A State-of-the-Art Review of Radioactive Decontamination Technologies: Facing the Upcoming Wave of Decommissioning and Dismantling of Nuclear Facilities

Abstract

The average share of nuclear energy in electricity production is expected to increase under the background of the global pursuit towards carbon neutrality. Conjugating with its rapid development, the wave of decommissioning and dismantling (D&D) of nuclear facilities is coming. The surface decontamination is a prerequisite to D&D, which will make it easier and reduce the volume of radioactive wastes. However, there are no comprehensive studies on the decontamination methods, which is not helpful for the sustainable development of nuclear energy and environment protection. Therefore, in this work, the current status and future trends of global energy and nuclear energy are first analyzed. Then, various decontamination approaches are comparatively studied, including cleaning mechanisms, application subjects, and intrinsic advantages and disadvantages. Finally, the criteria and factors for selecting a decontamination process, the challenges, and future studies are directed. Among the mechanical methods, laser-based cleaning is high-speed, having automation ability, and thus is promising, although it creates a dust and airborne contaminant hazard. In further studies, factors such as selecting a proper laser facility, optimizing operating parameters, and designing a high-efficiency dust collection system could be studied. Regarding the chemical method, chemical gels are good for decontaminating complex shapes and vertical and overhead surfaces. In addition, they can enhance other decon agents’ efficiency by improving contact time. However, the formulation of colloidal gels is complex and no gel type is useful for all contaminants. Therefore, novel and versatile gels need be developed to enlarge their application field. Combining various decontamination methods will often have better results and thus a reasonable and effective combination of these decontamination methods has become the main direction.

1. Introduction

The global energy demand is increasing rapidly with population and economic growth and is expected to increase by about 30% by 2050, although it declined in the context of the global COVID-19 pandemic. Recently, fossil fuels such as oil, coal, and natural gas made up 83% of the global energy mix and will shrink to below 50% in 2050. For China, the share of primary energy from fossil fuels was larger than 90% before 2012 and declined to approximately 84% in 2020. It is expected to decrease to 77% in 2030 and 47% in 2060 under the impact of China’s carbon neutrality pledge (Figure 1). The long-term utilization of conventional energy sources has a major impact on climate change, air quality, and public health [1,2,3]. In 2018, the global energy consumption increased by 2.3%, resulting in an increase in CO₂ emissions of 1.7%. According to the report from Harvard University [4], more than 8 million people died in 2018 due to the exposure to particulate matter from fossil fuel emissions, accounting for 18% of the total global deaths. Therefore, developing renewable energies to replace the predominant fossil energy has become the alternative strategy to address the impacts of climate change.

Nuclear energy offers advantages of low-cost operation, low carbon emission and reliability, and has been recognized as one of the most environmentally friendly resources [5,6]. The transition from conventional sources of energy to nuclear energy is expected to reduce demand pressures on crude oil and protect the environment from greenhouse gas emissions. The average share of nuclear power in global electricity generation declined steadily from a peak of 17.6% in 1996 and stagnated at 10.5% since 2013. France, with the second largest nuclear electricity generation capacity, had the largest share of about 70.6% in 2020. Ukraine, Belgium, Finland, and Switzerland also have a significant share of nuclear power in their mix accounting for 51.2%, 39.1%, 33.9%, and 32.9%, respectively. Finnish utility Teollisuuden Voima Oyj has received permission from the country’s Radiation and Nuclear Safety Authority to bring the Olkiluoto 3 EPR to first criticality and conduct low-power tests. The unit is expected to begin regular electricity production in 2022. Poland also plans to have nuclear power from about 2033 as part of a diverse energy portfolio, moving it away from heavy dependence on coal. In China, nuclear plants contributed only about 4.9% in 2020 after growing slowly from 1.8% in 2010. There will be tremendous room to grow under the background of carbon neutrality and it is expected to account for approximately 10% in 2030 and 19% in 2060 (Figure 2).

Currently, there are approximately 447 nuclear power reactors operating around the world, providing about 10% of the world’s electricity. The United States has 55 commercially operating nuclear power plants with 93 nuclear power reactors, followed by France, China, Russia, and Japan with 56, 51, 38, and 33 reactors (Figure 3). To achieve the carbon neutrality goal, China is on track to surpass the USA as the world’s top producer of nuclear energy with more than 100 commercial nuclear reactors as early as 2030. However, each nuclear plant must be decommissioned after the reactors reach their operational life. Of those reactors, nearly 70% are older than 30 years and 25% exceed 40 years. According to the International Energy Agency, around 200 commercial reactors may be shut down between 2020 and 2040.

Once a nuclear plant has been shut down, there is a transition phase during which the facility is prepared for decommissioning. It generally includes removing fuels and readily accessible radioactive materials and contaminated equipment. Following this transition phase, facilities are decontaminated, which will make the decommissioning and demolishing easier and reduce the volume of radioactive wastes. Recently, some sophisticated decontamination techniques, such as dry ice blasting, laser-based cleaning, and electrochemical decontamination have been developed [7,8,9,10]. The overall decontamination strategy should be optimized, taking into account the benefits which result from reduced public exposure, reduced exposures to workers, and the costs of operation. However, to the best of our knowledge, there are sparse comprehensive pieces of research on the surface decontamination. Therefore, in this work, the current status and future trends of global energy and nuclear energy are first analyzed. On this basis, various decontamination approaches are described and comparatively studied. Finally, the criteria and factors for selecting a decontamination process, the challenges, and future studies are directed.

