Disposal method of radioactive waste. I believe everyone is familiar with radioactive waste, which is harmful to nature, and the country needs to properly handle these radioactive wastes. Next, I will take you to know more about the disposal methods of radioactive waste.
Disposal method of radioactive waste 1
introduce
The operation process of changing the physical and chemical state of radioactive waste in advance for the safe and economical final disposal of radioactive waste, including collection, concentration, solidification, storage and waste transfer.
Radioactive waste sometimes produces new waste in the process of treatment, which is called secondary waste. For example, when dealing with radioactive waste liquid, it is often necessary to treat it many times by flocculation and precipitation, ion exchange and other methods. The specific activity can reach the level of allowable discharge, and the sludge precipitation and waste resin produced in the treatment process belong to radioactive secondary waste. These wastes still need further treatment.
The treatment effect of radioactive waste is usually expressed by decontamination coefficient and volume reduction ratio. Because radioactivity can only be weakened by the decay of radionuclides, the process of radioactive waste treatment is essentially just a process of dividing radioactive waste into two parts, one part is small but concentrates most radioactive substances in the original waste, and the other part is large but has low specific activity (or radioactive concentration). The treatment goal of the latter part is to make the radioactivity reach the allowable standard, so that it can be treated as general waste in the next step, and its treatment effect is often measured by decontamination coefficient. Decontamination coefficient, also known as purification coefficient, is defined as the ratio of specific activity (or radioactive concentration) before and after waste treatment. For the former, the treatment effect is usually measured by the volume reduction rate, because its treatment goal is to minimize the final disposal volume. Volume reduction rate, also known as volume reduction factor, is defined as the ratio of waste volume before and after treatment. Volume reduction ratio usually refers to the volume ratio of solid waste before and after compression treatment or liquid waste after solidification treatment.
Collection of radioactive waste
All kinds of radioactive wastes should be collected at the place where they are produced and stored in temporary storage facilities with different receiving methods and transportation equipment. The purpose of classified collection is to facilitate separate treatment and disposal by different methods. Usually, wastes are divided into liquid, solid and gas wastes according to their physical state, and then further divided into high, medium and low radioactive wastes according to their specific activity (or radioactive concentration), which is referred to as high, medium and low radioactive wastes for short. Some special radionuclides should also be collected separately, such as tritium-containing wastes and transuranic wastes (see transuranic elements). Solid waste can also be divided into combustible waste, noncombustible waste and compressible waste.
Volume reduction of radioactive waste
Concentration and volume reduction of radioactive waste liquid include flocculation and precipitation, ion exchange, adsorption and evaporation. According to specific activity, chemical composition, waste liquid volume and treatment requirements, one or more methods can be used in combination. In general, the decontamination coefficients of evaporation method, ion exchange method and flocculation precipitation method can reach 103 ~ 106, 10 ~ 103 and102 respectively. Radionuclides in the treated raw waste liquid are concentrated in a small amount of evaporation residue, waste resin and precipitation mud. Volume reduction of solid waste is usually treated by incineration or compression. The volume reduction rate of combustible waste after combustion can reach 40 ~100; Nonflammable. The volume reduction ratio of waste cutting compression can reach 2 ~ 10.
Solidification of radioactive waste
In order to safely store and reduce environmental pollution, it is necessary to convert radioactive waste liquid or its concentrate into solid. The basic requirements of radioactive waste solidification are: stable physical and chemical properties, sufficient mechanical strength, large volume reduction rate and low leaching rate in water; The operation process is simple and easy, and the treatment cost is low. Different types of waste can be solidified by different methods, among which cement solidification, asphalt solidification, plastic solidification and glass solidification have been applied in practice.
