Ion exchange refers to the phenomenon in which ions in a solution are exchanged or adsorbed onto exchangeable ions (active groups) in another phase, such as a solid-phase ion exchange material. Leveraging the high selectivity of ion exchange materials for specific types of ions, this method is widely used to remove radioactive nuclides existing in solution either in ionic or colloidal form, thereby achieving the effect of purifying waste liquids. Ion exchange is suitable for treating relatively clean radioactive liquids with low levels of soluble substances (typically less than 4 mg/L in insoluble content and less than 1 g/L in soluble content). The decontamination factor for radioactive liquids is generally between 10 and 100 (per single ion exchange bed).

Ion exchange technology is commonly used in pressurized water reactor nuclear power plants for preparing make-up water for the reactor primary loop, purifying primary coolant and fuel storage pool water, adjusting the concentration of chemical additives, treating blowdown water from steam generators, and handling radioactive liquid waste. Based on the matrix of ion exchange materials, these materials are mainly divided into two major categories: organic ion exchange materials (such as various types of ion exchange resins) and inorganic ion exchange materials (such as clay minerals and zeolites). In the treatment of radioactive liquids in pressurized water reactor nuclear power plants, the ion exchange materials most commonly used are artificially synthesized, granular organic ion exchange resins.

When treating high-salinity waste liquids, non-radioactive salt ions occupy a large number of functional groups in ion exchange resins, thereby increasing treatment costs and reducing efficiency. Therefore, the ion exchange method is more suitable for waste liquids with lower salinity and typically requires pretreatment of the incoming liquid to ensure normal operation of the ion exchange resin bed and maximize its service life. Currently, many ion exchange processes for treating radioactive waste liquids are combinations of several methods, such as coagulation-ion exchange, coagulation-electrodialysis-ion exchange, distillation-ion exchange, reverse osmosis-selective adsorption, ultrafiltration-reverse osmosis-electrodialysis, etc. These combined processes can achieve high decontamination factors and are relatively economical and reasonable^[5].

Evaporation and drying is currently one of the primary methods used in nuclear power plants for treating radioactive wastewater. During this process, the vast majority of radionuclides exist in the form of crystalline salts. The evaporation and drying method boasts high decontamination factors and removal rates, making it suitable for treating high-, medium-, and low-level radioactive wastewater. However, it also has certain drawbacks: it is not appropriate for wastewater containing volatile radionuclides; complete vaporization of all water results in significant thermal energy consumption, thereby increasing operational costs; and as the concentration of salt ions increases, the evaporator becomes prone to corrosion and scaling, with additional safety concerns such as explosions at high temperatures^[6]. Given the widespread use of evaporation and drying in nuclear power plants both domestically and internationally, some progress has been made in addressing its limitations. Shao Yanjiang et al.^[7]studied the effects of pH, evaporation temperature, Ca²⁺concentration, and feed flow rate on the scaling rate of evaporators, and developed an immersion-based acidic descaling method, achieving good results. Li Ming et al.^[8]improved the design and process structure of evaporators, increasing the system's decontamination factor to the order of 10⁵to 10⁶. To address the high energy consumption associated with traditional evaporation techniques, utilizing low-cost heat sources such as geothermal and industrial waste heat for low-temperature evaporation has also become a research hotspot. Guo Lingfei et al.^[9]constructed a self-made packed-tower low-temperature evaporation experimental setup, achieving a wastewater volume reduction ratio of 4% and an ion removal efficiency of 89.5% when the evaporation rate was 80.6 kg/h. Zhang Weiyu and Jin Chang et al.^[10-11]used a hydrophobic microporous membrane-based air-gap membrane distillation device to treat actual low-level radioactive liquid waste from the nuclear industry, with total α activity concentration of 868 Bq/L and total β activity concentration of 5610 Bq/L. The average retention rates of trace amounts of Sr²⁺and Cs⁺in the wastewater were both greater than 99.99%, and the decontamination factor reached above 10⁴. Moreover, this method exhibits low sensitivity to flow rate and temperature differences.