2. Methodology

2.1. Literatures Search

To identify the relevant literatures for this work, the search engines ISI Web of Science, ScienceDirect, and Google Scholar were used to search scholarly literatures encompassing the period from 2000 to 2022. The search terms included different combinations of the following words: “nuclear energy”, “decommissioning and dismantling”, “nuclear waste”, “carbon neutrality”, “radioactive decontamination”, “high-pressure water jetting”, “dry ice blasting”, “laser-based cleaning”, “nonthermal plasmas”, “reagent washing”, “foam decontamination”, “chemical gels”, “strippable coating”, and “electrochemical method”. To ensure that we got a comprehensive list of the studies, we also went through the citations and the reference list of the selected papers.

2.2. Screening

After every search, literatures were screened and included in the review database based on predefined criteria: (i) published in English, (ii) peer-reviewed, with full text available, (iii) data are reliable, comprehensive and not already included in a previous paper to avoid double counting. After these procedures, more than 300 literatures were checked and 93 were included in this review.

3. Radioactive Contamination

3.1. Surface Contamination Mechanism

Decontamination is defined as the removal of contaminants from surfaces by washing, heating, or chemical, electrochemical, or mechanical action. It is mainly applied in nuclear facility equipment and components, including building exteriors and interiors, equipment, pavement, and vehicles. Sometimes, it can also concern the removal of radioactivity situated deep in the material. During the decontaminating processes, two points need to be better understood. The first is to clarify the parts that need be decontaminated, and the second is to know the source of the contaminants. The radioactive contamination is caused by radionuclide contaminants on surfaces through mechanical adsorption, physical adsorption, or chemical reactions with different base materials, such as stainless steel, rubber, plastic, etc. [11,12,13]. The mechanical adsorption is resulted from the imbedding and adhesion of radionuclides in the defect of the material surface. The physical adsorption is generated because some materials’ surfaces have charges and the radionuclides with opposite charge are adhered on their surfaces. The chemical reaction is caused by various reactions, such as ion exchange and isotopic exchange between radionuclides with ionic form and materials. The type of contamination is determined by the radionuclides and base material, as well as the system.

The main forms of radionuclides binding to the surface include [8]: (1) Nonfixed contaminants attached to the surface by intermolecular forces. There is no reaction between radionuclides and base materials. The combination between them is weak and this contaminant is easy to be removed. (2) Weakly fixed contaminants formed through chemical adsorption or ion exchange. The combination is strong, and the radionuclides penetrate the base material with a considerable depth resulting the difficulty of radionuclides to be removed. (3) Deep surface contamination formed by radionuclides diffusing to the base or neutron irradiation activation of trace elements in the base. This contamination is difficult to be dealt with as well. Contamination can also be classified as being “fixed” and “loose (or free)”. Fixed contamination is that which is not transferred from a contaminated surface to an uncontaminated surface when two surfaces accidently touch. The radioactive material cannot be spread, since it is chemically or mechanically bound to structures. It cannot be removed by common cleaning methods. As a comparison, loose contamination is that which may be readily transferred under these circumstances. The radioactive material can be spread, and this contamination can easily be removed with simple decontamination methods. Therefore, in response to different levels of surface contamination and the requirements for decontamination effects, it is necessary to choose a reasonable and economic decontamination technique.

3.2. Sources of Radionuclide Contaminants

Various types of facilities, especially reactors, produce various radionuclides due to different moderators, coolants, structural materials, nuclear fuels, and auxiliary processes used. In nuclear reactors, the neutron moderator is designed to slow down high-energy neutrons to lower energies and thus achieve a self-sustained chain reaction involving ²³⁵U or ²³⁹Pu [14]. Radioactive nuclear fission products such as ¹³¹I, ¹³⁷Ce, and ⁹⁰Sr are created in this process. There are a limited number of radioactive sources that are large enough to cause widespread contamination. The radionuclide contaminants mainly include ⁶⁰Co, ^{134, 137}Cs, ⁹⁰Sr, ²³⁸U, ¹³¹I, ¹²⁹Te, ¹¹⁰Ag, ²³²Th, ^238,239,240Pu, ¹⁹²Ir, ²⁴¹Am, ^129,131I, ^97,98,99Tc, ^93,95Zr, ⁵⁵Fe, ⁹⁴Nb, etc. [15,16,17].

