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| Fig. 1: Nuclear Fuel Cycle. (Image Source: D. LaJoie, after the University of Michigan. [1]) |
Nuclear energy has become a significant and prominent resource in the United States and is currently responsible for roughly 19% of the country's electricity generation. With the first nuclear power plant in the U.S. beginning commercial function in 1958, there are now approximately 93 currently in operation, with an overall average capacity factor of 92.6%. Electricity is generated via controlled nuclear fission chain reactions that heat water and produce steam to power turbines. [1] Because this form of electricity generation does not depend on biofuels, no greenhouse gases are directly emitted from the process, making it an attractive alternative to other traditional processes. However, the drawback of nuclear energy is its inevitable generation of radioactive waste that presents an array of health and environmental issues. [2] The overall cycle of nuclear fuel and its processing requirements can be observed in Fig. 1. Currently, the U.S. accumulates approximately 2,000 mt of spent fuel each year. [1]
Nuclear waste is anything that becomes radioactively contaminated ranging anywhere from soiled clothing to chemical sludges to spent reactor fuel rods. The International Atomic Energy Agency (IAEA) categorizes radionuclides into two different groups depending on their half-lives of decay. The radionuclides are classified as either short-lived (half-life < 31 years) or long-lived (half-life > 31 years). As seen in Fig. 2, radioactive waste can further be categorized into the following five classifications: (i) Very-short-lived waste (VSLW), which can be stored for decay over a limited period; (ii) Very-low-level waste (VLLW), which does not require advanced containment and can be disposed of near the surface; (iii) Low-level waste (LLW), which has above-clearance levels but contains limited amounts of long-lived radionuclides; (iv) Intermediate-level waste (ILW), which contains both short-lived and long-lived radionuclides and requires a significant degree of containment and isolation at depths of up to 100 meters below the surface; and (v) High-level waste (HLW), which contains appreciable concentrations of long-lived α-emitting and/or β,γ-emitting radionuclides. Due to the significant quantities of heat generated from the radioactive decay process, HLW requires an advanced degree of isolation and containment at depths of several hundred meters below the surface and must be isolated for timescales on the order of hundreds of thousands, or even millions, of years. [2,3] However, per IAEA definition, nuclear waste classes are not strictly separated from each other. For example, the separation of LLW from ILW is not specifically defined because the total amount and concentration limits of radionuclides are based on safety report results and are therefore site and facility specific. Additionally, the varying durability of various waste containers from one facility to another will yield different allowable levels of radionuclides, further adding to the ambiguity of the cut-off line between waste classifications. [3]
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| Fig. 2: Classification of Radioactive Waste. (Image Source: D. LaJoie, after Fernandez et al. [2]) |
As seen in Fig. 3, the radioactive spent fuel accumulated in the U.S. each year, by volume, is 90% LLW, 7% ILW, and 3% HLW. Although, it should be noted that the small volume of HLW accounts for 95% of the total radioactivity of the waste produced. Most LLW and short-lived ILW, mainly contaminated equipment, is sent to near-surface disposal facilities. As for long-lived ILW and HLW, primarily in the form of spent reactor fuel rods, many countries reprocess this used fuel, but the U.S. does not. Ensuing their usage, freshly spent fuel rods emit approximately 92.5 TBq/kg of radiation. [4] Consequently, they are immediately sent to wet storage, which is basically multiple storage pools filled with circulating water, which acts to convect away the heat released from the decay of the fission products, before being evaporatively cooled by the surrounding air. [2] Currently in the United States, after a period of about ten years, the spent fuel is considered safe enough to be transported to dry storage, although, the surface of a spent fuel assembly still emits approximately 14.8 TBq/kg of radiation after this time period. [4] This form of storage utilizes dry casks, or large concrete and stainless-steel containers, which are designed to passively cool the radioactive waste while also preventing exposure to rainwater and the potential leaching of radioactive atoms into the ground. [2] In