Chemical precipitation primarily involves adding chemical reagents that combine with radioactive ions in the waste liquid to form precipitate products, thereby transferring the radioactive nuclides from the waste liquid into a small volume of sludge, achieving the effect of wastewater purification. The main factors affecting the efficiency of chemical precipitation include the selection of precipitant, solution pH, temperature, and the type of radioactive ions. Additionally, the amount of precipitant used must be appropriate for the types and concentrations of radioactive metal ions present in the waste water to achieve effective purification of radioactive wastewater. Chemical precipitation is simple, cost-effective, capable of removing a wide range of elements, and benefits from mature technology and equipment; it is often employed for purifying large volumes of waste liquids with less stringent decontamination requirements and as a pretreatment technique for membrane processes. If the wastewater contains short-lived radioactive nuclides, a large amount of stable isotope carriers must be added to meet discharge standards, resulting in significant precipitate production. Moreover, some precipitates tend to form colloids, making solid-liquid separation difficult, necessitating the addition of flocculants to ease this process. According to literature, the United States has developed a chemical additive combined with filtration and ion exchange technologies, which has been adopted by over 20 nuclear power plants, treating 265,000 m³annually, achieving a decontamination factor of 10⁴, with each cubic meter of medium capable of treating 800–1,000 m³of wastewater^[12]. Luo et al.^[13]improved the efficiency of Sr-90 precipitation using the PCM (Pellet coprecipitation micro-filtration) mechanism. When the strontium concentration was 12.0 mg/L, the decontamination factor reached 577, the precipitate separated easily from the liquid phase, and the concentration factor was 1958. Fang Xianghong et al.^[14]studied the removal of simulated radionuclides Cs, Sr, and Co using aluminum chloride, ferric sulfate, ammonium phosphate, and mixtures of these three reagents. The results showed that at a pH of 8, all precipitants exhibited good removal efficiency for Co and Sr.

Membrane treatment technologies for radioactive wastewater mainly include microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Microfiltration primarily treats groundwater contaminated with radioactivity, with membrane pore sizes and separation ranges between 0.1 and 1 μm. Ultrafiltration can remove proteins and smaller colloids but cannot retain ions, with membrane pore sizes and separation ranges between 0.01 and 0.1 μm. Nanofiltration can retain small-molecule substances and high-valence ions while allowing monovalent ions and water molecules to pass through, with membrane pore sizes and separation ranges between 0.001 and 0.01 μm. In nuclear power plants, nanofiltration technology is often used for the recovery of boric acid. Reverse osmosis technology has high decontamination efficiency and produces less secondary waste, commonly used in seawater desalination and the treatment of wastewater such as ground drainage from the nuclear island, reactor shutdown maintenance wastewater, and spent resin rinse water. According to the literature, the average decontamination factor of reverse osmosis membranes for Co-60 and Co-58 is around 150, for Sb-125 it reaches an average of 1000, and for Cs-134 and Cs-137 it is about 10. Reverse osmosis membrane separation technology offers significant advantages in terms of economy and operation^[15-16]. Wei Xinyu et al.^[17]reviewed the application of reverse osmosis membranes in the treatment of radioactive wastewater from nuclear facilities both domestically and internationally, analyzed the feasibility of applying current commercial reverse osmosis design evaluation models to the removal of trace radioactive nuclides from water, and suggested conducting analytical research on aspects such as the removal efficiency of Ag-110m and Mn-54 and their compounds, the mechanism of reverse osmosis membranes in removing trace radioactive nuclides from water, and the correlation evaluation model between nuclide concentration and membrane area.

With the development of the nuclear industry, new types of nuclear fuels and equipment are continuously being introduced, leading to increasingly complex pollutants in waste liquids and a growing number of recalcitrant substances, thereby increasing the difficulty of treatment. The radioactive waste treatment processes in nuclear power plants are also constantly being upgraded; for instance, new technologies such as oxidation treatment and microbial methods, as well as combined process technologies, have become promising directions for future advancements. Nuclear power plants typically employ multiple wastewater treatment techniques simultaneously to specifically address radioactive wastewater with different chemical characteristics.

(1) Adsorption method

Adsorption methods are characterized by low energy consumption, simple operation, and high efficiency. Some studies have shown that carbon-based nanomaterials, silicon-based adsorbents, MOFs materials, and COFs materials exhibit excellent adsorption performance for radionuclides. Graphene oxide, with its unique structure, numerous surface-active groups, and high stability, has attracted widespread attention in environmental remediation. Liu Hongjuan, Li et al.^[18-19]conducted a comparative study on the adsorption performance of various graphene oxide materials and their composites for radionuclides, finding that these materials can achieve a maximum adsorption capacity of up to 718 mg/g. Solution pH, reaction temperature, solution ionic strength, and adsorption time all influence the adsorption performance. The research results of Yu Jing et al.^[20]indicate that silica gel and mesoporous silica, due to their abundant pores, large specific surface area, and uniform surface functional groups, can serve as matrix materials for inorganic adsorbents. Carbon nanotubes and graphene, which have high chemical activity, demonstrate good volume reduction effects; however, their inherent chemical toxicity introduces additional environmental pollution^[21]. MOFs materials and COFs materials represent emerging crystalline porous materials in recent years and are also ideal candidate materials in the field of membrane separation^[22]. In recent years, many studies have focused on utilizing MOFs materials for the separation and adsorption of metal ions^[23]. Due to their selectively controllable pore size and specific functional group effects, MOFs materials significantly enhance the selectivity and adsorption capacity for radioactive metal ions^[24-26]. COFs materials are organic crystalline porous polymers formed in two or three dimensions through reversible polymerization and irreversible tautomerization reactions involving light organic elements such as C, O, N, and B^[27]. Their stable structure, high surface area, and controllable pore size enable efficient treatment of radioactive waste liquids even under conditions of strong acid and intense irradiation^[28]. Currently, both domestically and internationally, significant progress has been made in using these materials to treat radioactive nuclides such as uranium^[29-31].

