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| Fig. 1: The current test site for radioactive waste at Yucca Mountain in Nevada (Source: Wikimedia Commons) |
Radioactive waste, also known as nuclear waste, is a byproduct from fuel processing plants, hospitals and research facilities, however, it is most commonly associated with nuclear reactors and the processes of fission within nuclear reactors, along with the decommissioning and dismantling of nuclear reactors and other nuclear facilities. Radioactive has two common classifications: high level waste and low level waste. High level waste is mostly spent fuel removed from reactors, while low level waste mostly comes from other commercial uses of radioactive materials. [1] Looking more closely at the two different classifications of radioactive waste, and analyzing their backgrounds, we can come up with different options on where and how to store this radioactive waste for years to come.
There are significant differences between low level and high-level waste. In terms of discussing options for storage, high-level waste is the biggest focus due to how long it takes for high-level waste to become nonradioactive. Low-level waste (LLW) is generally a product of hospitals and industry, but can also come from nuclear fuel cycles. It is comprised of paper, rags, tools, clothing, filters, etc. which contain small amount of short-lived radioactivity. The difference between low-level waste and high-level waste is that low-level waste does not need to be shielded during handling and transportation and is suitable to be buried in shallow ground. Low-level waste comprises nearly ninety percent of the volume but only one percent of the radioactivity of all radioactive waste. [1]
On the other hand, high-level waste (HLW) is produced from the burning of uranium fuel in a nuclear reactor. It contains the fission products and transuranic elements generated in the reactor core. Unlike low-level waste, it is extremely radioactive and hot due to decay heat, and requires shielding and cooling. High-level waste has both long- lived and short-lived components, which depends on the amount of time it will take for the radioactivity of particular radionuclides to decrease to levels that are considered no longer hazardous for people and the environment. [2] The components of high-level waste are a major topic of discussion and there are a few particular options for storing this type of radioactive waste.
There are three main options for storing nuclear waste, and each comes with a different type of risk. The first option is to leave the radioactive waste where it is, and store it in temporary pools or dry casks across the US. According to the United States Government Accountability Office, The US has accumulated over 70,000 metric tons of spent nuclear fuel from nuclear reactors, generating 2,000 metric tons per year of spent fuel. By 2055 this amount is estimated to rise to 153,000 metric tons. Currently, spent fuel is being stored in these temporary pools or dry casks at 75 operating and decommissioned reactor sites in 33 states. Worldwide, there are an estimated 300,000 metric tons, generating 10,000 metric tons per year, also stored in temporary storage facilities. [2] After 5-10 year, radiation and decay heat levels are low enough that the fuel assemblies may be stored in large casks which can be cooled by air passing on the outside. Casks generally hold 24 to 40 fuel assemblies. Casks have double metal ring seals and are bolted so that radioactive material release should not be able to occur. Pressurized helium is used inside the cask to promote heat removal from the fuel assemblies to the cask wall.
A second option for radioactive nuclear waste is to bury the waste where it will be safe for hundreds of thousands of years. There have been many proposed sites across the world, but all efforts to create a permanent central storage site have failed. One option that is currently being explored is Yucca Mountain in Nevada, which is depicted in Fig. 1. The Nuclear Waste Policy Act of 1982 established plans for a total of 70,000 metric tons of spent nuclear fuel and solid high level radioactive waste to be placed in a dry, geologically stable repository. However, the cost to explore Yucca Mountain as a test site will nearly exceed 100 billion dollars and different approaches have been considered, such as a consent-based approach designed to build support at the local and state levels. [2,3]
A third and final option to store nuclear radioactive waste is to reprocess spent fuel for reuse. In other words, recycle the waste. Reprocessing the waste can separate out usable uranium and plutonium. The recycling of plutonium increases the energy derived from the original uranium by 12%, while on the other hand the reprocessed uranium must be enriched significantly to be used again. [3] The problem with this option is that, currently, reprocessing is illegal in the United States. Congress must first approve any reprocessing requests, including other countries that want to reprocess US-origin nuclear fuel. Although a viable option, the problem with reprocessing is more of a security issue do to the possibility of certain countries using spent fuel for bombs, and proliferation is a big concern with nuclear waste and reprocessing.
The reason that storing nuclear waste is a large topic of current discussion is due to the half-lives of certain radionuclides produced from fission of uranium, ranging from 1 day to 14 billion years. The radioactive isotope Pu-239 has a half-life of 24,400 years, meaning it is dangerous for quarter of a million years. As it decays, it becomes U-235, with a half-life of 710,000 years. [3] As more high-level waste is produced and stored in tanks and casks, other options will need to be explored further to store this waste for long periods of time without harming people and the environment currently and for the next thousands of years.
© Colton Hock. 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] Y. S. Tang and J. H. Saling, Radioactive Waste Management (Taylor and Francis, 1990).
[2] R. E. Berlin and C. C. Stanton, Radioactive Waste Management (Wiley-Interscience, 1989).
[3] L. E. J. Roberts, "Radioactive Waste Management," Ann. Rev. Nucl. Part. Sci. 40, 79 (1990).
