The German government's original plan to identify storage sites for nuclear waste by 2031 may now be delayed until 2046, according to a pessimistic estimate by the German Federal Radioactive Waste Company (BGE).20 In April 2023, Germany shut down its last three nuclear power plants, and the nuclear waste has all been temporarily stored in 16 temporary facilities. As of 2019, with the exception of Russia and Slovakia, more than 60,000 tons of nuclear fuel (excluding extractive and processed waste) is stored in Europe, most of it in France. Among EU countries, France has about 25% of spent nuclear fuel, Germany (15%) and the UK (14%).
Nuclear waste is generally defined as no longer required and radioactive waste from nuclear fuel extraction, production, processing, and spent fuel reprocessing, decommissioning of nuclear facilities, and from nuclear reactors. Usually referred to as nuclear waste, including low-level radioactive nuclear waste, medium-level radioactive nuclear waste and high-level radioactive nuclear waste three categories. The first is usually the nuclear power plant production process was radiated some of the items and some of the waste gas waste liquid; the second is usually generated in the power generation process of some of the waste liquid waste; the third is from the core of the spent fuel replacement, because of its utilization rate of only a few percent, with a very high level of radioactivity.
Nuclear waste is characterized by radioactivity, radiation hazards and heat release.
The radionuclides in the nuclear waste buried deep underground radiate decay heat during the decay process, which is equivalent to attaching a heat source to the underground media field. The existence of the heat source first changed the temperature distribution of the underground media field, and temperature changes by affecting the fluid viscosity, fluid density and the impact of fluid transportation, but also to make some of the chemical properties of the material changes, which directly affects the nuclide migration of the underground media field. Temperature changes may also cause the opening and closing of fissures, which affects the underground stress field. Thus, the presence of heat sources have a greater impact on the environment, but mainly concentrated in the local area of the near field.
In addition, the nuclides in the solidified body of the waste may leach out of the packaging containers, and with the migration of groundwater may enter the biosphere, thus affecting the human environment.
Nuclear waste is diverse and is categorized into radioactive waste gas treatment, radioactive waste liquid treatment, solidification treatment, and post-treatment or untreated final disposal methods.
1, radioactive waste gas treatment
Radioactive waste gas usually exists in the form of small droplets, aerosols and volatile gases. It mainly comes from the process system of the nuclear reactor and the exhaust system of each plant. The exhaust gas in the process system is mainly the noble gases neon and iodine which are higher than the release, while the exhaust system of the plant generally contains activated gases and aerosols as well as iodine-131 which is more hazardous but less in quantity. iodine-131 belongs to the long half-life, and although its content is very low compared with others, it is of high toxicity and has a concentration effect on human beings. Therefore, considerable attention should be paid to the treatment of iodine-131 in exhaust gas. The general treatment method is to first pass through the dust collector, condenser, mercury nitrate deiodization scrubber and NOx absorber, and then, in turn, pass through the second deiodization scrubber, silver-containing zeolite deiodization absorber and high-efficiency particulate filter, and finally be discharged into the atmosphere via a large chimney of more than 100 meters.
2, radioactive waste liquid treatment
Radioactive waste liquid due to its easy to soak, corrosive, not easy to store and other reasons, it is the most important treatment.
One method is the preliminary neutralization of oily wastewater, and lime milk at room temperature for stirring, until the PH reaches 10.0 ~ 10.5. In the process of neutralization, precipitation proceeds very quickly, the generation of insoluble hydroxides, are precipitated together. This method can effectively remove uranium, radium and other harmful substances in the waste liquid.
The radioactive waste liquid treatment system commonly used in nuclear power plants, is the use of ion exchange resin to deal with the process of dredging, drainage. In order to improve the service life of the ion exchange and purification efficiency, often before and after the ion exchange bed were set up pre-filter and post-filter. Pre-filter is used to remove suspended solids and solid particles; post-filter is used to block the dispersion of resin particles.
