. The lenses used in the scanner are typically crafted from high-quality materials, such as quartz or sapphire, to minimize the heat generation from the laser and ensure optimal performance. In this study, the laser power was fixed at 1.5 kW for the larger amount of aerosol particles generated.
Test specimens
The materials selected for the laser cleaning experiments included various surfaces of CS (grade SS400), SS (grade SS304), and concrete, chosen for their widespread use in numerous industrial applications, particularly in nuclear reactor pressure vessels and containment vessels. These materials exhibit distinct thermophysical properties that influence the laser cleaning efficiency. Additionally, different coatings were applied to these base materials to evaluate the effectiveness of laser cleaning on the coated surfaces. ZrO2 and CeO2 were used as coatings to simulate the nuclear decommissioning conditions [15]. The coating solution was prepared by mixing distilled water and nanoparticles in a 50:50 ratio, followed by sonication using an Ultrasonic Sonicator for 60 min to ensure uniform dispersion. The solution was applied to the specimen surfaces using a spray gun with three successive applications to achieve consistent coating coverage. This study aimed to assess the potential of laser cleaning for removing contaminants from coated surfaces and the applicability of advanced agglomeration techniques to subsequent aerosol spray scavenging. The selection of materials for the specimens and coatings reflects the practical demands of decommissioning applications, providing a thorough evaluation of the effectiveness of laser cleaning on diverse surface types and coatings.
Water spray system
As shown in Fig. 2, the spray system was installed at the top center of the vessel, featuring a nozzle for injecting water droplets to capture aerosol particles. The spray water flow rate was regulated at 2 L/min using a pump in current experiments. In this study, a single-hole full-cone nozzle (model: 1/8 GG-SS3004) fabricated by Spraying System Co. was used. Spray droplet characterization was performed in our previous studies [28, 29]. Table 1 lists the parameters of the nozzle and sprayed droplets in the current study. Figure 3(a) and (b) illustrate the size distribution of the droplets generated under 2 L/min spraying and the flow pattern inside the vessel during spraying.
| Model | 1/8 GG-SS3004 |
| Orifice diameter (mm) | 1.2 |
| Spray anglea (°) | 26-32 |
| Droplet velocity magnitude at 2 L/minb (m/s) | 20 |
| Droplet Sauter mean diameter (μm) | 309 |
| Spray pattern of the nozzle | Full cone |
| Distance from the top of vessel (hNozzle) (mm) | 300 |
| Water pressure for 2 L/min spray (MPa) | 0.53 |
Mist generation and charging systems
The water mist injection system was designed to horizontally introduce submicron water mist particles into the UTARTS facility from its sidewall. An ultrasonic mist generator (MZ-JH30, Yamazen Company, Japan) capable of producing varying levels of mist concentrations was employed for this purpose. As illustrated in Fig. 4, the size distributions of water mist particles at the lowest and highest mist concentration levels (Cmist) generated by the system were relatively consistent. Regardless of the concentration level, the water mist particle sizes were predominantly centered around 0.68 μm. The water mist particles are also in the submicron range.
According to the spray charging methods (e.g., induction, conduction, corona discharge) that have been widely applied in many industrial fields [61, 62], several methods might be applicable for charging water mist. Induction and conduction are the primary methods used to charge conducting liquids [63]. In induction charging, an electric charge is applied to droplets after atomization, whereas conduction charging involves transferring an electric charge to a liquid jet and its resulting droplets by directly connecting the atomizer nozzle to a high-voltage source. Although the droplets can also be charged after atomization, for example by gaseous ions generated by corona discharge, their charge is much lower than that produced by the other two methods. Both methods can achieve comparable charge levels for droplets [46]. However, in the context of this study, the water mist to be charged consists of much smaller particles (primarily submicron in size) than spray droplets, which typically range from tens to several hundred microns. In addition, the atomization process for generating water mist occurred within the mist generator. Given these characteristics, the induction-charging method is more suitable for charging mist in this setup.
Figure 5 shows the mist charging system used in this study. To charge the water mist, a cylindrical brass electrode, which was confirmed as the optimal electrode in our previous study [59], was installed at the outlet of the mist injection pipe inside the vessel. This electrode was connected to a high-voltage power supply (Matsusada Precision, HAR-30R10), which can provide an adjustable charging voltage and polarity (positive or negative). The mist injection pipe, made of stainless steel, was grounded to establish an electric field near the pipe outlet, facilitating the electrical charging of the water mist.