theory, after wet storage, the HLW can be transported to a deep geological repository, which consists of a multi-barrier system featuring carbon-steel waste containing canisters, sealing material, and geology that avails for the long-term isolation of the harmful waste. [2,5] As of now, no country has successfully constructed and operated a deep geological repository for HLW yet. However, Finland, which currently has a site under construction, is set to become the first country with a running HLW deep geological repository, with operation expected to begin within 2024. [6] These deep geological repositories plan to utilize bentonite, a very fine, highly expansive, and clay-like sealing material that acts as a buffer between waste canisters and restricts the migration of the toxic radiation. Additionally, these repositories must be constructed at great depths, roughly 500-1000 meters, within crystalline or sedimentary rock exhibiting low permeability (ie. opalinus clay, granite, etc.), in order to limit the exposure to fluids and leaching of radioactive material. [5]
Because no permanent geological repository has been dedicated in the United States, the government has had to fund utility companies for their interim storage of nuclear waste in the time-being. As of the end of 2021, the U.S. government has had to pay approximately $9 billion to these companies and the Department of Energy estimates it will cost another $30.9 billion until a permanent waste disposal option in the U.S. is found. [7] Compared to the amount of electricity generated, these nuclear waste management costs have been found to be in the range of $1.60-$7.10/MWh. [8]
The main environmental issue associated with nuclear power is the generation of radioactive waste, which may include uranium mill tailings, spent reactor fuel, and other radioactive materials. These substances can retain their radioactive and hazardous nature for thousands of years, posing a long- term risk to the environment and human health. Additionally, powering a standard one-gigawatt nuclear plant requires significant mining of uranium ore (20,000-400,000 mt/yr), a substantial amount of processing into fuel (27.6 mt/yr), and the inevitable disposal of the 27.6 t of fuel in the form of highly radioactive waste. [1] Over the lifetime of a plant, around 30 years, this corresponds to the need for mining about 1000 acre-feet of uranium ore. [8] In terms of gaseous emissions, the life cycle greenhouse gas intensity of nuclear power is estimated to be 34-60 gCO₂e/kWh, which is far below values for other common sources such as coal (1,001 gCO₂e/kWh). [1] Nevertheless, nuclear plants are associated with significant thermal pollution, do to the discharging of heat from radioactive waste into receiving waters. [9]
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| Fig. 3: Total Volume of Nuclear Waste. (Image Source: D. LaJoie, after the University of Michigan. [1]) |
Nuclear energy is a viable alternative method of electricity generation that has little to no GHG emissions and low mining requirements per kWh of electricity generated. However, the highly radioactive waste product created from this process poses significant environmental threats, requires a multitude of precautionary management strategies, and has substantial costs associated with its storage. With climate change and carbon emissions becoming increasingly popular political topics of concern, the argument for nuclear energy is clear. On the contrary, the lack of a permanent geological repository for nuclear waste disposal in the United States poses the question of how long nuclear energy will be sustainable for.
© Dominic LaJoie. 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] "Nuclear Energy," Center for Sustainable Systems, University of Michigan, Pub. No. CSS11-15, July 2023.
[2] P. Fernández-Arias, D. Vergara and A. Antón-Sancho, "Global Review of International Nuclear Waste Management," Energies 16 6215 (2023).
[3] M. I. Ojovan, "Nuclear Waste Disposal," Encyclopedia 3, 419 (2023).
[4] R. L. T. Thisirini and K. A. C. Udayakumar, "A Study to Find Out the Suitability of Nuclear Power Plant to Sri Lanka," JET-OUSL 6, No. 2, 27 (2018).
[5] R. C. Kale and K. Ravi, "A Review on the Impact of Thermal History on Compacted Bentonite in the Context of Nuclear Waste Management," Environ. Technol. Innov. 23, 101728 (2021).
[6] S. El-Showk, "Final Resting Place," Science 375, 806 (2022).
[7] 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, 19 Dec 21.
[8] V. Tsyplenkov, "Electricity Production and Waste Management: Comparing the Options," IAEA Bull. 35, No. 4, 27 (1993).
[9] E. E. El-Hinnawi, "Review of the Environmental Impact of Nuclear Energy," IAEA Bull. 20, No. 2, 40 (1978).