3.3. Behavior Characteristics of Radionuclides

During the surface decontamination of nuclear facilities, the focus is not only on the surface deposition layer but also taking into account the radionuclide migration. Recently, some methods have been developed to probe the diffusion and migration of radionuclides in the micro area, such as scanning proton microprobe, synchrotron radiation X-ray fluorescence, and analytical transmission electron microscopy, etc. These studies will provide critical information for decision-making regarding the decontamination and management of contaminated facilities. Shuai et al. [18] studied the migration of ²³⁵U in rubber and cable used for uranium storage by neutron activation analysis and γ spectrum analysis. The results showed that the permeation depth of ²³⁵U in rubber ranged from ten to one hundred micrometers. Gao et al. [19] studied the decomposition of a pipe storing intermediate level liquid waste using laser decontamination techniques. It was difficult to achieve the decontamination factor of more than 100× with radiation intensity larger than 10 J/cm² and high-frequency scanning. The deposition lay of samples after decontamination still had strong radiation. It indicated that the radioactivity of the deposition layer is just a proportion of radioactivity in the pipe and some radionuclides had penetrated into the sub-surface of the base material. Stainless steel is a common structural material used in a nuclear power plant due to their better mechanical properties and corrosion resistance. The chromium-enriched oxide film on the stainless-steel surface is an important barrier to protect the substrate from external contamination. However, the radionuclides could accumulate at the steel grain boundaries at high temperatures and penetrate into the steel subsurface. Lang et al., [13,20] probed the ⁹⁰Sr and ^134,137Cs uptake onto and distribution in AISI Type 304 stainless steel. Under passivating conditions, ⁹⁰Sr and ^134,137Cs were maintained at the steel surface by sorption/selective incorporation into the Cr-rich passive film. However, under transpassive conditions (12 M HNO₃), corrosion and severe intergranular attack led to ⁹⁰Sr diffusion into the passive layer and steel subsurface. ⁹⁰Sr and ^134,137Cs accumulation was also commensurate with corrosion product (Fe and Cr) re-adsorption. Xie et al. [21] simulated the contamination of 316L stainless steel with ⁹⁰Sr and ¹³⁵Cs. The ⁹⁰Sr and ¹³⁵Cs contents on the surface reached the maximum at 21 days. Due to dissolution of the oxide layer and diffusion of the contaminants to the matrix, the concentrations of ⁹⁰Sr and ¹³⁵Cs decreased and maintained at a stable state after 30 days. There were two species of ⁹⁰Sr on the steel surface, namely SrCrO₄ in the oxide layer, and SrCO₃ in the matrix. Taylor-Underhill [22] probed the mechanism of cesium and strontium contamination in AISI type 304H stainless steel. Stable Sr contaminations were more pronounced than stable Cs contamination on stainless steel surfaces. Sr was found to penetrate the steel and does not desorb with a change in pH. From these above reports (Table 1), it can be found that the depth distribution of radionuclides such as ²³⁵U, ⁹⁰Sr, and ¹³⁷Cs in some base materials have been investigated using the laser-induced breakdown spectroscopy (LIBS), X-ray photoelectron spectroscopy (XPS), and γ-ray spectrometry. The penetrating depth varied depending on the physicochemical properties of base materials, nature of radionuclides, etc. (Figure 4). Cs had similar contamination behaviors to Sr, but it was more inclined to surface than Sr. The majority of Sr remained near the base material surface and did not penetrate beneath the surface. Therefore, the penetration depth of Sr was smaller than that of Cs.

Tritium is considered as an important fuel for fusion reaction and stainless steel is commonly used as a candidate structural material to make tritium containers, the facility, and the pipe. During long-term application, the tritium penetrates the stainless steel through adsorption and diffusion, resulting in the structure performance degradation and threatening the safety of the stainless steel for use. Many pieces of research have been conducted to study the distribution and speciation of tritium in stainless steel. The tritium is mainly in the form of tritiated water vapor (HTO) and tritiated hydrogen (HT). The distribution of tritium varies based on the tritium storing condition and place. Hirabayashi et al. [23] investigated the sorption of gaseous tritium in 316 stainless steel. The tritium was released as HTO with a fraction of HT during gradual surface etching. The HTO mostly originated from the tritium present on the outermost surface and about 90% of it could be easily released. The rest was sorbed tightly and remained in the surface layer. Wang et al. [24] studied the distribution and forming of tritium in RAFM steel. The tritium retained in steel was mainly in the non-soluble form, whose content was significantly larger than that of soluble tritium. It was speculated the main form of tritium which existed in the defects of both steels was hydrogen isotope molecules, hydrocarbons (CH₃T), or an atomic tritium cluster trapped by a dislocation cluster. Sharpe et al. [25] also investigated the distribution of tritium in the near-surface of 316 stainless steel. The concentration profiles indicated that the adsorbed water layers contained tritium surface concentrations of 1.5 × 10¹³ Bq/cm³, which decreased by a factor of 10⁶ after etching to a depth of ~10 µm. Knowledge of tritium distribution in contaminated base materials and release is important in terms of safety of the facility’s maintenance and decommissioning. There have been some studies on the diffusion transport characteristics of deuterium and tritium in the base materials [26,27,28]; however, there are fewer reports on deuterium. The interactions of deuterium and tritium (dissolution, diffusion, and penetration) between bulk materials and the decay products of radionuclides in materials need be further studied.

Table 1. Characteristics of radionuclides in base materials.

Radionuclides	Bulk Materials	Species on the Bulk Surface	Depth Distributions	References
235U	Rubber, cable	-	Rubber tube: 6.5–35.0 µm Rubber floor: 2.1–220 µm Cable insulation material: 3.5–38.3 µm	[18]
90Sr	316L stainless steel	SrCrO4 in the oxide layer SrCO3 in the matrix	Penetrating depth of ∼150 nm in 12 M HNO3	[21]
137Cs	316L stainless steel	Cs2Cr2O7 in the oxide layer	Penetrating depth of ∼15 nm in 1 mM NaOH	[21]
	Concrete		Degraded concrete: several millimeters. Cracked concrete: >10 cm	[29]
90Sr 137Cs 90Y	Concrete	137Cs does not interact with cement hydrates. Sr interacts with cement hydrates through ion exchange with Ca. High pH of the cement hydrate forming a hydroxide of low Y solubility.	137Cs: 15 mm with concentration of approximately 1 × 10−8 mol/kg. 90Sr: 3 mm with concentration of approximately 1 × 10−7 mol/kg. 90Y: on the surface of the mortar.	[30]
3H	316 stainless steel	Majority in HTO; minority in HT	HTO: ~0.01 µm with amount of 1015 molecules/cm2	[23]
	CLAM steel CLF-1 steel	soluble tritium; non-soluble tritium	-	[24]
	SS316 stainless steel	-	<10 µm with concentration of 1.5 × 1013 Bq/cm3 in adsorbed water layer and ~5µm with concentration of <107 Bq/cm3	[25]
	Low-carbon steel	-	Painted steel: <40 μm with activity concentration of <0.4 Bq g−1. Unpainted steel: <40 μm with activity concentration of >0.4 Bq g−1 and <0.4 Bq g−1 in the 40–80 μm layer.	[31]

Table 1. Characteristics of radionuclides in base materials.

