Tepco's Fukushima Daiichi nuclear power plant continues to cool water into molten nuclear fuel produced in the March 2011 nuclear accident, producing daily contaminated water with high concentrations of radioactive material. In addition, the contaminated water with high concentration of radioactive substances trapped in the reactor plant will also be mixed with the groundwater and rainwater flowing into the plant to produce more contaminated water.
From 2011 to now, the Fukushima Daiichi nuclear Power Plant has purified the radioactive substances contained in the contaminated water caused by the accident through sewage treatment facilities (including polyuclide removal equipment ALPS, etc.). The water after ALPS treatment and strontium treatment is stored in the storage tank in the plant. In addition, there are 1,073 storage tanks in the plant. As of May 18,2023, there were 1,033 ALPS treated water storage tanks, 27 strontium treated water storage tanks, 12 seawater desalination plants (RO) treated water and 1 concentrated brine, with a total of about 1.334 million tons.
Sewage treatment process: After the cooling water from the reactor core is mixed with seawater, nearby heavy oil and turbine oil are mixed in, turning the sewage into radioactive waste liquid containing oil and salt. Among them, cesium 134 and cesium 137 have strong γ radioactivity and extremely high concentration of radioactivity, so the waste liquid must be treated first to ensure the radiation safety of the staff.
The sewage first passes through the first set of cesium adsorption device (oil-water separation device + adsorption device + flocculation sedimentation device, 600 tons / day 2 series), and the second set of cesium adsorption device (pre-filtration + cesium adsorption + medium filtration device, 1200 tons / day 1 series). Then after desalination of reverse osmosis RO membrane concentration and filtration device and transfer various radionuclide to concentrated water, liquid back to the reactor condensate tank, desalting concentrated water through mobile device (600 tons / day, 1920 tons / day), RO concentrated water treatment device (500-900 tons days, evaporation crystallization), three polynuclide removal equipment ALPS (existing 250 tons / day 3 series / improved 250 tons / day 3 series 50 tons / high performance 500 tons / day) after temporarily stored in ALPS treatment water tank and strontium treatment water tank.
Polynuclide removal equipment (ALPS) is short for (Advanced Liquid Processing System), mainly using adsorption process, with selective adsorption treatment of radionuclide ions, colloids. After the removal of cesium 137, cesium 134 and desalination treatment, iron salt and carbonate were added in two steps for coprecipitation (pretreatment) to remove the components of the wastewater that may affect the adsorption effect. Iron salt co-precipitation mainly removes α radionuclide, cobalt 60, manganese 54, and carbonate co-precipitation removes calcium and magnesium. The sludge produced by iron salt coprecipitation and carbonate coprecipitation is concentrated and discharged into the HIC storage tank for centralized storage.
The ALPS preprocessing equipment
① Iron salt co-precipitation treatment equipment :
Drug addition process: after adding sodium hypochlorite and ferric chloride, add caustic soda to produce iron hydroxide to adjust the PH value, and then add the polymer as a flocculant; the main component of the agent : iron hydroxide (Ⅲ)
② Carbonate coprecipitation treatment equipment :
Drug addition process: add sodium carbonate and caustic soda to the sedimentation tank to produce divalent metal carbonate; the main components of the agent : calcium carbonate, magnesium carbonate, and the ratio of calcium carbonate to magnesium carbonate in the precipitation sludge is about 3 / 5.
Because the sludge has smaller particles than the adsorbent, it is difficult to remove water from the sludge once it is stored in HIC. The sludge is concentrated during the pretreatment process to reduce the amount of water before loading it into the HIC
What other treatment technologies for nuclear wastewater are summarized in this paper:
1, the chemical precipitation method
Chemical precipitation is a method of co-precipitate precipitant with trace amount of radionuclides in wastewater. The hydroxide, carbonate, phosphate and other compounds of radionuclide in wastewater are mostly insoluble, so they can be removed in the treatment. The purpose of chemical treatment is to transfer and concentrate the radionuclides into small volumes of sludge, so that the deposited wastewater has little radioactivity, so as to meet the discharge standard.
The advantage of this method is low cost, good removal effect of log radionuclide, can treat those non-radioactive components and their concentration and considerable flow of wastewater, the use of treatment facilities and technology have quite mature experience.