Aerosol sampling and measurement system
For aerosol measurements, we installed a sampling line above the laser target. During the measurements, aerosol particle-laden gas was sampled to evaluate variations in aerosol concentrations. Based on prior uncertainty analyses [29, 30], the sampling efficiency for submicron aerosol particles in this experimental system exceeded 90%. To remove water mist from the sampling flows before they entered the aerosol analyzer, a heater-wrapped tube with a fixed temperature and of 1.2 meters in length was installed upstream of the analyzer. After passing through the heated tubes, the aerosol sample flows were directed into a Palas Welas 3000 aerosol analyzer, which measured the aerosol particle number concentration (Cn) and size distribution during the experiments. The detailed specifications of Palas Welas 3000 are listed in Table 2. Concurrently, time-resolved measurements of temperature, humidity, pressure, and spray flow rate were conducted along with aerosol analysis. Table 3 summarizes the uncertainties associated with these measurements, all of which fall within acceptable ranges. Temperature and pressure remained stable throughout the experiment. The water pressure and spray flow rate exhibited fluctuations during the initial 20–30 s of spraying but stabilized thereafter. Additionally, the humidity increased progressively and reached saturation a few minutes after the initiation of mist or spray injection [27].
| Working principle | Light-scattering spectrometer |
| Particle type | Solid, liquid |
| Measurement range for aerosol diameter (μm) | 0.2 ~ 10 |
| Absolute measurement error (μm) | 0.01 |
| Relative measurement error | 5% |
| Number concentration measurement range (P/cm3) | 1 ~ 106 |
| Withstand temperature (°C) | up to 250 for aerosol sensor |
| Sampling flow rate (L/min) | 5 |
| Parameter | Sensor type | Range | Uncertainty |
|---|---|---|---|
| Humidity | Hygrometer | 0 ~ 100% | ±5% |
| Water pressure | Strain gage | 0 ~ 10 MPa | ±0.45 MPa |
| Vessel pressure | Strain gage | 0 ~ 1 MPa | ±0.045 MPa |
| Temperature | Type K thermocouple | 200 ~ 1260 °C | ±2.9 °C |
| Spray flow rate | Ultrasonic | 0 ~ 60 L/min | ±0.24 L/min |
Experimental procedures
The general procedures for laser-induced aerosol generation and spray scavenging experiments are illustrated in Fig. 6. For different experiments, the 1.5-kW laser irradiated the test specimen inside the UTARTS vessel for 30 s from an optical window. When the laser beam impacted the test specimen, material evaporation occurred, producing a substantial quantity of aerosol particles that were sampled through a line positioned 500 mm above the irradiation zone and 1000 mm below the spray nozzle. To ensure the homogeneity of aerosol dispersion within the vessel, dry compressed air was injected at the bottom of the vessel at a flow rate of 70 L/min for 300 s. This air injection was conducted with the aerosol outlet valve open to prevent the pressurization of the vessel. It is important to note that the interaction between the laser and specimen is expected to generate residual energy during laser irradiation. However, given the size of the UTARTS vessel and the continuous flow of mixed air, this residual energy is unlikely to have any significant effect. By the time the aerosol scavenging process commenced after laser irradiation, the residual energy was expected to dissipate. Furthermore, the temperature and pressure sensors installed inside the UTARTS facility to monitor the experimental conditions recorded no significant variations, confirming the stability of the system during the experiments.
Regarding the subsequent aerosol scavenging processes, generally three conditions, including “spray & no mist”, “spray & neutral mist” and “spray & charged mist”, were considered. Mist injection was performed for cases investigating mist effects. For cases using “spray & neutral mist”, the neutral mist injection was performed for 300 seconds, and idling was kept for another 300 seconds for aerosol-mist agglomeration. While for cases using “spray & charged mist”, the high voltage supply was activated to the electrode before the mist injections to ensure mist charging during their injections. A neutral water spray was then injected to remove aerosols. During spray activation, the aerosol particles were sampled and their concentrations were monitored using an aerosol analyzer (Welas 3000). The size distribution data were presented as normalized number concentrations per logarithmic diameter interval.