Radionuclides	Bulk Materials	Species on the Bulk Surface	Depth Distributions	References
235U	Rubber, cable	-	Rubber tube: 6.5–35.0 µm Rubber floor: 2.1–220 µm Cable insulation material: 3.5–38.3 µm	[18]
90Sr	316L stainless steel	SrCrO4 in the oxide layer SrCO3 in the matrix	Penetrating depth of ∼150 nm in 12 M HNO3	[21]
137Cs	316L stainless steel	Cs2Cr2O7 in the oxide layer	Penetrating depth of ∼15 nm in 1 mM NaOH	[21]
	Concrete		Degraded concrete: several millimeters. Cracked concrete: >10 cm	[29]
90Sr 137Cs 90Y	Concrete	137Cs does not interact with cement hydrates. Sr interacts with cement hydrates through ion exchange with Ca. High pH of the cement hydrate forming a hydroxide of low Y solubility.	137Cs: 15 mm with concentration of approximately 1 × 10−8 mol/kg. 90Sr: 3 mm with concentration of approximately 1 × 10−7 mol/kg. 90Y: on the surface of the mortar.	[30]
3H	316 stainless steel	Majority in HTO; minority in HT	HTO: ~0.01 µm with amount of 1015 molecules/cm2	[23]
	CLAM steel CLF-1 steel	soluble tritium; non-soluble tritium	-	[24]
	SS316 stainless steel	-	<10 µm with concentration of 1.5 × 1013 Bq/cm3 in adsorbed water layer and ~5µm with concentration of <107 Bq/cm3	[25]
	Low-carbon steel	-	Painted steel: <40 μm with activity concentration of <0.4 Bq g−1. Unpainted steel: <40 μm with activity concentration of >0.4 Bq g−1 and <0.4 Bq g−1 in the 40–80 μm layer.	[31]

4. Recent Decontamination Technologies

Decontamination is a pretreatment procedure before conducting the decommissioning and dismantling operations. It allows the removal of or reduction in radioactive contamination in or on structures or systems. It can minimize the total amount of materials to be sorted or processed as radioactive waste, and offers the potential to greatly expand the amount of recycled material. Recently, many reagents and sophisticated techniques have been developed for decontamination. They are basically grouped into physical and chemical processes according to the mechanism.

4.1. Mechanical Methods

Mechanical decontamination techniques are physical methods that are based on different mechanical forces, such as scraping, brushing, wiping, and scrubbing to release radionuclides through mechanical agitation or physical removal. They mainly include high-pressure liquid jetting (high pressure and ultra-high pressure) [32,33], dry ice blasting [34,35], laser-based cleaning [10,36], nonthermal plasmas [37], low-pressure arc plasma [38], supercritical fluid cleaning [39] and air-blast cleaning [40].

4.1.1. High-Pressure Liquid Jetting

Among these mechanical methods, the high-pressure liquid jetting is a process employing water (HPWJ) or liquid nitrogen (HPLN) pumped through a rotary nozzle at pressures of 10–100 MPa (Figure 5). These techniques have been proved effective to clean substrate surfaces. Nedyalkova et al. [41] investigated the decontamination efficiency of water jetting for stainless steel surfaces and the potential implications for steel recontamination. After decontamination, water jetting at 45° more efficiently removed the passive layer than that at 90°; thus, it was more suitable for Sr decontamination treatment. The mechanism of this method for material removal is considered to be a complex process including plastic deformations and crack initiation, pit development, granular erosion, and water permeation into cracks and pores. The HPWJ is an effective, customizable, reliable process and uses environmentally friendly water as a cleaning medium. However, the main problem arises from the resulting contamination of the water, which can lead to deep cross-contamination, especially in cracks and joints. During the study of Nedyalkova et al. [41], there was a linear correlation between decontamination and water pressure at a 90° incident angle. It has the potential to create larger volumes of liquid waste that would require effective management.

4.1.2. Dry ice Blasting

Dry ice blasting is a process applying CO₂ pellets as an impact medium for removing surface contaminants. The solid pellets sublime directly after impact on the surface, leaving no solid residue after blasting. It has been extensively used for cleaning surfaces, removing paint, or stripping contaminants from a surface. Masserant [42] designed a system for cleaning components contaminated with radioactive materials which uses dry ice blasting in a chamber. The bead reaction and thermal quenching by dry ice blasting was used to remove oxide layers formed on the stainless steel and carbon steel surfaces. The EDX analysis confirmed that oxide layers formed on the samples were successfully removed. Zeng et al. [43] prepared LaMgAl₁₁O₁₉ coatings with implanted vertical microcracks utilizing the ‘quenching’ effect of in situ dry ice blasting during the atmospheric plasma spraying process. Najafabadi and Khajavi [44] produced micro cracks on the primary wall of raw cotton through a thermal shock process by using frozen carbon dioxide. The blasting mechanism of dry ice was based on the cooling down of the surface up to a temperature of −78.51 °C, which caused cracks and separation between the interface of different layers. Dry ice blasting is a simple, nonabrasive cleaning method. Compared to other blast cleaning processes, it does not accumulate contaminants in the waste stream and thus significantly reduces the amount of secondary waste for disposal. However, some painted surfaces can be damaged by the process due to its high pressure.