At present, iron salt, aluminum salt, phosphate, soda and other precipants are most commonly used. In order to promote the condensation process, add coagulants, such as clay, active silica, polymer electrolyte, etc. Cesium, ruthenium, iodine and other radionuclides that are difficult to remove should be removed by special chemical precipants, such as cesium, which can be precipitated with iron ferrocyanide and copper ferrocyanide. Some people use insoluble starch xanogenate to treat metal-containing radioactive wastewater, the treatment effect is good, wide applicability, radioactive removal rate> 90%, is an excellent performance of ion exchange flocculant, in the treatment of wastewater because there is no residual sulfide, so it is more suitable for wastewater treatment.
2, and the ion-exchange method
Many radionuclides are in an ionic state in water, especially radioactive wastewater after chemical precipitation, due to the removal of suspended and colloidal radionuclides, the rest are almost ionized nuclides, most of which are cations. And radionuclides exist in trace in water, so they are suitable for ion exchange treatment, and ion exchange can work effectively for a long time in the absence of non-radioactive ion interference. Most cation exchange resins have high removal capacity and large exchange capacity for radioactive strontium; phenolic Yang resin can effectively remove radioactive cesium, large porous Yang resin can not only remove radioactive cations, but also remove zirconium, niobium, cobalt and ruthenium in the form of complex through adsorption. However, this method has a fatal weakness. When the content of radionuclide or non-radioactive ions in the waste liquid is high, the resin bed will soon penetrate and fail, and the resin that usually treats radioactive wastewater is not regenerated, so once the effect is replaced immediately.
Ion exchange method adopts ion exchange resin, which is suitable for waste liquid with low salt content. When the salt content is higher, the cost of using ion exchange resin is higher than the selective process. This is mainly low selectivity resin has a large association for radionuclides. In the purification of radioactive wastewater, the method of electrodialysis can increase the utilization efficiency of the ion exchange process.
3. Adsorption method
Adsorption method is an effective method to remove heavy metal ions in water by porous solid substances. The key technique of the adsorption method is the choice of the adsorbent. The commonly used adsorbents are activated carbon, zeolite, kaolin, bentonite, clay and so on. Among them, zeolite is low, safe and easy to obtain. Compared with other inorganic adsorbents, zeolite has greater adsorption capacity and better purification effect. The purification capacity of zeolite is up to 10 times than other inorganic adsorbents, so it is a very competitive water treatment agent. It is often used as an adsorbent in the water treatment process, and has the role of ion exchanger and filter agent.
Activated carbon has a strong adsorption capacity, high removal rate, but the activated carbon regeneration efficiency is low, the treatment of water quality is difficult to meet the reuse requirements, the price is expensive, the application is limited. In recent years, various adsorbent materials with adsorption capacity have been gradually developed. Relevant studies have shown that chitosan and its derivatives are good adsorbents for heavy metal ions. After crosslinking of chitosan resin, it can be reused for many times, and the adsorption capacity is not significantly reduced. The modified segravite to treat heavy metal wastewater has a good adsorption capacity for Co and Ag, and the content of heavy metal in the treated wastewater is significantly lower than the comprehensive discharge standard of sewage.
4. Evaporation and enrichment
Evaporative concentration method has high concentration factor and purification coefficient, which is mostly used for the treatment of medium and high level of radioactive wastewater. The evaporation method works by sending radioactive wastewater into an evaporation unit and introducing heating steam to evaporate the water into water vapor, while the radionuclide remains in the water. The condensed water formed during the evaporation process is discharged or reused, and the concentrated liquid is further cured. Evaporative concentration method is not suitable for treating wastewater containing volatile nuclides and easy foam; high heat consumption and high operation cost; and potential threats such as corrosion, scaling and explosion should be considered in design and operation. In order to improve the steam utilization rate and reduce the operating cost, countries have spared no effort in the development of new evaporators, such as steam compression evaporator, film evaporator, vacuum evaporator and other new evaporators have achieved remarkable results.
5, the membrane separation technology
Membrane technology is a more efficient, economical and reliable method to treat radioactive wastewater. Because the membrane separation technology has the characteristics of good water quality, no phase change, low energy consumption and so on, the membrane technology has been actively studied.
