“Nuclear entombment”的意思、由来-开放百科全书

Nuclear entombment (also referred to as "safe enclosure") is entombment of a radioactive site and a method of nuclear decommissioning in which radioactive contaminants are encased in a structurally long-lived material, such as concrete. The idea is that the entombment will last for a period of time to ensure the remaining radioactivity is no longer of significant concern.{{citation needed|date=November 2018}} The method has usually been used in relation to nuclear reactors, but also in relation to some nuclear test sites. Regarding decommission of nuclear power plants, entombment is one of three various ways: dismantling, safe enclosure and entombment. Nuclear entombment is the least used of the three options. The use of nuclear entombment is more practical for larger nuclear power plants that are in need of both long and short term burials. Entombment is used as 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.{{citation needed|date=November 2018}}

Preparation

The first procedure is to properly shut down the site and stow any spent fuel or 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 they 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 shut down 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 then properly qualified contractors are brought in. After this procedure comes the next one which deals with the radioactivity and radioactive waste.{{citation needed|date=April 2017}}

The second procedure is the dismantling of the site. First, the radioactive fuel is removed, some of which can be recycled and refined for further use in a reactor. 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 through various means. 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.

Entombment

The procedure of entombment is a time extensive process. 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 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 site 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. An example would be a waste disposal facility in El Cabril Spain that uses a multi-concrete barrier concept. The concept includes placing the radioactive waste drums inside concrete boxes and placing those boxes inside a reinforced concrete vault. The Vault is then sealed with a waterproof coating to prevent any hazardous liquid from escaping the drums.{{citation needed|date=April 2017}}

Concerns

Much of the concern using nuclear entombment is the worry of nuclear leakage. Nuclear leakage occurs when there is a slight crack in the cementitious material or the clay for any radiation to seep through. Entombment does not solve the problem for every type of isotope of waste. Many different types of nuclides outlive the lifetime of the materials containing it by a thousand years or more. There are some disagreements among state governments on nuclear entombment, as it does not uphold the polluter pays principle.{{citation needed|date=April 2017}} It is often not possible to make an exact estimate of total decommissioning costs, leaving a certain financial liability for a future generation. It is also difficult to guarantee that the necessary expertise for final decommissioning will be available in due time, or that the decommissioning fund has earned sufficient interest.^[1] The main concern for many of the infrastructures being used to entomb the waste, is that the facilities being used to entomb the radioactive waste were built for operations and not for disposal and burial of the waste. Some of the facilities may be too small for nuclear waste to be entombed because of the financial aspect going into all the materials needed to build safe housing for the hazardous material. The only other option in the case of the facility being too small is for the nuclear waste to be transferred to another disposing facility or the facility will have to be reinforced. Many U.S. state governments chose to steer clear of entombment because of its potential hazards of not being properly contained.{{citation needed|date=April 2017}} If any of the nuclear waste were to come in contact with any water source, it has the potential to spread through rivers and lakes into highly populated cities to radioactively contaminate it and anyone who drinks from it.

Benefits

The surveillance cost will be lower than the surveillance cost for SAFSTOR (safe storage) option. For example, Yankee Rowe Nuclear Power Station was finally decommissioned in 1960 at the cost of 39 million with the idea of making it to a safe storage.{{citation needed|date=April 2017}} The cost to surveillance the power plant will add up to 8 million every year. In 2007 The Yankee Nuclear Power Station in Rowe was fully dismantled. The cost for entombment is less than the cost or dismantling which rends it to be more of a financial choice. By using the facility in which the waste came from helps the entombment process cost to be less. The use of entombment requires fewer workers and prevents them from being in major contact with the nuclear waste.^[2] In addition to using the same facility to reduce cost, it also reduces public interaction with the project and the amount of nuclear radiation emitted from the waste. By disposing of the nuclear waste in the same facility it will allow engineers to reinforce the facility to ensure safety for the public and the environment. By using the entombment method nuclear waste will be able to decompose, which will then be allowed to be transferred and the facility will be able to be dismantled.{{citation needed|date=April 2017}}

USNRC (United States Nuclear Regulatory Commission)

The USNRC provides licensing for the entombment process, as well as providing license they research and develop programs to help decommission nuclear power plants. USNRC will continue the development of rule making for entombment. NRC will ask the company that are running power plants to set some money aside while the power plant is being run for future shut down cost.^[3] The NRC has decided that in order for nuclear entombment to be a possible, a long-term structure must be created specifically for the encasing of the radioactive waste. If the structures are not correctly built water can seep into them and come out with radioactive waste that can infect the public. The NRC itself imposes acts such as the Nuclear Waste Policy Act of 1982 and the Low-level radioactive waste policy to help regulate state governments on the procedures and precautions needed to dispose of the nuclear waste. The Nuclear Waste Policy of 1982 states that both the federal government's are responsible to provide a permanent disposal facility for high-level radioactive waste and spent nuclear fuel. As well with the Nuclear Waste Policy Act of 1982, which gives states the responsibility to dispose of low-level waste and provides facilities to the states that will be regulated by the NRC or by states who have agreed to follow §274 of the Atomic Energy Act.

Chernobyl 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.{{citation needed|date=April 2017}}

Other examples