Experimental parameters
Table 4 lists the parameters used in the current scavenging experiments for aerosols generated from laser cleaning. Regarding the parameters used for laser-induced aerosol generation, the laser power was fixed at the maximum value (1.5 kW) for the 30-second irradiation to attain an adequate concentration of generated aerosols from varying surface conditions. Similarly, CS, SS, and concrete surfaces coated with CeO2, ZrO2, and an oxidized layer (typically for the CS surface) were employed. For the aerosol scavenging parameters, no-mist and high-concentration mist conditions were considered. For mist charging, –10 kV and +10 kV were both considered to further investigate the effect of polarity on aerosol scavenging in more realistic scenarios.
| Type | Parameter | Value |
|---|---|---|
| Aerosol generation | Irradiation power, Pir (kW) | 1.5 |
| Irradiation time, tir (s) | 30 | |
| Irradiation area | Circle (Φ=5 mm) | |
| Line distance in each scan [mm] | 0.1 for CS&SS; |
|
| Test specimens | CS: 50 mm×50 mm×5 mm; |
|
| Surface coating material | Oxidized layer; CeO2; ZrO2 | |
| Aerosol scavenging | Spray nozzle | Nozzle 1 |
| Electrode model | 1B for spray charging; |
|
| Voltage and polarity for spray charging | -15 kV | |
| Voltage and polarity for mist charging | -10 kV; +10 kV | |
| Spray flow rate, Q (L/min) | 2; 3; 4 | |
| Mist concentration level | 0; High Cmist |
Results and discussion
Aerosol size distribution and variation during spray
Figure 7 shows the aerosol size distributions and variations during sprays for experiments under different specimen and mist conditions. Focusing on the aerosol concentrations before spraying, it was found that for metal and concrete surfaces, the laser-generated aerosol sizes ranged within the Greenfield gap. It can be seen that the aerosol number concentrations peak at approximately 0.28 μm for metal surfaces and 35 μm for concrete surfaces, caused by the mechanism of aerosol generation with laser interaction. If a laser beam interacts with the material surface, the absorbed energy causes the surface temperature to increase. As the energy density of the laser increased, the temperature eventually reached the vaporization threshold of the material. At this stage, the surface underwent melting and vaporization by laser irradiation, inducing recoil pressure on the molten layer. This recoil pressure (Pr) is intrinsically linked to the saturated vapor pressure, which is determined by the temperature within the molten region [64]. The recoil pressure generated by vaporization intensified with increasing surface temperature. Once this pressure exceeds the surface tension of the molten surface, it drives the vapor-liquid mixture away from the molten pool. Simultaneously, the molten liquid was displaced along the cavity sidewall until expulsion. During laser cleaning, submicron aerosol particles, droplets, and fragments are produced. The formation of fine particles smaller than 1 µm arises from rapid expansion and substantial cooling of the vapor, which is subsequently followed by nucleation and condensation processes [65-67].
Figure 8 shows a schematic of various laser-irradiated surfaces. Notably, our experiments revealed the formation of a substantial number of fine particles. These particles are generated from liquid droplets within the molten layer [65, 66]. For concrete surfaces, a series of chemical and physical reactions occur, primarily driven by the dehydration of calcium hydroxide and decomposition of calcium carbonate, resulting from the release of gases (such as CO2 and H2O) from the irradiated zone [12]. With an increase in the energy density, the corresponding temperature rise leads to the melting of the cement and aggregates, forming a thick SiO2-rich molten pool within the cavity. The pressure generated in this phase propels molten concrete along the sidewalls of the cavity until its ejection. Furthermore, gases released during concrete decomposition encounter the high viscosity of the molten pool, causing them to escape from the surface. Sustained gas production significantly increased the pressure beneath the molten layer, creating a gas bubble within the melt that progressively expanded over time. The difference in the aerosol formation mechanisms for different surfaces leads to different size distributions after their generation.
Regarding the aerosol concentration variation during spraying, as shown in Fig. 7, an obvious reduction in aerosol concentration for all sizes can be observed irrespective of experimental conditions. Aerosol particles can be captured by neutral sprayed droplets by mechanical (e.g., Brownian diffusion, inertial impaction, and interception) and phoretic (e.g., diffusio- and thermo-phoresis) effects. It is noteworthy that, based on our prior experimental measurements [68], the relative humidity within the UTARTS chamber increases from the ambient level (approximately 65%) to 99% following the mist injection or a 6-minute spray without mist injection, which is driven by the production of small droplets. Owing to their high velocity relative to the chamber dimensions, the falling time of these droplets was brief. For thermophoresis, the experiments were conducted at room temperature. While localized temperatures at and around the irradiated surface are expected to be very high during laser irradiation (inducing melting, evaporation, and aerosol generation), significant heat removal occurred during the 5-minute air mixing phase prior to mist injection. Furthermore, the specimen cooled rapidly once spray activation began. Consequently, the influence of diffusiophoresis and thermophoresis on the capture of aerosol particles near falling spray droplets is expected to be negligible compared to other mechanisms, such as Brownian diffusion, initial impaction, and interception.