Storage of radioactive waste
Unsolidated radioactive waste liquid and concentrate, as well as radioactive wastes such as solidified bodies for which the final disposal scheme has not been selected, should be stored in special containers at fixed locations, and attention should be paid to safety during storage to prevent leakage of radioactive wastes. Wastes with different specific activities need different storage tanks. For example, when storing alkaline medium and low level radioactive waste liquid, carbon steel storage tanks are generally used; Double stainless steel tanks must be used to store acidic high-level radioactive waste liquid. Compared with the high-level radioactive waste liquid storage tank with high activity and large heat release, there are particularly strict requirements: the material should be corrosion-resistant, the structure should be firm and reliable, and there should be ventilation and heat dissipation devices, leak detection systems, feed liquid conveying devices and so on. , it should be monitored.
Transshipment of radioactive waste
The key to the transfer of radioactive waste is the packaging container of waste. Safety inspection should be done in advance, and strict regulations should be made on the strength, shielding protection, sealing system and packaging marks of containers. Safe transportation is required to prevent radioactive waste from leaking and polluting the environment due to fire, container subversion and packaging damage.
Separation and recovery of radioactive waste
In the late 1940s, the research on separation and recovery of fission product nuclides from high-level radioactive waste liquid began. From the late 1950s to the early 1960s, some countries set up intermediate plants to separate and recycle fission products. The separation process has developed from early precipitation-extraction method to solvent extraction and ion exchange method (especially inorganic ion exchange materials). Compared with precipitation method, solvent extraction method and ion exchange method have higher recovery rate, better separation and purification effect, and are convenient for large-scale continuous operation and remote control. The following are the separation and recovery methods of various common radioactive wastes.
Strontium is a mature separation and extraction process. The organic extractant bis (2- ethylhexyl) phosphoric acid (HDEHP) was used to extract from high-level radioactive waste liquid under acidic conditions, or it was separated and recovered by ion exchange displacement chromatography.
Cesium in high-level radioactive waste liquid was separated by precipitation-extraction process in the early stage, but the radiation resistance of organic extractant was not ideal. The process of separating and extracting cesium from high-level radioactive waste liquid with inorganic ion exchange materials such as zeolite and zirconium phosphate has low recovery cost and good radiation resistance.
The process of separating and recovering promethium from high-level radioactive waste liquid is to extract and separate rare earth nuclides and transuranic nuclides with HDEHP, and then separate promethium and rare earth nuclides with ion exchange displacement chromatography.
Precious metals mainly adsorb technetium, rhodium and palladium from neutral or alkaline high-level radioactive waste liquid by ion exchange method, and then recover them with different eluents.
Neptunium 237 in transuranic radionuclide high-level radioactive waste liquid can be separated and extracted by extraction or ion exchange. When americium and curium are separated, HDEHP can be used to extract with rare earth nuclide * * * under the condition of low acidity (pH 1 ~ 2), and then the rare earth nuclide can be separated by extraction or ion exchange displacement chromatography.
Radioactive waste treatment is an important measure of radioactive waste management. The selection of treatment methods should be based on technical feasibility, economic rationality and code permission. The treatment process should prevent environmental pollution and minimize the generation of secondary waste. In addition, comprehensive utilization of radioactive waste should be actively carried out.
Treatment and preparation of radioactive solid waste
There are many kinds of radioactive solid waste, which can be divided into wet solid (evaporation residue, precipitation mud, waste resin, etc.). ) and dry solids (contaminated labor protection articles, tools, equipment, waste filter elements, activated carbon, etc.). ). More than 40% of solid waste from nuclear power plants is combustible or compressible. In order to reduce the volume and be suitable for transportation, storage and final disposal, solid waste should be burned, compressed, purified, solidified or fixed.
(1) Incineration refers to the oxidation of combustible waste into ashes (or residues). Incineration can achieve great volume reduction and weight reduction (10 ~ 100 times), and can convert wastes into inorganic substances; Avoid the danger of thermal decomposition, decay, fermentation and fire; Incineration can also recover useful substances, such as plutonium and uranium.
Incineration is divided into two categories, namely dry incineration (such as excess air incineration, controlled air incineration, pyrolysis, fluidized bed, molten salt furnace, etc.). ) and wet incineration (such as acid digestion and hydrogen peroxide decomposition, etc. ). For the incineration of radioactive waste, it is required to adopt a specially designed incinerator with adequate protective measures and maintain a certain negative pressure in the incinerator. After incineration, more than 70% of radioactive substances enter the furnace ash. Ash should be solidified or directly put into a highly integrated container for treatment.