Electrodialysis treatment of low emission waste liquid method, is usually carried out in two steps, the first step is the radioactive waste liquid containing high salt first electrodialysis method to reduce the concentration of salt to a sufficiently low degree, the second step and then use ion exchange resin to remove the remaining salts and radioactive substances.
Qingshan third nuclear power plant is unique in that it adopts foreign heavy water reactor technology, is China's first commercial heavy water reactor nuclear power plant, the design of nuclear wastewater treatment is extremely innovative, with significant features. The design of the nuclear wastewater treatment is very innovative and has significant features, which can greatly reduce the amount of secondary waste generated, and greatly reduce the cost of site use and wastewater treatment costs of the nuclear power plant.
Qingshan third nuclear power plant in two storage tanks of medium and high discharge wastewater, three storage tanks of low discharge wastewater. If the waste water in the storage tanks to a certain height, the short-lived radioactive material in which the complete decay, this time to turn on the waste liquid storage tanks of the circulating pump, so that its continuous operation for more than 1 hour, so that the waste water in the storage tanks can make the full mixing. Sample and analyze the waste water in the tank, and if the indicators meet the discharge standards, the waste water can be discharged directly to the outside.
Radioactive medium wastewater after treatment, if it does not meet the criteria for direct discharge, it must again go through the purification and decontamination process. The purification circuit for radioactive wastewater is shown in Figure 1. If the differential pressure at the filter ports is abnormal during operation, this indicates that the filters are clogged and the filter elements must be replaced in a timely manner. If the absorber material fails, it needs to be replaced. Sampling and analysis is a direct reference to determine the number of purification cycles and the purification effect.
May 2011 Issue 5 "Urban Road and Bridge and Flood Control" has a low-key news, a fast, efficient adsorption, filtration of nuclear contaminated wastewater in China's development of new technologies, can be used to prevent and control the proliferation of radioactive substances iodine-131 and other radioactive isotopes of iodine, can be widely used in nuclear accidents, nuclear wastewater treatment, nuclear facilities, medical radioactive wastewater treatment. It can be widely used in nuclear accident emergency, nuclear waste water treatment, nuclear facilities protection, medical radioactive waste water treatment, etc. The high adsorption efficiency of this material on iodine-131 is shocking. The 10g of new material - catalytic biotoxic particles made by this new technology, immersed in nuclear wastewater containing 12,640 Bq/L radioactive iodine-131 for 20 min, can be adsorbed and fixed as high as 99.97% of the radioactive material iodine-131l. Tests show that the use of this new material filtration of radioactivity up to 1,850,000 Bq/L. Tests show that the use of this new material filtration radioactivity up to 1.85 million Bq / L of iodine 125 wastewater, only 5 rain, radioactive iodine 125 removal rate of up to 2%.
The core of the principle of ALPS treatment of wastewater is through the adsorption of activated carbon, titanate, ferrous cyanide, impregnated activated carbon, titanium oxide, chelating resins and resins, such as seven kinds of adsorbents. According to some studies, the contaminated wastewater from the Fukushima nuclear plant in Japan can only dilute the radioactive element "tritium" even if the water is treated with an antinuclide system (ALPS), but it does not remove it in any way. The South Korean government believes that the Fukushima nuclear wastewater after timely treatment, the contamination value still exceeds the standard by 20,000 times, and the Fukushima nuclear power plant's multi-nuclide treatment system has had as many as eight failures.
3, radioactive waste solidification
Radioactive waste solidification must achieve two purposes: one is to fix the waste liquid, and the other is to be able to confine radionuclides for a long time. In order to achieve the above requirements, the curing product should have sufficient damage resistance. The cured product should be easy to transport, store and finally dispose. The performance is usually measured in terms of irradiation stability, thermal stability, mechanical stability and chemical stability. The curing process includes processes such as evaporation and concentration of waste streams, denitrification, drying, calcination, solidification of the melt and annealing. Methods include cement, plastic, asphalt, glass and artificial rock curing.