By comparing the results of metal and concrete aerosols, it can be found that the spray system can easily remove the concrete, as shown in Fig. 7(g) and (h), where almost all aerosols are removed after 30-minute sprays. This can be attributed to the more hydrophilic nature of the concrete aerosol particles (e.g., SiO2) compared to the SS and CS particles. The concentration peak shifting to smaller sizes is also observable for concrete cases, indicating faster removal of larger-sized aerosols. This variation in aerosol size distribution can be explained by the greater influence of inertial impaction and interception exerted by the spray droplets on larger-sized aerosols. However, for the metal surface cases, as shown in Fig. 7(a)–(f), although faster removal of larger-sized aerosols was found, no apparent peak shifting was found within the 30-minute spray due to the low removal efficiency of hydrophobic metal aerosols. Since the aerosol size distribution were found to be concentrated within 0.2 to around 0.6 microns, the aerosol removal efficiencies for 0.2~0.6 μm are analyzed and discussed in the following sections.
Quantification for aerosol removal efficiency with different sizes
For the quantitative evaluation of the removal process for aerosol particles of different sizes, the aerosol removal efficiency E is defined as
In our experiments, it was observed that, during the initial transient spraying phase (e.g., 1–3 min), the aerosol concentration was not spatially consistently distributed when the flow pattern was still developing. Nevertheless, as the spray continued (e.g., 5–30 min) and the flow pattern stabilized, the discrepancy in the spatial distribution of aerosols was significantly reduced. In addition, preliminary tests conducted at the UTARTS facility indicated that the background loss of aerosol particles (e.g., due to deposition or agglomeration) was negligible when the spray duration exceeded approximately 10 min [68]. Consequently, for the quantitative analyses presented in the following sections, the experimental results obtained at 30 min were considered for the metal surfaces. For the concrete surfaces, as the removal process is comparatively faster and all aerosols are mostly removed after 30-minute sprays irrespective of experimental conditions (as shown in Fig. 7(g) and (h)), we consider a 10-minute removal efficiency for concrete cases.
Effect of neutral and charged water mist for metal surfaces
Figure 9 shows the removal efficiency under different mist conditions after 30 min sprays for aerosols generated from different metal surfaces irradiated by the laser. As shown in Fig. 9, it can be found that for both metal surfaces, injecting water mist is helpful for improving the aerosol removal efficiency as compared to the aerosol removal efficiency in case without mist injection. This phenomenon can be explained as follows. In the presence of water mist, the diffusiophoresis mechanism compels the aerosol particles within the experimental chamber to migrate towards the water mist surface. Additionally, mist-aerosol coagulation may also occur due to different mechanisms, especially gravitational and Brownian coagulation [27]. Furthermore, the interaction with mist enhances the surface wettability of aerosol particles, thereby increasing the water uptake capability. As a result, the agglomerated aerosol mist particles exhibit a stronger affinity for spray droplets, accelerating their removal process. The experimental results validated the feasibility of our aerosol-mist agglomeration method for improving the removal of aerosols generated by laser irradiation. This reveals the effectiveness of pre-injection of water mist in improving aerosol scavenging in a more realistic industrial application, such as the Fukushima Daiichi decommissioning.
However, for the SS surface, as shown in Fig. 9(a), compared with the neutral-mist case, the aerosol removal efficiency is higher when using a negative charging supply to water mist, and lower when using a positive charging supply. By using the negative charging supply to water mist, the average improvements in SS aerosol removal efficiency were 40% and 3%, respectively, compared to no-mist and neutral-mist cases, respectively. For the CS, as depicted in Fig. 9(b), the situation is the opposite. By using the positive charging supply to water mist, the average improvements in the CS aerosol removal efficiency were 44% and 9% compared to the no-mist and neutral-mist cases, respectively. This may be because the crystal structure of the SS surface without coating shows a lower capacity for electron retention and leads to the dominance of negative ion emission during laser irradiation [69, 70], thereby improving the agglomeration between generated aerosols with negative charges and injected water mist with positive charges, which is charged by a negative voltage supply from induction charging. On the other hand, the CS surface is supposed to retain electron emission during laser irradiation, predominantly generating aerosol particles with positive charges. As a result, the application of a positive voltage supply, which induces negative charges in injected water mist particles, shows a better aerosol scavenging efficiency for the CS surface owing to the enhanced aerosol-mist agglomeration as compared to the neutral-mist case. Conversely, the negative voltage supply to charge the water mist results in a slower removal process for laser-induced aerosol particles from the CS surface.
