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| Fig. 1: The Onkalo nuclear waste repository in Finland. (Source: Wikimedia Commons) |
The proper disposal of nuclear waste is critical for encouraging the adoption of nuclear powerplants. Nuclear plants and equipment produce contaminated streams of materials that are radioactive and harmful to humans if not properly disposed. These radioactive materials usually remain toxic for long periods of time. For example, Pu-239 has a half life of 24,100 years and U-235 has a half life of 700 million years. [1] As a result, it is important to take the characteristics of nuclear waste materials into account during disposal. Nuclear waste is generally disposed into geological storage sites, and those sites are broadly divided into two types: shallow geological sites and deep, mined geological sites. [2] In selecting the appropriate storage site for nuclear waste, numerous factors are considered, some of which include: the amount of heat produced by the nuclear waste, the lifetime of the radioactive elements in the waste, and the decay products of the initial radioactive components in the waste stream. [1] While most countries have their methods for classifying nuclear waste, the consensus is that waste materials with high thermal output and long lifetime are suitable for deep, mined geological repositories, while waste materials with short lifetime are suitable for shallow geological sites. Fig. 1 shows the Onkalo nuclear waste repository in Finland. Here, we discuss the scientific and engineering strategies behind shallow and deep geological waste disposal sites.
Shallow geological repositories are typically a few tens of meters below ground surface, while deep geological repositories are significantly deeper. [2] In both types of repositories, the nuclear waste containment strategies can be broadly divided into engineered barriers and geological barriers.
Engineered barriers are physical waste barriers designed by humans to provide an additional layer of protection over disposed waste. For example, MgO and high-pH cement are typically installed in waste sites to remove CO2 and reduce the solubility of radioactive materials, ultimately preventing waste materials from seeping into geological formations and escaping into ground water and the atmosphere. [1] One factor that is considered when engineering waste barriers is the waste form. Barriers must be designed to handle the rate of radioactivity release associated with different waste forms. For instance, UO2 is usually stored in reducing environments because it is unstable in oxidizing environments. [1] It is also noteworthy that temperature changes can affect the properties of waste. [3] Finally, engineered barriers also rely on rational design of waste packages. A waste package is the architecture in which waste is encapsulated. For instance, a waste package concept developed in Sweden is detailed as follows: Spent fuel is placed in iron holders that are encapsulated in copper containers and subsequently inserted into holes that are drilled into crystalline rock and lined with bentonite. [1]
Geological barriers rely on the attributes of rocks that surround the waste disposal site to slow down the transport of radioactive materials into ground water. The transport of radioactive materials is dependent on the permeability of the rocks within the storage site vicinity. Because most disposal sites are close to bodies of water, surrounding host rocks must be impermeable to water. Some of the geological barriers that meet this criterion are strong rocks with low porosity like igneous and metamorphic rocks, clays that do not sustain open fractures for long timeframes, and self-healing rock salts. [4-6]
Good waste storage sites are products of carefully designed engineered and geological barriers. In particular, the interactions between engineered and geological barriers are also important considerations when ensuring that waste is properly contained. For instance, the geochemistry of ground water in certain terrains may not be conducive to certain waste package materials. [1]
Once the science of waste disposal is properly established, regulatory frameworks and public acceptance must be considered. Regulatory frameworks are put in place to examine the safety of waste disposal sites over extended periods of usage. Public acceptance is also critical, from the process of selecting waste disposal sites until the point of obtaining operational licenses. As with all things, it is often recommended that the science behind waste disposal be communicated to the public in simple terms.
© Solomon Oyakhire. 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] R. C. Ewing, R. A. Whittleston, and B. W.D. Yardley, "Geological Disposal of Nuclear Waste: A Primer," Elements 12, 233 (2016).
[2] I. G. Crossland, "Near-Surface, Intermediate Depth and Borehole Disposal of Low-Level and Short-Lived Intermediate-Level Radioactive Waste," in Geological Repository Systems for Safe Disposal of Spent Nuclear Fuels and Radioactive Waste, ed. by M. J. Apted and J. Ahn (Woodhead Publishing, 2010), p. 43.
[3] R. C. Ewing, "Long-Term Storage of Spent Nuclear Fuel," Nat. Mater. 14, 252 (2015).
[4] A. Hedin and O. Olsson, "Crystalline Rock as a Repository for Swedish Spent Nuclear Fuel," Elements. 12, 247 (2016).
[5] B. Grambow, "Geological Disposal of Radioactive Waste in Clay," Elements. 12, 239 (2016).
[6] T. von Berlepsch, B and Haverkamp, "Salt as a Host Rock for the Geological Repository for Nuclear Waste," Elements 12, 257 (2016).
