The disposal and storage of nuclear waste in the USA
In the US radioactive waste is differentiated into low and high level waste. This accounts for the different demands as to disposal and storage. The Department of Energy, the Nuclear Regulatory Commission and the Environmental Protection Agency are responsible for the nuclear waste disposal. Civil low and moderate nuclear waste is stored near to the surface in the three final storages in Barnwell, Richland and Clive, military waste in Waste Isolation Pilot Plant. For highly nuclear waste neither a final storage nor a plan, where such a storage could be built, does exist, after the hitherto explored region of the Yucca Mountains was rejected by the change of Government.
Possibilities for the reduction of nuclear waste do exist, however, they are hardly ever used in the US. Furthermore, the final disposal in a geological deep storage is the solution aspired by the US for the nuclear waste pro, however, the search for it is difficult.
Definition and origin of radioactive waste
First of all it has to be defined, what is conceived as nuclear waste. These are radioactive substances, with an activity which exceeds the exemption limit and cannot be used any further.
The IAEA (International Atomic Energy Agency) prepares a classification on an international level and divides 3 classes, which are again categorized in sub-classes. There is the Exempt Waste (EW), which can be disposed of conventionally, as it only contains a low body of radioactive substances. The second class consists of the Low and Intermediate Level Waste (LILW), which is subdivided into short- (LILW-SW) and long- living (LILW-LW). The last class is the high level waste.
The classification of radioactive waste in the U.S. is made in only in two main categories: The first one is Low Level Waste (LLW), the other one is High Level Waste (HLW). Additionally, there are special categories, such as the Transuranium Waste, Spent Nuclear Fuel (SNF) and Naturally Occurring Radio nuclides (NORM).
In the U.S. the activity classes „low-“ and „medium-active” are categorized according to the limit value for single radio nuclides and their sum. The activity class “highly active” is categorized according to the limit values which are above medium-active.
The major part of the radioactive waste, about 80%, arise during the mining of uranium, furthermore at nuclear power plants, nuclear research centers, during the recycling of used fuel assemblies and military activities in connection with nuclear weapons.
Risks due to nuclear waste
Already during the operation of nuclear power plants, radioactivity poses a high risk, as you can easily see with accidents like Chernobyl. The consequences of such accidents are enormous. Within a huge distance the soil and thus everything living in and on it, is contaminated with radioactivity. In humans this radiation leads to the creation of cancer and gene mutation, which can also take place in plants.
In addition, every nuclear power plant produces 20 to 30 tons of highly radioactive waste per year. So far there is known no absolutely save method to dispose of this waste and no place, where nuclear waste can be stored completely save till the end of the half-life period of its elements.
Plutonium-238 is the most poisonous element in the world with a half-life period of 24.000 years. With its short-wave alpha-radiation it destroys any tissue and is thus highly dangerous, even millenniums after its use in the reactor.
The two main dangers at the storage of nuclear waste are the air- and water- contamination. As a slow or explosive escape of gas cannot be excluded, there exists the possibility of air-contamination with radioactive gases. In case of water entering a repository for radioactive waste, a contamination of drinking water is possible.
The hazardous potential also exists during the transportation radioactive waste. In the case of an accident in which a container is damaged, radioactive material might escape and contaminate the surrounding area.
Disposal of radioactive waste
Per year worldwide arise more than 10.000 tons of highly radioactive waste. These have to be disposed of to be innocuous for the environment till their radioactivity degraded as far as possible.
Already during the 1950s the U.S. started to tilt nuclear waste – cast in concrete and packaged in steel barrels – into the ocean. That it cannot be this easy, as this exposes the environment to a huge danger is known by this time and alternatives to this easy way of disposal have to be found.
However the previous possibilities are rather insufficient and not satisfying.
One option to dispose of nuclear waste is the conditioning. In doing so, the radioactive waste is brought to a chemically stable condition, in which it is difficultly soluble or insoluble, so the danger of water contamination can be minimized. Low or medium active waste is burnt or crimped in the process and cast in steel barrels together with concrete.
Highly radioactive waste is merged to vitrified waste block containers and thus prepared for the reprocessing treatment. Vitrified waste block containers inclose the non-reusable by-products of the spent fuel elements, which are melted with glass granulate under high temperature and casted in a container of high quality steel.
Already mentioned was the recycling. In this process the remnants of elements (Uranium 235 and Plutonium) of old fuel rods are detached and re-used for new fuel elements. However at this low and medium radioactive waste occur in great quantities, while the amount of highly radioactive waste is only slightly reduced.
Prior the aspired final storage the radioactive waste has to be stored temporarily till the radiation is decreased to the extent that it can be packed. After several years in the cooling pond, a basin filled with water, in which the radiation gradually reduces, the final storage is carried out
Here one has to differentiate between repositories for low and medium active material and terminal storage for highly radioactive material. There are also several other criteria which have to be considered with the choice of a suitable final storage. It should be located in a geological formation and possess the following characteristics:
- The geological situation of the final storage can be characterized for the period in which the waste has to be isolated from the biosphere
- The components of a multibarrier system all add to the long-term safety of the repository
- The terminal storage should be as robust as possible
Possible rocks are mudstone and saltstone as well as crystalline rocks. These possess special features, which make them eligible as final storage and result in various technical approaches. With repositories in crystalline rock much importance will be attached to the technical barrier (often for the simplification of the choice of location). In contrast repository concepts for saltstone are based on the non-existent groundwater in the host rock. With final storage concepts for clay relevance of geological and geotechnical barriers are almost the same. Of course it is difficult to find a suitable repository, which meets all requirements und additionally finds acceptance in public.
For low and medium radioactive waste there are already final storages, most times near to the surface, thus at or slightly below the surface. After the storage they are covered. The repository of highly radioactive waste presents a much bigger problem, for which until now there is no solution – there is no final storage for highly radioactive waste.