Nuclear entombment (also referred to as "safe enclosure") is a method of nuclear decommissioning in which radioactive contaminants are encased in a structurally long-lived material, such as concrete. This prevents radioactive material and other contaminated substances from being exposed to human activity and the environment. Entombment is usually applied to nuclear reactors, but also some nuclear test sites. Nuclear entombment is the least used of three methods for decommissioning nuclear power plants, the others being dismantling and deferred dismantling (also known as "safe storage"). The use of nuclear entombment is more practical for larger nuclear power plants that are in need of both long and short term burials, as well as for power plants which seek to terminate their facility licenses. Entombment is used on a case-by-case basis because of its major commitment with years of surveillance and complexity until the radioactivity is no longer a major concern, permitting decommissioning and ultimate unrestricted release of the property. Considerations such as financial backing and the availability of technical know-how are also major factors.
Preparation The first step is to cease operations and stow any spent fuel or waste. Nuclear reactors produce high-level waste in the form of spent nuclear fuel, which continues to release decay heat due to its powerful radioactivity. Storing this waste underwater in a spent fuel pool prevents damage and safely absorbs the radiation. Over a period of years the radioactivity and heat generation declines, until the spent fuel can be removed from the water and stored in casks for burial. When a reactor is decommissioned, partially spent fuel can be treated the same way. The reactor is sealed in order to allow no escape of radioactive particles or gases. Lastly the heating water is then pumped out and put in containers to await proper decontamination. Decontamination is the process of removal of radioactive contaminants on the remaining surface. Washing and mechanical cleaning are processed during the decontamination process by using the chemical reactors, and the global objective is to protect public safety and the environment. The coolant is also removed and stored for proper disposal. This procedure is often performed by the company that owns the plant, and if the company is unable to then properly qualified contractors are brought in. After this procedure comes the next one which deals with the radioactivity and radioactive waste. The second procedure is the dismantling of the site. The decommissioning project is for removing the radioactive materials. Thermal cutting and mechanical cutting are two technical ways to dismantle and demolish. Thermal cutting is used for the metals by burning with high energy in one concentration area. The mechanical cutting takes place in the workshop with mechanical force and cuts reactive materials into two parts or in small pieces. The most dangerous waste is placed inside radioactive-resistant containers, after which the containers are transported to storage facilities. The rest of the site can then be decontaminated. The site is then checked thoroughly for any signs of radiation. Most of the remaining waste onsite can be disposed of normally as it is either not contaminated or radioactivity levels have dropped to within safe limits. This process is often completed using robots, which are able to access the difficult to reach areas deemed too radioactive for human workers. The robot was made by WWER-440-type-NNR and is mostly in central and Eastern of Europe, Russia. The main idea of using robots in decontamination is to reduce the radioactive to a level, therefore workers can be exposed. The robot's energy was provided from the robot control system and was placed in the manipulator. The manipulator can be controlled by the remote. The "Decomler" robot works in decontamination by using the wheel system and track system. Also, the robot needs to be strictly licensed by national regulating authorities, because the materials processed by the robot need to ensure they are not discharged to outside. Otherwise, it will cause nuclear pollution to both the environment and humans.
Entombment Entombment is a more time-intensive process than protective storage and dismantlement as a decommissioning mode. The simplest of the procedures is entombing the radioactive waste source at the site itself. After containment and disposal of lower-level radioactive spent fuel sources, the entombment process of high-level radioactive parts of the plant may begin. The entombment itself is accomplished by numerous layers of sturdy materials, concrete usually among them. The first step is to cover the area with a protective shield which is usually made up of radioactive-resistant materials - this allows workers to continue working with a significantly lower radioactive environment. The second step is the most crucial and time-consuming. Cementitious materials are used to encase the site in cement, absorbent grout, and/or infills. Each layer of cement, grout or infills must set and cure before the next layer is added. Time and proper testing is required to ensure the safe containment of radiation within the layers of cement. The final step is often to surround the site in a clay or sand/gravel mixture and then soil is laid on top of the site. Entombment designs must be defined and agreed upon by an authorized organization, like the NRC. These designs must also be an approved alternative to other decommissioning methods. Furthermore, because the nuclear facility is often in close proximity to other public environments, the public must accept entombment as a decontamination & decommissioning (D&D) option before proceeding. Small-scale tests will sometimes be performed to prove to organizations like the NRC that a standard process can be transferred. A consortium approach is also necessary to ensure a broader understanding and funding of nuclear entombment. Sites for potential entombment have been identified in the U.K., Japan, Lithuania, Russia, and Taiwan but further research and development of nuclear entombment methods has been called for as of the early 21st century. Sites must be routinely checked for breaches in the containment barrier for decades. Therefore, entombment is often considered as a last resort solution to the decommissioning of a nuclear power plant or nuclear disaster site.
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