Radioactive wastes are the leftovers/wastes that contain radioactive elements which are mainly obtained from nuclear substances that have already been used. Nuclear materials are used in various sectors such as in the medical sector for treatment, in agriculture, in manufacturing to test for flows in materials, in research, and in the energy sector for the production of energy production. After the radioactive materials have been used there a lot of waste is produced. These radioactive materials even though they are radioactive are highly radioactive and remain in this state for many years. These materials are harmful to almost any living thing as well as the environment. They have both immediate side effects and long-life affects the environment. Therefore, government agencies take the lead in regulating the use of radioactive materials with the main purpose being protecting human lives and the environment in which they live in ( Malcolm, 2005 ). This calls for the waste to be carefully disposed of. Due to the longtime of decay of the radioactive materials, they have to be isolated and confined in disposal facilities where they are stored under water for several years until the radiation level poses no threat to the people and the environment and then they are disposed of. The time for storage of the radioactive waste varies from one waste product to another and it depends on the type of radioactive isotopes and waste. There are various methods of disposing of radioactive materials and are classified in two; the short-lived wastes are disposed by segregation and storage, the low-level and intermediate-level wastes are disposed of near-surface disposal, and the high-level wastes are disposed of by deep burial and transmutation/partitioning ( Rogner, 2010 ). Among the large-scale energy-producing technology, nuclear power is the only section that takes full responsibility for all their wastes. Comparing nuclear energy with other forms of power production such as thermal electricity production it has the least amount of generated waste. The used nuclear fuel may at some point be considered as a resource rather than waste ( Ojovan, et al. 2006 ). Among the approaches of the disposal of high-level radioactive waste, the best and technically proven method is deep geological disposal. The main reason behind disposing of radioactive waste is to safeguard human health and reduce the impact on the environment. Before the wastes are disposed of they have to pass through a process that involves isolating them and diluting the concentration of radionuclides to a state where they are not harmful to the environment. This is achieved by containing and managing the waste by burying them deep not allowing them to cause any harmful pollution. This process is called radioactive management.
Very Low-Level Waste these are the radioactive materials that are considered not harmful to human health and the environment in general. It is mainly composed of the materials from produced during the rehabilitation of a nuclear industrial site. Food processing, steel and chemical industries also produce these radioactive wastes from the concentration of natural radioactivity. These wastes are disposed of together with domestic wastes. Countries France has of late developed new facilities designed to specifically store these Very low-level wastes.
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Low-level Waste has a low content of radioactive elements of alpha activity that do not exceed 4 GBq/t or beta-gamma activity of 12 GBq/t. during the transportation and handling of these type of waste, shielding is not necessarily required and the near-surface method is most suitable for the disposal of these wastes. Most of the hospital's and industries' radioactive wastes lie in this category. These wastes comprise of paper, tools, rags, filters and clothing containing short-lived radioactivity. Among these wastes, only one percent is radioactive waste. The incineration of these materials is considered before disposal to reduce the volume.
Intermediate-Level Waste has a higher level of radioactivity than low-level waste and has a radioactive decay heat less than 2KW/m 3 which is not sufficient to cause a temperature rise to the surroundings but it requires some shielding. These materials comprise of chemical sludge, resins, metal fuel cladding, and contaminated materials from the reactor. To reduce the volume of the non-solids waste are solidified in concrete or bitumen before disposal. It comprises up to seven percent of the radioactive waste.