In 1978, the world's first industrial-scale, continuously operated glass curing unit (AVM) was put into operation at the Marcoule plant in France.AVM has processed more than 2,000 m?of waste liquid. The AVM unit proved to be a success, not only in terms of process refinement, but also in terms of the lifetime of the calciner components, which exceeded 10,000 h. In France, an AVH unit was developed for the curing of highly discharged waste liquids from the reprocessing of light water reactors (LWRs) for oxidized fuel elements, which is similar in process to the AVM, with the main components scaled up in relation to those of the AVM, but with the main difference being the R7 glass curing plant at UP-2, which has been built for the purpose of curing the waste liquids from the LWRs. One of the main differences is the R7 glass curing plant built at UP-2, where a different calcination additive is used to reduce the volatilization of ruthenium (R7 uses a bath). In France, three glass curing lines were built for T7 and R7 at UP3 and UP2-800, respectively, with AVH units. Glass curing has proven to be flexible because fine particles from spent fuel dissolution and alkaline waste from melt processing are cured into the glass body.
Britain studied the crucible curing method of intermittent glass, after curing the crucible as a storage container for the glass body. This method uses the crucible's different sections to realize the evaporation of high discharge waste liquid, the calcination of the concentrate, as well as the vitreous body melting, melting section of the temperature of 1050 ° C. The crucible is used as a storage vessel. The UK later decided to adopt the French continuous AVM process for the treatment of Sellafield's waste by building the WVP unit.
Germany, the U.S. and Japan began researching the use of ceramic furnace curing in the mid-1970s. Two curing units were built in Karlsruhe, the first with a capacity of 20-40 kg/h and the second with a capacity of 100 kg/h. Waste liquid was added to the ceramic furnace along with the glass, where it was vaporized and concentrated, the glass was melted, and the glass was poured into containers.
The results of the U.S. experiments proved that the glass industry with the furnace similar to the ceramic furnace has a wide range of prospects for application. The United States based on the results of the study decided to build all the curing units to be used in a one-step ceramic furnace process. The United States in the Savannah River base established a defense waste disposal unit - high discharge waste liquid glass curing device, is the world's largest glass curing device. The United States of America's West Valley glass curing plant has been the West Valley reprocessing plant of 2300m? high discharge waste liquid treatment, *** production of 250 glass curing tanks.
Japan in the experimental study of many kinds of glass curing high discharge waste liquid method, for the curing of Tokai reprocessing plant waste preferred ceramic furnace glass curing method.
Russia has studied two methods of glass curing of liquid wastes, called two-step process and one-step process, since 1974. 1987, Russia built a glass curing facility, EP-500 Joule-heated ceramic furnaces in Mayak, and three furnaces have been put into operation, and by the end of 1999, 12,500m?of high-level wastes had been cured.
At present, not only the United States, Russia, France and the United Kingdom have constructed a high discharge waste liquid glass curing device, Japan, Belgium and India and other countries have also built such facilities.
4, disposal methods
Disposal methods include ocean dumping, near-surface disposal, geological disposal, space disposal.
Ocean dumping has been banned by the International Atomic Energy Agency, but Japan has dared to be the first. Calculations by a German marine science research organization show that radioactive material will spread to most of the Pacific Ocean in 57 days from the date of discharge, and that the United States and Canada will be affected by nuclear contamination three years later.
Near-surface disposal of the main object to medium and low-level radioactive waste is the main, burying depth from the ground within ten meters can be. The safety monitoring period is 300 to 500 years.
The main objects of geological disposal are high-level waste and medium-lived waste, which are buried in the crustal rock layer hundreds or even thousands of meters below the ground, but the geological disposal of high-level waste and medium-lived waste is still a worldwide problem.
Space disposal is the process of placing nuclear waste on a carrier rocket, launching it into space and storing it there permanently. This idea was initially proposed by the former Soviet Academy of Sciences academician Kabi rubbed in 1959. 1989, this program again by the famous American physicist Schlezinger proposed, and add a relevant argument. Actual data show that the launch of the launch vehicle accident rate is usually about 2%, in order to solve the resulting nuclear disaster, experts in the design and construction of sealed containers quite a lot of effort, and decided to use high-strength titanium steel to make the shell. The surface is then covered with multiple layers of insulation. According to the design, the sealed container is in the shape of a bullet, with a height of 3.4 meters and an inner diameter of 3 meters, divided into three isolation bins. But this kind of disposal in the current situation is only a vision.