4.1.3. Laser-Based Cleaning

Laser-based cleaning is removing contaminants deeply embedded within the material surface by physically stripping the entire upper layer of the base material using laser irradiation. It is a noncontact/nonabrasive process that can replace the use of chemicals or abrasive cleaning. The cleaning process can be conducted remotely by a fiber-optic beam delivery system, and the laser head can be manipulated by robots, thus avoiding exposing workers to a potentially radioactive area. Typical mechanisms of laser cleaning are evaporation/spallation/ablation/shockwave generation. The resulting mechanisms cause the removal of the contaminants without causing any damage to the substrate. Demos et al. [45] investigated the mechanisms governing the interaction of metallic particles with nanosecond laser pulses. The formation of the pit was attributed to the interaction of the substrate with the plasma. A dependence of pit size on the pulse shape and pulse durations was observed. Within the area of laser cleaning, several working mechanisms and processing types have been proposed, including dry laser cleaning, wet laser cleaning, angular laser cleaning, shock laser cleaning, and hydrodynamics-based laser cleaning. In wet cleaning [46], the thin layer of liquid is sprayed onto the porous encrustation prior to laser irradiation. When laser irradiation starts, both the encrustation and liquid are heated. Once the temperature reaches the boiling point of liquid, it is vaporized immediately. If the pressure is larger than the reduced tensile strength of encrustation, the encrustation may be stripped away locally. Related with the laser shock cleaning process, laser-induced breakdown (LIB) occurs in a liquid solution and the shock wave generated in the liquid enhances the cleaning process significantly. However, it is difficult to control the purity of the cleaning solutions as the particle size decreases. Therefore, much effort has been devoted to dry laser cleaning due to its simplicity. AJ Potiens Jr et al. [47] studied the decontamination of the ²⁴¹Am-contaminated metal scrap generated in the treatment of radioactive lightning rods. The laser used was a nanosecond Nd:YAG, operating in 1064 nm, with a pulse duration of 5 ns and 300 mJ per pulse. It confirmed that this process was effective and generated the lowest volume of secondary waste. The secondary wastes generated were only the stripped paint and the HEPA filters. Greifzu et al. [10] applied a Nd:YAG laser with pulse width of 105–230 ns and wavelength of 1064 nm during epoxy paint surfaces’ cleaning. The effects of laser fluence and pulse duration on the decontamination factor was studied. The optimized parameters enabled the selective removal of the decontamination paint without any heat impact to the metallic substrates. The reduction in ⁶⁰Co, ¹³⁷Cs-137, and ⁹⁰Sr-90 concentrations after decontamination were 62%, 98.7%, and 96%, respectively. The commonly used lasers include CO₂ laser, Nd: YAG laser, excimer laser, and diode laser. It has been proved that the laser intensity, laser frequency, laser angle, laser scanning speed, and laser peak power have effects on the decontamination factor. Recently, there have been many studies on the laser-based decontamination of different materials (stainless steels in loop cooling system, high-level pipes for fluid transport, contaminated reactor components) in Japan, South Korea and France, including the Japan Atomic Energy Agency, FUGEN Decommissioning Engineering Center, Osaka University, French Alternative Energies and Atomic Energy Commission, etc. In China, some research on contaminant cleaning using low-power laser were conducted by institutes, such as the China Institute of Atomic Energy, Nuclear Power Institute of China, and China Academy of Engineering Physics. Li et al. [48] investigated factors, pulse width, gas pressure, sweep path, and scan times on the decontamination depth and roughness. The results showed that the laser width was proportional to the single removal depth and the shading roughness. The scanning path had effects on the removal efficiency and roughness of the shading. The auxiliary gas cannot effectively deepen the decontamination depth and improve the shading roughness. Hu and Wang [49] studied the effects of laser pulse frequency, laser power density, and laser cleaning rate on the laser cleaning effect of cerium oxide coating on the stainless steel. There was no significant cleaning effect with pulse frequency of 20–100 kHz. The optimal cleaning parameters were determined as a laser energy density of 4.67 W/mm, and cleaning rate of 10 mm/s. Ma et al. [50] also investigated different parameters such as laser intensity, frequency, angel, and scanning speed in the decontamination factor. The decontamination factor of 216 and peeling depth of 15.86 μm were achieved with a radiation intensity of 7 J/cm², pulse frequency of 10 Hz, scanning speed of 1.0 mm/s, and irradiation angle of 15°. In recent years, laser-based cleaning has attracted much attention due to its advantages of precise treatment, high selectivity, and flexibility in comparison with conventional cleaning techniques. However, further studies need to be conducted to select the proper laser facility and parameters, and design protective devices and waste collection systems during cleaning according to the radionuclide characteristics and practical demand.