Effect of neutral and charged water mist for concrete surfaces
Figure 10 shows the removal efficiency under different mist conditions after 10-minute sprays for aerosols generated from concrete surfaces irradiated by a laser. As shown in Fig. 10, it can be found that the pre-injection of water mist is also beneficial for accelerating the aerosol removal process by water spray in comparison with the case without mist injection. Again, this could be attributed to the enlargement of aerosol sizes and improvement in wettability by agglomeration with water mist. The results again indicate the applicability of aerosol-mist agglomeration in facilitating aerosol removal in more realistic engineering scenarios, irrespective of the charging configurations and surface conditions.
However, unlike the metal cases depicted in Fig. 9, as shown in Fig. 10, both negative and positive voltage supplies for charging the water mist seem to improve the scavenging efficiency of concrete aerosol particles as compared to the water mist without charging. For both negatively charged and positively charged cases, the average improvements in the concrete aerosol removal efficiency were 21% and 10%, respectively, compared with the no-mist and neutral-mist cases, respectively. For the concrete case, the differences in aerosol removal efficiency seem to be insignificant for different polarities of voltage supply. This might be because of the large composition of SiO2 (>60%) with low electrical conductivity (or even insulation) in concrete, which is different from the conductive metal cases. Therefore, the aerosol particles generated from laser cleaning of concrete surfaces are supposed to be neutral or have a low charging level in predominance compared to those generated from metal surfaces. This leads to an insignificant effect of the charging polarity of the water mist on the scavenging of concrete aerosols. Compared to the case using a neutral mist for the concrete surface, the improvement shown in the cases using a charged mist was found. One possible reason could be the polarization and induction charging from the charged mist to conductive aerosols generated from the concrete, facilitating the agglomeration of water mist and aerosol particles. The results from the metal concrete surfaces may indicate the different dominant polarities of aerosols generated from distinct materials during laser cleaning operations, which is of great significance for the evaluation of the application of mist/droplet charging techniques to the Fukushima Daiichi decommissioning and other environmental chemical applications.
Effect of neutral and charged mist for metal surfaces with different coatings
A comparison of the performances of neutral and charged mist in the scavenging of aerosols generated from different surfaces with coatings is shown in Fig. 11. Our experimental results suggest predominant charge states for aerosols generated from metal specimens under different surface conditions. Table 5 summarizes the estimated polarities of aerosol charges under different surface conditions. By comparing the cases using neutral mist and mist charged by negative voltage shown in Fig. 9(a) and Fig. 11(a), it can be observed that although the applied negative voltage is helpful for the improved scavenging of aerosols generated from the pure SS surface, the CeO2 coating on the SS surface seems to reduce this beneficial effect triggered by the pre-injection of mist charged by negative voltage, or may even result in worse performance. For the cases using ZrO2 coatings, it is clear that the ZrO2 coating on SS leads to the ineffective application of mist charged by a negative voltage. Similar oppositeness between the uncoated and CeO2 coated surfaces can also be found for the CS cases by comparing Fig. 9(b) and Fig. 11(b). This may reveal the influence of surface coating on the predominant polarity of aerosols generated during laser cleaning. When an aerosol particle collides with a charged water mist particle, three forces generally act on the aerosol particle: water surface tension, particle inertial force, and electrical attractive force between the aerosol and mist. The Coulomb attractive force accelerates the particle towards the mist, and the electrical attractive force continues to act on the particle until it comes into mechanical contact with the mist surface. After mechanical contact, the Coulomb force still acts for a short time owing to charge relaxation. This electrical force counteracts the rebounding force of the water surface tension, leading to a gentle landing and capture of particles on the droplet surface. This mechanism decreases the probability of particle re-entrainment by rebounding compared to when the particle and droplet are neutral and provides a higher capture efficiency for opposite-charged mist droplets [71].
