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| Fig. 1: Geological Disposal in a Borehole [6] (Courtesy of the DOE) |
With the rapid expansion and development the world has faced in its recent decades, the demand for energy has increased exponentially. The first nuclear power plant was commissioned in 1954, and over the history of over 60 years nuclear energy has grown in size and efficiency. It is not surprising to see that as of 2015, a number of countries depend on nuclear power plants to meet over 30% of their energy demands. [1] While nuclear energy has been stable source of energy, it comes at the cost of handling byproducts. With the increasing number of nuclear power plants, handling their byproducts have grown to be an issue.
There are roughly three levels of wastes, divided by their immediate danger to nature and humankind: [2]
Low-level waste (LLW) is compromised of daily objects used in nuclear power plants, such as clothing, filters, tools, etc. While it accounts for 90% of the volume of all nuclear wastes, it constitutes only 1% of the radioactivity. Hence it does not need to be isolated from environment during handling and transportation, and is subject to shallow land burial.
Intermediate-level waste (ILW) includes materials or tools used in facilitation of nuclear power plants, such as resins, chemical sludge, and contaminated materials from reactor decommissioning. It takes up 7% of the total volume and 5% of the radioactivity, as it includes higher concentrations of beta/gamma contamination and sometimes even alpha contamination. It has low heat-emission, but depending on its level of contamination, may require deep disposal.
High-Level Waste (HLW) is the direct byproduct of operating nuclear power plants. It includes the used fuel rods or wastes from reprocessing them. Unlike other levels of waste, HLW emits decay heat that requires cooling in its handling. While its volume only takes up 3%, it accounts for over 95% of radioactivity. It is composed of both short-lived and long-lived components, decided by the length of their half-lives. Handling of HLW is affected by the availability of technology to separate these components.
Radioactive atoms go through natural decay and are gradually neutralized. For every half-life period, the number of radionuclides residing in wastes is halved. Hence for those with short half-life periods, it does not take long before the disintegration takes it sufficiently below the levels detrimental to humans and nature. However, radionuclides such as uranium have a half-life period of 4,500 million years and pose high threat to health issues. The primary form of uranium has a short range of penetration; it is the group of uranyl ions such as uranium trioxide that cause detrimental issues to public health. Hence these wastes must undergo additional treatments before being subject to disposal.
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| Fig. 2: Geological Disposal with Clay/Shale Buffers [6] (Courtesy of the DOE) |
Possible options for treating high-level wastes include secured storage (Long-term surface storage), direct disposal (Geological disposal, borehole disposal), and closed cycle. [3] For the interests of this report, it will focus on the deep geological disposal.
Deep geological disposal is done at depths between 250m and 1000m, or between 2km and 3km for boreholes. The main motivation for such disposal is that the future generations should not bear the costs of the current generation's use of nuclear energy. Hence the disposal site is chosen where it is geologically stable, naturally sustaining, and has natural barriers, part of the "multi-barrier" that protects the current and the future generations. [4] The depositories are designed so that the contents can be retrieved if needed. The wastes are put in a container and placed in the underground depositories. Once the containers are filled, they are again surrounded by clay buffers to provide additional radioactive protection. Fig. 1 shows an example of a geological depository in a deep borehole. Fig. 2 shows a depository with clay and shale acting as buffers around the container.
One more thing to consider is, how can we prevent future humankind thousands of years from now from finding one of these depositories and opening them, just as we explored the Pyramids or the oil reservoirs? [5] Languages are changing quickly over time, and also we cannot rule out the possibility that these nuclear depositories would be forgotten. Hence the warning messages in front of these depositories should be durable, easily found, understandable, and believable. The pyramids were not successful in the last criteria, and we cannot risk it happening again to these depositories.
Over the progress of time, humankind has gradually increased its dependency on nuclear energy for its prosperity and development. They are not without cost, however. Managing the byproducts from the nuclear development has always been one of the biggest concerns that could impact the environment and people's lives. While geological disposal is one of the options taken by many countries, we must make sure this strategy keeps the current and the future generations safe from nuclear threats.
© Simon Kim. 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] "Nuclear Power Reactors in the World", International Atomic Energy Agency, IAEA-RDS-2/36, May 2016.
[2] "Current Issues in Nuclear Energy: Radioactive Waste," International Nuclear Societies Council, August 2002.
[3] "RD&D Programme 2010," Swedish Nuclear Fuel and Waste Management Co, TR-10-63, September 2010.
[4] "Radioactive Waste in Perspective," Nuclear Energy Agency, NEA No. 6350, 2010.
[5] J. Conca, "Talking to the Future - Hey, There's Nuclear Waste Buried Here!," Forbes, 17 Apr 15.
[6] "Evaluation of Options for Permanent Geologic Disposal of Spent Nuclear Fuel and High-Level Radioactive Waste, Vol. 1," Sandia National Laboratory, SAND2014-0187P, April 2014.