4.1.4. Nonthermal Plasmas

Plasma is usually generated by electrical discharges and can primarily be divided into thermal (hot) or nonthermal (cold) plasmas according to their gas temperature. In thermal plasmas, the gas temperature can reach several thousand degrees Kelvin, while in the nonthermal case the temperature is close to ambient temperature. Since thermal plasma has a very high temperature, it would melt substances rather than perform any chemical etching. By contrast, nonthermal plasmas generate plasma at atmospheric pressure while maintaining their nonthermal nature [51]. Windarto et al. [52] were the first to use atmospheric pressure nonthermal plasma for radioactive decontamination. They applied CF₄-O₂-based microwave discharge to remove radioactive CoO₂ from the stainless-steel surface. After that, other groups worked on this, confirming that atmospheric pressure nonthermal plasma etching with CF₄-O₂ plasma mixture was efficient for surface decontamination [53,54]. Kar et al. [55] showed that it is very much possible to make a microwave-based atmospheric pressure plasma jet device for radioactive decontamination, and only 100 watts of microwave power are sufficient to remove 92% Pu from solid surfaces. Petrovskaya et al. [56] proposed a dry method by the ion sputtering of the nano- and micro-sized radioactive layers from the contaminated surface. The nonthermal plasma technique is of a high efficiency and capable of cleaning a wide range of contaminants in a few seconds. However, the initial cost for a facility is slightly higher.

4.2. Chemical Methods

Chemical methods are mainly based on reactions such as dissolution, oxidation/reduction, complexation, and sequestration to remove contaminants from the surface. This method mainly includes reagent washing, foam decontamination, chemical gel, strippable coating, electrochemical gel decontamination, etc. Reagents used for chemical decon approach include water alone or with soap, surfactants, acids, bases, chelating agents, or redox changing agents. Foams, gels, or pastes are used to provide a longer contact time and thereby enhance removal.

4.2.1. Reagent Washing

Reagent washing is a simple decontamination method, which is more effective for smooth nonporous surfaces. Many decon solutions have been applied, the use of which depends on the contaminant and surface chemistry, as well as the secondary waste generated. Typical reagents used in decontamination solutions include water, detergents and surfactants, acids, chelators, redox agents, foams and gels, and hybrid and proprietary solutions (

Table 2. Common reagents used in decontamination.

Type	Species	Advantages	Disadvantages	References
Water	Alone or with soap	Safe, inexpensive, and few handling problems	Low cleaning efficiency	[59,60]
Detergents, surfactants	Anionic, cationoid, nonionic	Safe, mild	Limited effectiveness by themselves	[61,62,63]
Alkali	NaOH, KOH	Relatively safe and nontoxic	Limited working range	[64,65]
Chelators	Oxalic acid, citric acid and ethylenediaminetetraacetic acid	Relatively safe and nontoxic	Cost and limited working range	[66,67,68]
Inorganic acids and salts	HCl, HNO3, H2SO4, H3PO4	Being inexpensive and readily available	Compatibility issues and can cause fires with incompatible materials	[57,69]
Organic acids and salts	Acetic acid, citric acid	Low corrosion and safer handling	Higher cost and slower reactivity	[57,70]
Redox agents	KMnO4, K2S2O8, H2O2	Inexpensive and few handling problems	Requires more skilled workers and good engineering/chemistry support	[71,72,73]
Foams and gels	Foam, gel	Improving contact times	Poor penetrating ability	[74,75]

). Among them, water is available for most ionic compounds and mainly applied to smearable contamination. Its decontamination efficiency can be improved by increasing temperature or adding wetting agents and detergents. Most commercial detergents have a detergent acting as a wetting agent or surfactant. While detergents have limited effectiveness by themselves, they are effective at enhancing other decon solutions. Surfactants can decrease the surface tension and increase liquid contact with the contaminated surface. The role of acids is to react with and dissolve metal oxide films containing contaminations or to etch the base metal and release the contaminant. Compared with inorganic acids, the organic acids offer advantages of safer handling and the ability to sequester contaminants. Kim et al. [57] developed a washing–electrokinetic technology with the combination of inorganic and organic acids. The concrete was first washed with hydrochloric acid to decompose CaCO₃ and thus decrease the pH of concrete. The ⁶⁰Co and ¹³⁷Cs removal efficiency were 85% and 76.3% at this stage. Then, the electrokinetic method was applied with acetic acid with ⁶⁰Co and ¹³⁷Cs removal of 99.7% and 99.6%, respectively. Chelation techniques are best used on nonporous surfaces and generally applied to fixed contamination that is not readily removed by simpler methods. Complexing agents are often used in combination with detergents, acids, and oxidizing agents. Reduction and oxidation agents are used to change the oxidation state of a metal and make it more soluble or more conducive to other decon methods. Foams and gels are used as carrier media for other decontamination agents. They have little decontamination ability by themselves but can enhance other agents’ efficiency by sticking to a surface and providing longer contact times. Hybrid and proprietary solutions can increase the decontamination factor over conventional washes. These systems combine several solution types and use gels, foams, pastes, and combinations of each for delivery and enhanced efficiency. Argonne National Laboratory (ANL) developed a Supergel system that incorporated nanoparticle technology. Idaho National Laboratory (INL) revealed a long-lasting foam and clay paste system that remained in place for extended times [58].