| Specimen material | Coating condition | Estimated aerosol charge |
|---|---|---|
| SS | Uncoated | Predominantly negative charge |
| CeO2 coating | Reduced negative charge or predominantly positive charge | |
| ZrO2 coating | Predominantly positive charges | |
| CS | Uncoated | Predominantly positive charges |
| CeO2 coating | Predominantly negative charges | |
| Oxidized layer | Predominantly negative charges |
Because the negative voltage power supply to the electrode would lead to the sprayed droplets being charged positively by induction charging, the results shown in Fig. 11(a) and (b) may indicate that the ZrO2 and CeO2 coatings on the SS surfaces are expected to cause less generation of negatively charged aerosols during laser cleaning. In contrast, as shown in Fig. 11(c), the surface coatings seem to result in more generation of negatively charged aerosols on the CS surfaces. These different influences of the surface coating on the dominant polarity of aerosols generated from different substrate materials may result from the different crystal structures of the materials. During laser irradiation, the efficiency and predominance of electron emission processes, such as thermionic emission (electron emission induced by high temperatures), photoelectric effect (electron emission due to photon interaction), and field emission (electron emission influenced by strong electric fields), can be affected by the crystallographic orientation of the metal surface owing to variations in the electronic properties and atomic arrangement [72-77]. In addition, as the metal surface is heated and potentially melted or vaporized by a laser, the ejected material can form particles via condensation and solidification. The charge state of these particles depends on the nature of the charge carriers involved in their formation. The coatings on the SS surface probably enhance the predominance of positive ion emission due to more effective electron retention, resulting in the generation of more positively charged particles, whereas CS may have the opposite effect.
Effect of metal surface condition
Figure 12 displays the effect of surface conditions on the scavenging of aerosols generated from different surfaces with mist charging. The injected water mist was electrically charged by a –10 kV voltage supply, which means that the water mist carried positive charges. As shown in Fig. 12(a), for the SS substrate, the surface coating seems to restrain aerosol scavenging when using mist with positive charges. This may indicate that the CeO2 and ZrO2 coatings on the SS surfaces can probably reduce the generation of negatively charged aerosols during laser cleaning owing to the change in surface conditions. However, as displayed in Fig. 12(b), the surface coatings on the CS surfaces seem to intensify the generation of negatively charged aerosols. Corresponding to the findings in the previous sections, this may have resulted from the different crystal structures of the materials and electron emission characteristics during laser irradiation. These results may again reveal the influence of the surface coating on diminishing or even changing the predominant polarity of aerosols generated during laser cleaning, although further detailed investigation and exploration are still required. The current findings on the influence of surface conditions on the charging characteristics of generated aerosols could provide critical insights for developing technical strategies for the dismantling and decommissioning processes of the Fukushima Daiichi nuclear power plant.
Conclusion
The efficient removal of harmful aerosols plays a crucial role in gas purification processes aimed at mitigating environmental pollution across various chemical industrial applications, including the decommissioning of nuclear reactors. In particular, during the dismantling of the Fukushima Daiichi nuclear reactors, it is essential to control greenfield-gap radioactive aerosol particles to prevent the release of radioactivity into the environment and to minimize radiation exposure risks for both workers and the public. Techniques for improving the aerosol removal efficiency must be validated in realistic scenarios.
In the present study, an experimental investigation on the removal of aerosols generated from laser cleaning using spray with pre-injection of neutral/charged water mist was performed in our UTARTS facility to validate the applicability of the aerosol-mist agglomeration technique and attain more knowledge on laser-induced aerosol generation and spray scavenging under more realistic conditions in view of the Fukushima Daiichi decommissioning. The experimental results validated the feasibility and applicability of the pre-injection of water mist for accelerating the scavenging processes of aerosols generated by laser irradiation. Proper mist charging was also beneficial for facilitating aerosol scavenging. The average improvements were 40%, 44%, and 21% for SS, CS, and concrete, respectively. Furthermore, different polarities of the charged mist showed opposite influences on the aerosol removal efficiency for different irradiated materials, indicating that the dominant polarity of aerosols generated may be different for different materials during laser irradiation, which is attributed to the different crystal structures of the materials and electron emission characteristics during laser irradiation. Based on experiments using a negative charging voltage, the surface coating was found to be effective for affecting (or even changing) the dominant polarity of aerosols generated during laser cleaning. An investigation of the effects of positive charging on coated surfaces will be conducted in the future to further elucidate these polarity-dependent mechanisms. The current study provides important knowledge on the improvement of aerosol removal and gas purification in the Fukushima Daiichi decommissioning and various environmental chemical engineering processes. Future studies should focus on the detailed characterization of aerosols generated from laser irradiation (e.g., dominant polarity), including laser cleaning and cutting. The understanding and data from this investigation are expected to be used for formulating technical strategies for Fukushima Daiichi decommissioning and developing relevant models of laser-induced aerosol generation and aerosol spray scavenging for wide applications in environmental and chemical engineering, such as gas decontamination, air purification, and pollution control.
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