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| Fig. 1: Image of casks storing radioactive waste. (Source: Wikimedia Commons) |
In the United States, no solution has been found to the problem of radioactive waste disposal. High-level radioactive waste - mostly used uranium fuel from a reactor that is thermally hot, very radioactive, and incapable of efficiently generating electricity - is first stored underwater for up to ten years. [1] This waste is then placed in reinforced concrete casks depicted in Fig. 1. [1] Essentially all of the United States' radioactive waste remains in these casks at nuclear plants. As on-site waste accumulates, the need for permanent radioactive waste disposal is becoming increasingly salient. Permanent waste disposal requires inexpensive technology that can immobilize the waste and a place to permanently store the immobilized waste.
Most materials capable of immobilizing high-level waste are glasses or ceramics. The most widely used immobilization material is borosilicate glass, which can incorporate high-level waste by vitrification - by melting the waste with glass-forming additives and then allowing the mixture to cool down until it has solidified. [2,3] The final solid does not have a particularly stable crystal structure, as glasses are susceptible to crystallization. [4] On the other hand, ceramics possess a more stable crystal structure making them well-suited for long-term storage. One promising ceramic is Synroc, which consists of small quantities of metal along with hollandite, perovskite, rutile, and zirconolite minerals. [5] For Synroc, immobilization involves mixing predefined proportions of the high-level waste and several ceramic components and then calcining and hot-pressing the combination. [5]
Compared to glasses, Synroc is less likely to crack due to thermal stress, which one should consider because radioactive waste generates heat. [5] Another comparative advantage of Synroc is that it offers greater resistance to leaching of radioactive material by groundwater, which may come into contact with immobilized waste if it is stored underground for 1000 years or more. [4,5] Both borosilicate glass and Synroc also exhibit good resistance to radiation damage caused by the decay of the radioactive waste they incorporate. [4-7]
In order for the immobilization technology to be used in practice, it cannot be expensive. Ceramics like Synroc are known to be more costly than glasses. [8] While glasses are cheaper, vitrification still requires high initial investment and operational costs. [9] Consider the Defense Waste Processing Facility, a vitrification plant in South Carolina. The plant had cost $2.4 billion before it even opened in 1996. [10] Its cost of producing a cylinder of vitrified waste (10 feet tall and 1 foot in radius) was estimated to be $1.4 million. [10]
The technology and cost of immobilization only matter if there is a place to store the waste in the long term. In the United States, no permanent radioactive waste dump exists, primarily because of state and national politics. [11] In 1987, Congress selected Yucca Mountain in Nevada as a permanent geological repository site for radioactive waste storage. [12] However, Senator Harry Reid of Nevada wielded his political power to prevent the establishment of this repository by regularly blocking its funding. [11,13] In 2010, the Obama administration decided to shut down Yucca Mountain. This decision is often attributed to the political influence of Senator Reid, who at the time was also the Senate Majority Leader. [13]
Radioactive waste immobilization cannot be reduced to a technical issue involving the materials science of glasses and ceramics; it is also a political and economic issue. The high cost of immobilization technology, combined with the politics of waste disposal, prevent the technology from being widely used in the United States.
© Ajay Ravi. 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] N. Barber, "Yucca Mountain and the U.S. Nuclear Waste Storage Problem," Physics 241, Stanford University, Winter 2021.
[2] L. Thompson, "Vitrification of Nuclear Waste," Physics 240, Stanford University, Fall 2010.
[3] W. E. Lee et al., "Immobilisation of Radioactive Waste in Glasses, Glass Composite Materials and Ceramics," Adv. Appl. Ceram. 105, 3 (2006).
[4] A. Palke, "Ceramic Materials for Long-Term Sequestration of Radioactive Waste," Physics 241, Stanford University, Winter 2011.
[5] A. E. Ringwood and P. M. Kelly, "Immobilization of High-Level Waste in Ceramic Waste Forms," Phil. Trans. R. Soc. London 319, 63 (1986).
[6] C. Corkhill and N. Hyatt, Nuclear Waste Management (IOP Publishing, 2018).
[7] A. E. Ringwood et al., Immobilization of High-Level Nuclear Reactor Wastes in SYNROC: A Current Appraisal," Nucl. Chem. Waste Manage. 2, 287 (1981).
[8] P. M. Boffey, "Nuclear Waste: U.S. Prepares Momentous Step Toward Disposal," New York Times, 4 May 82.
[9] M. I. Ojovan and W. E. Lee, "Glassy Wasteforms for Nuclear Waste Immobilization," Metall. Mater. Trans. A 42, 837 (2011).
[10] M. L. Wald, "Factory Is Set to Process Dangerous Nuclear Waste," New York Times, 13 Mar 96.
[11] C. Clifford, "The Feds Have Collected More Than $44 Billion For a Permanent Nuclear Waste Dump - Here's Why We Still Don't Have One," CNBC, 18 Dec 21.
[12] C. Druzgalski, "The Fate of Yucca Mountain," Physics 241, Stanford University, Winter 2012.
[13] L. Ketterer, "The Nuclear Waste Policy Act of 1982's Relation to Yucca Mountain," Physics 241, Stanford University, Winter 2018.