4.2.2. Foam Decontamination

Foam decontamination, also known as decon foam, is achieved by spraying the detergents and wetting agents with hydrophilic groups or hydrophobic groups with a high pressure [76]. The foam, as a carrier for chemical decontamination agents, is formed on the facility surface, which collapses after decontamination and drains away. It produces low-volume secondary waste and has been applied to a series of large carbon steel valves having complex internal configuration. Yoon et al. [77,78,79,80] prepared a foam for the decontamination of stainless-steel specimens contaminated with radionuclide ⁶⁰Co. The Zonyl TBS and mesoporous silica nanoparticles combined with chemical reagents had a significant effect on the decontamination efficiency. Approximately 73% of the ⁶⁰Co was removed, close to that obtained with the chemical decontamination solution alone, i.e., 80%. As compared to the use of chemical decontamination solution, the decontamination foam decreased the secondary waste by 80%. To overcome the low efficiency for conventional water-based foam in winter, Zhang et al. [81] fabricated an antifreeze foam using a biomass-based surfactant, a polysaccharide foam stabilizer, and an antifreeze agent. With the temperature dropping from 10 to −10 °C, the viscosity of the foam solution significantly increased, and the half-life of the foam increased from 181.5 to 2758 min. It is worth noting that decontamination efficiency may be decreased if foams are unstable. Therefore, it is the key issue for a successful decontamination process to stabilize the decontamination foam. Improving foam stability can lead to the increase in decontamination time, which is conducive to improving the decontamination efficiency. Sonn et al. [75] used coupling agent dimethyldichlorosilane (DMDCS) to modify the silica nanoparticles, and the foam stability reached the maximum with a DMDCS concentration of 0.025 wt%. This may be attributed to the fact that hydrophobic silica particles adsorbed around the foams and prevented foam coalescence and disproportionation.

4.2.3. Chemical Gels

The use of chemical gels aims to overcome problems associated with chemical-based decontamination techniques, such as reagent baths, foaming solutions, or solvents. The gels are commonly prepared by dispersing thickening agents, such as silica or alumina particles in solution, forming a gel-like suspension. The excellent rheological properties for decontamination gels allow them to be sprayed and remain attached to surface. This allows the implementation of this technique over large surfaces at large scale. The gels crack after drying, forming non-dust flakes where the contaminants are trapped and are easily removed by brushing or vacuum cleaning (Figure 6). It offers advantages of safe handling, high penetration, and small volumes of secondary waste [74,82]. Gurau and Deju [83] used commercial DeconGel™ 1101 to remove ⁶⁰Co and ¹³⁷Cs radionuclides during the decommissioning of a nuclear research reactor. The decontamination process was more efficient for nonporous materials and the decontamination factors could reach more than 90%. Yang et al. [84] developed a polyvinyl alcohol–borax complex-based spray coating containing adsorbents (Prussian blue, bentonite, and sulfur-zeolite) for the decontamination of ¹³⁷Cs-contaminated surfaces. The gel-like coating adhered to vertical surfaces with a ¹³⁷Cs removal efficiency of 56.9%, as compared with 27.2% for DeconGel. In addition, the coating could be easily removed by rinsing with water leaving no residue. Moore et al. [82] prepared a nitric-acid-loaded polymer hydrogel with a high removal of ¹³⁷Cs and ⁹⁰Sr on stainless steel. Yang et al. [85] prepared a strippable hydrogel composed of polyvinyl alcohol and phenylboronic acid grafted alignate for the decontamination of ¹³⁷Cs-contaminated surfaces. It displayed high removal efficiencies of 91.61% for painted cement, 97.505% for aluminum, 94.05% for stainless steel, and 53.5% for cement, 2.3 times higher than that of commercial Decongel. The adsorbent can be separated from the used hydrogel by a simple magnetic separation, which can reduce the waste disposal cost.

The chemical methods possess easy application and increased contact time. In addition, they can reach remote and hidden areas. However, it may require repeated applications to achieve maximum effectiveness.

4.2.4. Strippable Coating

Some methods are a combination of chemical and mechanical or a hybrid of the two. Strippable coatings use chemical and adhesive methods to remove the contamination from the surface and again require mechanical peeling of the coating [86]. Wang et al. [7] prepared a strippable coating using acrylate emulsion as the main film-forming agent and lauryl sodium sulphate as surfactant. The decontamination rate reached 92.26% for uranium dust on the concrete surface with a dosage of 2.5 kg m⁻². Pulpea et al. [87] employed a biodegradable strippable coating to surfaces contaminated with ⁶⁰Co, ¹³³Ba, ¹³⁷Cs, and ²⁴¹Am. Up to 95% of decontamination factors were obtained for these radioactive isotopes. Pozo et al. [17] explored the feasibility of applying chitosan gels with or without Fe₃O₄ nanoparticles to deal with radioactive contamination. A removal efficiency of 85% was achieved for noncompactible waste contaminated with uranium. For the strippable coating method, it has no airborne contamination and secondary liquid waste. However, it is best suited for smaller decontamination activities and only worked for easily removed contaminants.

4.2.5. Electrochemical Method

Electrochemical decontamination includes electrolysis, electrophoresis, and electro osmosis [88], in which processing the contaminated surface is immersed in a certain electrolyte as the anode. The electrolysis corrodes the anode, so that the nuclide pollutants in the metal surface layer are dissolved into the electrolyte solution achieving the purpose of removing nuclides. Compared to other chemical decontamination technology, the solution can be reused by filtering and adding an electrolyte. It is suitable for deep decontamination of carbon steel, stainless steel, aluminum, and other metal surfaces produced during the decommissioning of nuclear facilities. Pavlyuk et al. [89] investigated the electrochemical decontamination of irradiated nuclear graphite in an acid medium. The reduction in ⁶⁰Co activity by a factor of 2–10 and Cs activity by a factor of 7–100 was achieved. Lu et al. [88] coupled electrochemical decontamination with ultrasonic technology, and the decontamination effect was compared with that for individual electrochemical decontamination. The results indicated that the ultrasonic-assisted electrochemical decontamination had advantages such as good decontamination efficiency, simple equipment, fewer chemical reagents, etc. Pujol et al. [90] used the electro-coagulation technique to remove uranium from stainless steel. The removal efficiencies of 90% uranium were obtained in using a molar solution of H₂SO₄ as a support electrolyte and a potential of 2.4 V. The electrochemical method has a high efficiency with a small volume of secondary waste. However, the saturated solutions require appropriate processes for final treatment.