The membrane technologies used in abroad mainly include: microfiltration, ultrafiltration, nanofiltration, water-soluble polymer-membrane filtration, reverse osmosis (RO), electrodialysis, membrane distillation, electrochemical ion exchange, liquid membrane, ferrite adsorption filtration membrane separation and anion exchange paper membrane and other methods.
6, the biological treatment method
Biological treatments includes phytoremediation and microbiological methods. Phytopremediation is a new in situ treatment technology that uses the combination of green plants and their rhizosphere indigenous microorganisms to remove pollutants from the environment.
According to the existing research results, the applicable types of biological remediation technology mainly include constructed wetland technology, rhizosphere filtration technology, plant extraction technology, plant curing technology and plant evaporation technology. The results show that almost all uranium in water can be enriched in the roots of plants.
Microbial treatment of low radioactive wastewater is a new process developed in the 1960s. There are some studies in removing uranium from radioactive wastewater at home and abroad, but most of them are in the experimental research stage at present.
With the development of biotechnology and the further study of the interaction mechanism between microorganisms and metals, people gradually realize that the use of microorganisms to control radioactive wastewater pollution is a very promising method. Using microbial bacteria as biological treatment agent to absorb and recover radionuclides such as uranium in aqueous solution with high efficiency, low cost, less energy consumption, and no secondary pollutants. Therefore, the reduction of radioactive waste can be achieved and create favorable conditions for the regeneration or geological disposal of nuclides.
7, and the magnetic-molecular method
The Mag-American Electric Power Research Institute (EPRI) has developed the Mag- Mole-cue method to reduce the production of radioactive waste such as strontium, cesium and cobalt. This method is based on a protein called ferritin, modified by using magnetic molecules to selectively bind the pollutant, removed from the solution with a magnet, and then the bound metal is recovered through a backwash magnetic filter bed. Ferritin (Fer-ritin) is a multifunctional multisubunit protein ubiquitous in organisms. This protein has dilute acid resistance (pH <2.0), dilute alkali resistance (pH= 12.0) and high temperature resistance (invariance under 70~ 75 C water temperature). With the development of ferritin research, great progress has been made in studying the new functions using its protein shell nanospace in vitro. In vitro studies have shown that ferritin has the ability to store heavy metal ions in vitro. In addition, previous studies have focused on the use of other heavy metal ions as a competition with iron ion probe to study the mechanism of ferritin storage and release of iron, and the latest research shows that can use ferritin this capture metal ions and the characteristics of the resistance, build ferritin reactor and used for field continuous monitoring the degree of water pollution by heavy metal ions. Under specific conditions in vitro, some metal nuclei such as FeS, CdS, Mn3O4, Fe3O4 magnetic iron cores and uranium nuclei of radioactive materials have been successfully assembled into the nanospace of the ferritin protein shell.
8, Inert curing method
The Penn State University and the Savannah River National Laboratory have developed a new method to process certain low-grade radioactive waste fluids into solidified bodies for safe disposal. This new process uses the low temperature (<90 C ) solidification method to stabilize the high alkaline, low activity radioactive waste liquid, namely the waste liquid into an inert curing body. Scientists called the final cure "hydroceramic" (a plain-fired porous ceramic). The final curing body, they say, is very firm, stable and durable, and holds the radionuclides in its zeolite structure, a process similar to the formation of rocks in nature.
9, zero price iron leach reaction wall technology
Perfiltration reaction wall (permeable reactive barrier, PRB) is a new method used in situ to remove contaminated components of contaminated groundwater in developed countries in Europe and the United States. PRB is generally installed in the underground aquifer, perpendicular to the direction of groundwater flow. When the polluted groundwater flow passes through the reaction wall under the action of its own hydraulic gradient, the pollutants have physical and chemical reaction with the reaction materials in the wall and are removed, so as to achieve the purpose of pollution remediation.
It is a passive repair technique that rarely requires manual maintenance and has a very low cost. As an important branch of PRB technology, Fe0- PRB technology has been studied and developed in many countries and many aspects of groundwater pollution treatment, and has achieved gratifying results in the research of reaction mechanism, the structure and installation of PRB, and the research of new active materials. Chinese scholars have begun to study the active leach wall technology represented by zero-price iron for the restoration (treatment) of radioactive wastewater from uranium tailings, and the research has achieved some results.
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