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| Fig. 1: Yucca Mountain south portal entrance. (Source: Wikimedia Commons) |
Nuclear waste is a broad term that includes three categories of waste: low-level waste, intermediate-level waste, and high-level waste. Low-level waste accounts for 90% of the total nuclear waste and only contains 1% radioactive content. Low-level waste usually includes lightly contaminated items, usually referring to tools used in the plant, or clothing worn by workers. Low-level waste could also be produced in hospitals, and research facilities. [1,2] Intermediate-level waste accounts for 7% of the total nuclear waste volume for a power plant and it contains 4% radioactive content. Examples of intermediate-level waste usually includes components from the inside of reactors, such as used filters and steel components as well as effluents from reprocessing.The term nuclear waste is most commonly associated with with high-level waste, synonymous with radioactive waste. High-level waste contains 95% radioactive content, but it only accounts for about 1% of total nuclear waste. High-level waste is nuclear fuel removed from the reactor after three to five years of generating electricity. [1] Nuclear waste can be dangerous because of its radioactivity so its proper treatment is crucial to safeguard human and environmental safety.
Nuclear waste treatment is one of the safety concerns of using nuclear energy to generate electricity, and it is therefore very important that all necessary precaution measures are taken. Treatment methods differ for high and for low and intermediate-level nuclear waste.
High-level waste is very hot and radioactive, therefore rendering it the most dangerous type of waste. Some of the most concerning byproducts from spent fuel are Sr-90, Cs-137, Pu-239 (half-life 24,000 years), Tc-99 (half-life 220,000 years), and I-129 (half-life 15.7 million years). [3] There are two crucial aspects of handling high-level waste: cooling it, and shielding plant workers from the radiation it produces. The most common storage method used by plants is dry cask storage, which uses steel cylinders along with inert gas or water to seal and store radioactive waste. 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 reaching the outside. If further reprocessing is needed, it is convenient to retrieve the waste from those storage cylinders for future reprocessing.[4]
The treatment of low and intermediate-level waste is easier and more straight-forward. There are two options for handling low and intermediate-level waste. The first is that it gets stored on site for long enough time that its radioactivity decays completely, in which case it can afterwards be disposed as regular trash in a landfill. Secondly, if amounts are large enough for shipment, there exist low-level waste disposal sites, where the waste can be transported to and disposed off. [2]
Just as waste treatment depends on its categorization as high, intermediate, or low-level waste, so does its disposal. Low and intermediate-level wastes can be buried close to the ground level. Depending on their radioactivity levels at the time of disposal, they can either be disposed off at a landfill, like regular trash, or they have to be transported to one of the four currently operating low-level disposal facilities in Barnwell (North Carolina), Clive (Utah), Hanford (Washington) or Andrews (Texas). [2]
High-level waste, on the other hand, remains radioactive for many more years, which is why it need to be disposed off in deep underground repositories, built in stable geological formations. No such repositories have been built in the United States as of today. In the US, the Nuclear Waste Policy Act of 1982, amended in 1987, directed the Department of Energy to design and construct an underground geologic repository at Yucca Mountain, Nevada, which is a project still in progress. Fig. 1 shows the South Portal entrance to Yucca Mountain. The tunnel beyond is U-shaped and 5 miles long. This facility would serve as the disposal site for all of the US's used nuclear fuel. Consolidated interim storage sites have also been proposed so that used fuel can be more efficiently managed until a disposal site becomes available. [2].
To sum up, nuclear waste treatment and disposal can be a dangerous process because of radioactivity emitted by nuclear waste. Nuclear waste can be classified into three categories of high, intermediate, and low-level waste depending on their percentage of radioactive content. Treatment and disposal of nuclear waste depends on its category. Although the most dangerous type of waste, high-level waste, containing 95% radioactive content, accounts for less than 1% of the overall volume of nuclear waste, the amount of used nuclear fuel around the world is non-negligible. Approximately 270,000 metric tons of high level radioactive waste has accumulated in 30 countries and an additional 9,000 metric tons are being added annually.[5] Therefore, finding a long-term solution for high level nuclear waste disposal remains a pressing issue.
© Nefeli Ioannou. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. Kumar, "Commercial Nuclear Energy Production and Nuclear Waste," Physics 241, Stanford University, Winter 2016.
[2] "Backgrounder: Radioactive Waste," U.S. Nuclear Regulatory Commission, April 2015.
[3] "Radioactive Waste Disposal," in Encyclopedia of Physical Science and Technology, ed. by R. A. Meyers (Academic Press, 2001), p. 633.
[4] B. C. Sales and L. A. Boatner, "Lead-Iron Phosphate Glass: A Stable Storage Medium for High-Level Nuclear Waste," Science 226, 45 (1984).
[5] S. E. Hasan, "International Practice in High-level Nuclear Waste Management," in Concepts and Applications in Environmental Geochemistry, ed. by D. Sarkar, R. Datta and R. Hannigan (Elsevier, 2007).