High-Level Waste has a high level of radioactivity and has a radioactive decay heat greater then 2KW/m 3 sufficient to cause a significant rise in its temperature as well as that of the surrounding therefore it requires shielding and these factors are put into consideration during the design or selection of storage and disposal facilities. Their main sources are the burning of the uranium fuel in a nuclear reactor and comprises of fission products and transuranic elements that come out of the reactor. The commonly accepted methods of disposal are near-surface disposal at the ground level or else in the below the ground but the material is confined in caverns the depth in usually tens of meters. It is implemented for low-level waste. The other method is deep geological disposal between 250 m and 1000m and is implemented for defense-related and Intermediate level waste and high-level waste. Facilities for low-level disposal are mainly situated at the ground level. These facilities are constructed as waste containers comprising a protective covering that is several meters thick. The containers containing the waste materials are placed in the vault until it is full where it is covered completely with an impermeable membrane and covered with a layer of soil (Gan, & Yang, 2017). The facility has gas venting as well as a drainage system incorporated in them. Where the facility involves caverns below the surface, it requires shallow excavation of caverns done by the use of drills. The facility is various meters below the earth’s surface and is accessed through drifts. During the design and construction of near-ground facilities long time climate change is taken into consideration so safety purposes. These facilities are used for radioactive waste of have a short life of up to 30 years mainly low and intermediate-level wastes. Countries that implement the near-surface disposal approach include the UK, Spain, France, Japan and U.S.A. countries that implement near-surface disposal facilities in caverns include Sweden and Finland ( Cherkashin, 2004 ). The approach involves disposing of the radioactive material underground in repositories where the geological conditions are stable. Natural barriers are improvised to provide the required isolation they include salt, rock, and clay. By using these barriers the need for regular maintenance is eliminated. Repositories for deep geological disposal are discussed below.
Mined repositories comprise mined tunnels or caverns where the wastes are then disposed of after they have been packaged. These repositories are excavated using the standard technology of mining. The waste containers are covered with a backfill/buffer which can either be cement or clay. The choice of waste container material depends on the type of waste being disposed of. Mainly the material used is copper since it remains in its unchanged original state for a very long time inside the bedrock. Therefore it is preferred when disposing of long-term radioactive waste since it has high corrosion resistance (Bryan, 1987).
Deep Boreholes have also been considered in the disposal of radioactive waste as geological isolation. It comprises drilling boreholes of about 5000 meters in crystalline bedrock and disposing of the nuclear waste confined in canisters. The canisters containing the waste are placed to occupy up to 2000 meters and the rest 3000 meters are covered with concrete, asphalt or concrete. The canisters are also separated by the backfill material. The boreholes can be drilled in a variety of places such as offshore and onshore increasing the range of disposal locations for the disposing wastes (Chalmers, et al, 2011).
Disposal in Clay is an approach implemented in Europe where spent fuels containing High-level wastes are stored in steel containers and placed in tunnels 230 meters below the surface of ductile clay which offers self-sealing providing an impermeable lead for a long time (Sounds, 2014).
Disposal in Layered Salt Strata or Domes since salt environments have a reduced rate of flowing underground water and they as well provide continuous self-sealing due to the creep of salt. This method is suitable for high-level wastes that produce heat. This approach is implemented in Germany, the Netherlands, and Mexico ( Milsted, Friedman, Stevens, 2004 ). These are regional repositories that are located in a willing country to allow other countries that are incapable of disposing of their own waste in their own country to access them and dispose of them there. This is because there are some countries that are geologically disadvantaged, are not adequately equipped to either store or dispose of radioactive wastes, or small countries that do not have the resources to ensure the required safety and security. These are specifically designed sub-surface waste storage facilities that aid in ensuring there is the required safety and security of harmful radioactive waste materials that are yet to be disposed of. They are used to hold materials such as intermediate and high-level waste pending long-term disposal to be available so as they can be disposed of. The transport casks are used for the transport of used nuclear fuel it is shielded with steel or sometimes a combination of steel and lead (Tollefson, 2014).
Conclusion
Radioactive wastes have a diverse impact on human health and the environment. They, therefore, need to be properly handled stored and disposed to prevent radioactive pollution. The disposal of radioactive wastes aims at placing the wastes in facilities that ensure there is long-term safety and security by implementing a combination of artificial and natural barriers to preclude the escape of radioactivity. This has been enhanced by various methods and the most common ones include of disposal are near-surface disposal at the ground level or else in the below the ground but material confined in caverns a depth in usually tens of meters. It is implemented for low-level waste. The other method is deep geological disposal between 250 m and 1000m and is implemented for defense-related and Intermediate level waste and high-level waste. Geological methods include Mined repositories, Deep Boreholes, Disposal in Clay, and Disposal in Layered Salt Strata or Domes. Other methods include Multinational Repositories, Interim waste Storage and Transport for countries that are incapable of disposing of their own radioactive products due to various reasons.
References
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