|Fig. 1: There is much public concern about nuclear waste. (Source: Wikimedia Commons)|
The emergence of nuclear energy offers promising opportunity for low cost and highly efficient energy sources. However, the proper disposal of nuclear waste is still highly challenging. Nuclear waste is one of the most difficult kinds of waste to managed because it is highly hazardous. According to the U.S. Environmental Protection Agency (EPA), nuclear waste is sorted into six general categories. [1-2] These include (1) spent nuclear fuel from nuclear reactors, (2) uranium mill tailings from mining and milling of uranium ore, (3) high-level waste from spent nuclear fuel reprocessing, (4) low-level waste, (5) transuranic waste from defense programs, and (6) naturally occurring and accelerator-produced radioactive materials.
The health concerns and safety issues associated with nuclear waste present important constraints on the widespread use of nuclear energy. Due to its radioactivity and highly hazardous properties, nuclear waste is required to be very carefully stored or reprocessed. The storing and reprocessing are further complicated by the long half life of the radioactive materials in the nuclear waste. For example, some of the components can remtain half of their dangerous levels even one million years later after production.  This makes the nuclear waste extremely difficult to be controlled and stored. Even if the waste is well stored, it is difficult to guarantee that the storages can be maintained properly for a long time period.  Therefore, in order to reduce the amount of nuclear waste produced, the use of nuclear power is limited. In this work the current methods of nuclear waste disposal and issue associated with them are reviewed.
The disposal methods for nuclear waste most used is simple storage. For example, dry cask storage uses steel cylinders along with inert gas or water to seal and store radioactive waste from spent fuel pool.  The steel cylinder is usually further placed in a concrete cylinder. These cylinders serve as radiation shield for the nuclear waste, stopping the radiation from reachin the outside. This is a relatively inexpensive way for storing radioactive waste. It doesn't require special location and transportation. The radioactive waste can be easily stored at a on-site reactor facility or adjacent to the source reactor. In addition, it is convenient to retrieve the waste from those storage cylinders for future reprocessing.
The other storage methods involve the selection of appropriate geologic location for the storage of high level radioactive waste. In this method, deep and stable geologic formations were selected to store the nuclear waste for long term.  In this process, large and stable geologic locations are first located and then excavated to form long tunnels (~kilometers) under the surface using conventional mining technology. The spent fuel and radioactive waste are then placed in the tunnels. Since the geologic formations chosen in this method are far from human population centers, the nuclear wastes are expected to be stably isolated from human living environment for the long term. This technique is still under investigation and development. Several countries in the world (e.g. England and France) were on the way of using this geologic disposal technique. However, there are still many concerns about this geologic disposal technique, because the stored nuclear waste has potential to leak into the environment if any huge geologic changing occurs (e.g. an earthquake). Moreover, even very low leakage or migration of nuclear waste may result in a huge disaster because the half-lives of the nuclear waste are so long. In this case, the nuclear waste would become a human catastrophe. Therefore, many countries in the world still don't agree with the using of this deep geologic disposal technique.
A similar technique is storaging the nuclear waste under then ocean. Essentially this is a kind of geologic disposal.  However, considering that the nuclear waste would leak and migrate more easily in the ocean, this technique has the higher risk for nuclear waste spreading. Moreover, these ocean disposal may be more difficult to monitor for leakage of the nuclear waste, making the control and management of the nuclear waste further challenging.
In summary, proper disposal of nuclear waste is still a challenging issue that constrains the growth of nuclear power. The main issue is that the half-lives of the radioisotopes produced are very long. Some of them are greater than a million years. This makes control and management of the nuclear waste extremely difficult. The most currently-used method for nuclear waste disposal is storage, either using steel cylinders as radioactive shield or using deep and stable geologic formations. However, the disposal of nuclear waste by storage still has many concern, since the leakage of the nuclear waste may cause huge environmental disaster. These techniques are still under development.
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