In the early days, carbon steel tanks were used in the U.S. to store alkaline and neutral highly discharged waste streams. More than 20 of the 183 carbon steel tanks at the Hanford and Savannah River plants have been found to be leaking. Neutral waste liquids produce slurry deposits that carry most of the radionuclides. This has occurred at the Hanford Plant, the Savannah River Plant, and the West Valley Plant. At the Hanford Plant carbon steel lined storage tanks with a diameter of 23 m, depth of 6-12 m and capacity of 1800-3700 m? are used. The Hanford plant allows the waste liquid to boil in the storage tank and the heat of decay is removed by the exhaust condenser. If self-evaporative concentration is allowed, further precipitation occurs in the storage tank. These solids settle to the bottom of the tank, causing collapse boiling in tanks without internal cooling. To solve the problem of boiling, the Hanford plant uses internal air lift for agitation to mitigate the boiling after the waste liquid is full.
The United States, the United Kingdom and other countries in the storage of high release concentrated waste liquid experience has proved that the stainless steel tank storage of acidic high release waste liquid is currently the only large-scale application of intermediate storage technology. In order to prevent possible leakage, two safety measures must be taken. One is that the tanks must be housed in a stainless steel clad underground equipment room capable of holding the entire tank. The second is that the tank in use is connected to an empty tank to transfer the waste liquid in the event of a leak. In order to prevent boiling of the concentrated waste liquid at high discharge and to maintain its temperature below 60 °C, the storage unit shall be equipped with a cooling system with sufficient margin. The cooling system is to be connected to an external heat exchanger. The storage tank is also equipped with a pressure-air mixing system, automatic monitoring and control system in addition.
Nanomaterials in the management of nuclear waste has obvious superiority, in some aspects of the conventional materials can not be replaced, the relevant aspects are pending systematic and in-depth research.
5, spent fuel management
Nuclear waste in the most difficult to solve, while the greatest harm, the longest half-life is spent fuel.
Spent fuel is naturally the most important part of nuclear waste management. Spent fuel generally requires reprocessing for final disposal.
Spent fuel reprocessing is also an important part of ensuring the sustainability of nuclear power. Through reprocessing, useful uranium and plutonium can be recovered from irradiated spent fuel, and then made into new fuel elements to be returned to hot or fast reactors, which can greatly improve the utilization rate of uranium resources.
In the future, if the closed-circuit cycle of nuclear fuel for fast reactors is realized, the utilization rate of uranium resources can be increased by about 60 times, which means that natural uranium, which can be used for 50-60 years, can be used for more than 3,000 years. Reprocessing spent fuel not only recovers and recycles large quantities of useful uranium and plutonium, but also greatly reduces the toxicity and volume of high-level waste that needs to be disposed of.
Spent fuel reprocessing technology has a history of more than 50 years, at present the world engaged in commercial reprocessing countries, including France, Britain, Russia, Japan, India, etc., France, Britain and the two countries of the large-scale commercial reprocessing level is in the world's leading position, the United States in the mid-1970s because of the political reasons to completely stop the commercial reprocessing activities, but has never ceased to be the study of reprocessing technology, 2006 clearly announced the restart of the reprocessing technology, and the United States is the first time to start the reprocessing technology. In 2006, it was clearly announced that the reprocessing program had been restarted.
The large commercial reprocessing plants currently in operation are: the Ager plant in France, with a capacity of 1700 tons of heavy metals/year; and the Sellafield reprocessing plant in the United Kingdom, with a capacity of 900 tons per year. The reprocessing plants currently undergoing hot testing are the six Sokomura reprocessing plants in Japan, with a capacity of 800 tons per year.
More than 50 years of operating experience has shown that Purex is an excellent reprocessing process. As the Purex process has been refined, it is now possible to design a process that can handle a wide variety of spent fuel and produce products that meet a wide range of purity and concentration requirements.