(2) Compression Compression depends on mechanical force to densify waste and reduce waste volume. Although the volume reduction coefficient obtained by compression treatment is relatively low (2 ~ 10), compared with incineration treatment, compression treatment is simple in operation, with lower equipment investment and operating cost, so compression treatment is widely used in nuclear power plants. At present, there are many kinds of compressors used in various countries, some of which are compressed in barrels, and some are packed in barrels after flattening. High-pressure compressors with pressures of tens, hundreds and thousands of tons can compress metal scrap to near theoretical density.
(3) Decontamination Decontamination is to completely or partially remove unwanted radionuclides. Decontamination can reuse contaminated equipment or components or treat them as non-radioactive waste to reduce the amount of waste; After decontamination, the radiation level can be reduced, the harm to human body can be reduced, and it is convenient for maintenance, accident treatment or decommissioning. Decontamination activities of nuclear power plants include periodic and irregular decontamination of loops, accidental decontamination and decommissioning decontamination.
There are many decontamination methods, and different methods should be selected according to the treatment object and requirements, pollution degree and objective conditions. Commonly used methods are: ① Chemical method: using acid, alkali, redox agent, complexing agent, surfactant, corrosion inhibitor, etc. to prepare decontamination solution, foaming agent, paste, etc. The decontamination process includes soaking method, circulating washing method and spraying method. ② Mechanical methods: including vacuum cleaning, manual or mechanical wiping, high-pressure water or steam spraying, abrasive spraying (such as sand, steel grit, alumina, boron oxide, dry ice particles), ultrasonic decontamination, etc. ③ Electrochemical methods: such as electrolytic decontamination. In addition, after melting, most of the polluting nuclides enter the slag and can be reused after monitoring.
(4) Solidification of wet solids such as fixed sludge, evaporation residue and waste resin, and dry solids such as incinerator ash are dispersive substances, which are not suitable for safe transportation, long-term storage and final disposal and need to be solidified. The cured product should be a solid block. Pressure resistance, impact resistance, firmly containing radioactive nuclides, leaching resistance, radiation resistance and decay heat resistance, no corrosion to packaging containers, insensitive to bacterial erosion. Many curing methods have been developed and studied. In addition, the polluted waste filter element and the polluted equipment which are cut and disintegrated are packed in steel drums or boxes, and cement mortar or molten asphalt needs to be poured to fill the pores and fix them.
Radioactive waste treatment method 2 Radioactive waste disposal
fundamental principle
The basic principle of radioactive waste disposal is to establish a disposal system, which can effectively contain radioactive waste within a certain safe period. Even though radioactive waste will migrate and be diluted in various diffusion forms through natural processes, there is no unacceptable harm to the diluted concentration. Waste rocks in uranium mines are generally backfilled in situ, and low-level radioactive wastes with short life are generally disposed near the surface, in caves or by hydraulic fracturing and deep well water injection. The effective period of the disposal system is 300-500 years. For high-level radioactive waste, D waste, spent fuel and low-level radioactive waste with long life, some methods have been proposed, such as cosmic disposal, deep-sea disposal, seabed disposal, ice sheet disposal and rock melting disposal. However, it is recognized that the effective and feasible way is deep geological disposal, and the effective period of its disposal system should reach 6.5438+0.000 years to 6.5438+0.000 years.
Other definitions
Waste disposal refers to putting the waste into an approved facility and adopting a multi-shielding system combining engineering shielding and natural shielding to provide safe isolation for the waste to be disposed, so as to ensure:
The short-lived radionuclide contained in (1) decays to a harmless level;
(2) The release of long-lived radionuclides and other toxic substances is extremely low, and the concentration entering the environment is at an acceptable level.
Broadly speaking, disposal also includes approved air-borne or liquid effluents discharged directly into the environment, such as wastewater discharged into water and waste gas discharged into the atmosphere.