(2) Biological method

Microalgae adsorption technology has been a research hotspot in the field of radioactive wastewater treatment in recent years. The key to microalgae adsorption lies in screening advantageous algal strains with good adsorption performance and recovering and treating microalgae cells. Kalin et al.^[32]studied the U(Ⅵ) adsorption capacity of 23 different algal species and found that 13 strains, accounting for 56.5% of the total, had adsorption capacities ranging from 100 to 600 mg/g. Li Xin et al.^[33]carried out a screening process for superior algal strains among 11 candidate species and discovered that Scenedesmus LX1 exhibited the highest uranium adsorption capacity at 40.7 mg/g, followed by Chlorella vulgaris, which showed better settling performance during the stationary growth phase, with a settling rate of 45.3%. Jiang Zheng et al.^[34]found that amino, hydroxyl, and carboxyl groups on the cell walls of baker's yeast and Escherichia coli were involved in the chemical adsorption of I^-, with theoretical maximum adsorption capacities of 120.7 and 37.2 μmol/g, respectively. Since radioactivity can induce strong mutations in biological genetic material, potentially leading to unforeseen consequences for ecosystems, this poses a challenge in developing bioremediation technologies for radioactive pollutants.

(3) Combined treatment

With the continuous maturation of new technologies, selecting a combination of various techniques tailored to the characteristics of radioactive liquid waste often improves overall efficiency and brings significant economic and effectiveness benefits. Research in this area can generally be categorized into three types. The first category involves combinations of precipitation, adsorption, and membrane technologies. In the United States, a complete treatment process combining precipitation filtration, membrane technology, and ion exchange has gradually been developed and is already applied in several nuclear power plants^[35-36]. Ren Meng and Dulama et al.^[37-38]mentioned in their literature that adding a certain amount of flocculant to the waste liquid promotes flocculation and sedimentation. After sedimentation, the waste liquid is treated with an ultrafiltration membrane to remove suspended solids, and the resulting ultrafiltrate is further subjected to adsorption using inorganic adsorbents, reducing the specific radioactivity of the radioactive waste liquid from 2.18×10⁴to 0.29 Bq/L. Wu et al.^[39]used a combined co-precipitation and microfiltration technique to remove strontium from radioactive wastewater, designing a hydraulic rotary flow mixing reactor coupled with a microfiltration filter. Experimental results showed that the purification coefficient for strontium could reach 842 to 1000. The Radiochemistry Laboratory of China Nuclear Power Engineering and Design Institute studied the use of an ultrafiltration-reverse osmosis (UF-RO) membrane combination process for treating low-level radioactive wastewater. Results indicated that desalination rates for Cs⁺, Sr²⁺, Co²⁺, Ni²⁺, and Fe³⁺all exceeded 95%, with a decontamination efficiency reaching 95.7%^[40]. The second category involves combinations of organic and inorganic treatments. The Faculty of Nuclear Chemistry at the Czech University of Technology published research on a combined technique that uses photocatalytic degradation of organic ligands in conjunction with adsorption to remove radionuclides. The adsorbent used was either inorganic or composite, while the organic ligands subjected to photocatalytic degradation primarily included citric acid, oxalic acid, and ethylenediaminetetraacetic acid (EDTA). The catalytic material used in heterogeneous photocatalysis under UV irradiation was TiO₂ ^[41]. The third category involves combinations of non-traditional technologies. The French Atomic Energy Commission has developed a new combined plasma pyrolysis-photocatalytic oxidation technology for treating radioactive organic waste liquids. The primary method involves immersing a plasma torch into the waste liquid, breaking down organic compounds into small-molecule organic fragments, which are then further oxidized and degraded by ultraviolet light, ultimately decomposing completely into clean water and carbon dioxide. Results indicate that when the pyrolysis temperature is approximately 2700 K and the gas flow rate at the nozzle reaches 400 m/s, the degradation rate can reach 99.8%. The entire process produces neither smoke nor secondary waste liquid, making it a clean and highly efficient treatment method^[42].