5. Perspective

Each technique has advantages and disadvantages in certain scenarios (

Table 3. The comparison of different decontamination techniques.

Techniques	Advantages	Disadvantages/Limitations
High-pressure water jetting	1. Using environmentally friendly water as a cleaning medium. 2. No chemical solvents or cleaning agents necessary. 3. Can be used for places that are hard to reach and for complex structures. 4. Fast, effective, and customizable process.	1. Minimal material removal on the base material. 2. Having the potential to create larger volumes of liquid waste that would require effective management.
Dry ice blasting	1. A simple, nonabrasive cleaning method. 2. The solid pellets sublime directly after working, significantly reducing the amount of secondary waste for disposal. 3. No size limitations to the parts to be cleaned.	1. Permitting direct release of CO2 to the environment via sublimation. 2. Some painted surfaces can be damaged by the process. 3. Requires large volumes of air and corresponding air compressors.
Laser-based cleaning	1. Precise treatment, high selectivity, and flexibility. 2. High speed, the possibility of remote control, minimal risk for personnel, less manual labor.	1. Larger volume of particulate matter, including aerosol particles are generated. 2. High cost and high noise emission. 3. Required specialized equipment, safety measures, and trained personnel.
Nonthermal plasmas	1. Ability to operate at atmosphere pressure and room temperature. 2. Capable of cleaning a wide range of contaminants in a few seconds.	1. Complicated technology with numerous factors affecting the efficiency. 2. High initial cost for facility.
Reagent washing	1. Relatively safe and nontoxic. 2. More effective on smooth nonporous surfaces.	1. Cost and limited working range. 2. Having the potential to create larger volumes of liquid waste that would require effective management.
Foam decontamination	1. Low-volume secondary waste. 2. Lengthens the contact duration of the medium.	1. Lifetime of foam is limited. 2. Care must be taken when flushing.
Chemical gels	1. Easy application and increased contact time. 2. Can reach remote and hidden areas. 3. Minimal secondary waste generation.	1. May require repeated applications to achieve maximum effectiveness. 2. The formulation of colloidal gels are complex.
Strippable coating	1. Produce a single solid waste. 2. No airborne contamination. 3. No secondary liquid waste.	1. The spray gun nozzles clog. 2. Best suited for smaller decontamination activities. 3. Only works for easily removed contaminants.
Electrochemical method	1. Quick processing time, high efficiency. 2. Small volume of secondary waste, thanks to the recycling/regeneration.	1. Even with recycling, the saturated solutions require appropriate processes for final treatment. 2. Protection against chemical hazards with high corrosive products (acid, gas, etc.) and by products (H2, HF, etc.).

). The selection of proper technologies in practice mainly depends on: the type of facilities, involved isotopes, activity level of the equipment and parts, and physical/chemical properties of the equipment/parts to be dismantled. The perspectives, such as safety, efficiency, cost-effectiveness, waste minimization, and feasibility of industrialization need be taken into account in the selection. Among the mechanical methods, the laser-based cleaning is high-speed, having automation ability and thus is promising, although creates a dust and results in airborne contaminant hazard. Further studies, such as selecting a proper laser facility, optimizing operating parameters, and designing a high-efficiency dust collection system (e.g., bag filter, high-efficiency particulate air filter) could be studied to promote its large-scale application. Among the chemical methods, the chemical gel is good for decontaminating complex shapes, and vertical and overhead surfaces. In addition, they can enhance other decon agents’ efficiency by allowing them to stick to surfaces and improving contact time. However, the formulation of colloidal gels is complex, and no gel type is useful for all contaminants. Therefore, novel and versatile gels need be developed to enlarge their application field. As far as the current technologies are concerned, there is no single one to address all kinds of problems. Combining various decontamination methods will often have better results. Therefore, a reasonable and effective combination of these decontamination methods has become the main direction for the development and application of decontamination technologies for decommissioning nuclear facilities in the future.

6. Conclusions

Decontamination is a prerequisite to the decommissioning and dismantling of contaminated facilities, removing the contaminants from the base material’s surface, reducing dose level in the installation, and minimizing the potential for contamination spreading during further dismantling. The decontamination methods mainly include mechanical and chemical decontaminations, which are diverse from the perspective of decontamination mechanism, application scope, the cost and volume of secondary waste generated, etc. Among them, the mechanical decontamination methods are generally more effective than chemical decontamination but requires the surface to be readily accessible. Corners, cracks, and crevices are difficult to decontaminate using mechanical techniques. These approaches also tend to create dust, resulting in airborne contaminants. In addition, laser-based cleaning has remote operation capability, which can reduce time consumption and worker exposure to irradiation. Dry ice blasting is a simple, nonabrasive cleaning method. The solid pellets sublime directly after impact on the surface, significantly reducing the amount of secondary waste for disposal. Regarding the chemical methods, they have the advantages of allowing the treatment of complex materials (e.g., hidden parts, inside parts of tubes, etc.), rich practice experience, and removing almost all radionuclides with the proper chemicals. However, there are some drawbacks, such as different isotopes require different solvents, the poor performance on porous surfaces, and corrosion of the surfaces being cleaned. Chemical gels and strippable coating offer the advantages of easy application and increased contact time which aid performance, no airborne contamination, and they can reach remote and hidden areas. The problem is that the formulation of colloidal gels are complex, which may require repeated applications to achieve the maximum effectiveness.