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Nuclear entombment - Stub Article

Chernobyl Incident Containment:

The Chernobyl Nuclear Incident is one of the most notable Nuclear disasters. The current containment building, known as the Sarcophagus, does not classify as a proper entombment device; However, due to lack of repair and maintenance, because of extremely high levels of radiation, a new tomb is in the process of construction and will be considered a proper tomb. Upon completion in 2017 the structure will measure 110 meters tall and span 260 meters. The main arch will be composed of triple-layered radiation resistant panels made up of stainless steel coated in polycarbonate, this will provide the shielding necessary for radioactive containment. The structure will weigh over 30,000 tons and completely cover Reactor number 4. This new tomb is designed to last over 100 years and has special ventilation and temperature systems to prevent condensation of radioactive fluids on the inside which could result in a compromised containment. Overall this new containment structure will act as band-aid and only buy time for the Ukrainian Government to develop ways of properly cleaning up the site.

Procedures:

Procedures Before Entombment:

The first procedure is to properly shutdown the site and stow any spent fuel/waste. The waste and reactors are often at extremely high temperatures due to the fission reaction that occurs. The waste is often placed in cooling pools filled with treated water where the await to cool to handling temperatures. Once the waste is cool enough it will often be stored in radioactive resistant containers to await being disposed of. The reactors are shutdown using special control rods to deter the fission reaction and allow for the cooling of the reactor and fuel inside. Once cool the fuel is taken out and dealt with like the waste, while 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; the coolant is also removed and stored for proper disposal. This procedure is often performed by the company that owned the plant, and if the company is unable to than 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. First, the radioactive fuel is removed, some of which can be recycled, basically the old fuel can be refined and put back into another reactor to be used as more fuel. The most dangerous waste, is placed inside a radioactive resistant container for long term storage, these containers are then taken to storage facilities for long term stowage such as Yucca Mountain nuclear waste repository. Then the rest of the site can be decontaminated through means of cleaning. The site is then checked thoroughly for any sign of radiation. Most of the remaining parts of the site can be disposed of normal as they are not contaminated or radioactivity levels have dropped to within the safe limits. This process is often completed using robots. The robots are able to access difficult to reach areas that may contain radioactive elements that would pose a threat to a human worker.

Procedure For Entombment:

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 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 next step is the most crucial and time consuming, it involves encasing the site in cement, absorbent grout, or infills. Each layer of cement, grout or infills must set and cure before the next layer is added and thus, time and proper testing is required to ensure the safe containment of radiation. Once the encasement is complete the final step is often to surround the site in a clay or sand/gravel mixture and then the soil is laid on top of the site. The use of many layers allows for the maximum shielding of radioactivity. The process of entombment is also a permanent process, the site must also 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.