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| Fig. 1: The nuclear fuel cycle. [7] (Courtesy of the NRC. Source: Wikimedia Commons) |
Worldwide, around 440 nuclear reactors provide approximately 10 % of the worlds electricity, contributing significantly to low-carbon energy production. [1] However, nuclear energy is not risk-free as it has led to the accumulation of radioactive waste. High-level waste is commonly understood as highly radioactive waste material that is generated during the reprocessing of spent reactor fuel. Although the exact composition depends on the composition of the fresh fuel, the level of burn-up, or the reactor type, high-level waste always contains fission products (short-lived and long-lived) and actinides. [2] High-level nuclear waste (HLW) poses a particularly significant challenge to environmental safety and human health due to its high level of radioactivity and long half-live. Thus, its long-term management has been a particular concern for governments around the world. [3] Transmutation of high-level waste, which could break down long-lived radionuclides, has emerged as a promising technology for addressing the long- term storage problem surrounding HLW.
High-level waste is generated from the uranium fuel and transuranic elements in the nuclear reactor core. As can be seen in Fig. 1, spent nuclear fuel is currently first put into temporary waste storage before being reprocessed and stored in permanent disposal facilities. Temporary waste storage consists either of storing nuclear waste in storage pools next to the reactor for a few years or transporting it to separate nuclear waste storage sites where it is stored in dry caskets. However, neither wet storage in pools nor dry storage in casks are viable long-term solutions for nuclear waste. [3] For that reason, researchers have explored several long-term storage options, ranging from launching waste into outer space to placing it in subduction zones. [3] The two most realistic long-term solutions for high-level waste that have emerged are Deep Geological Repositories and the transmutation of the most dangerous elements and subsequent mid-term storage of the waste. [2]
Many governments have moved toward Deep Geological Repositories as a long-term storage solution for their high-level nuclear waste. These underground structures are meant to safely store high-level waste until the radioactivity of the waste has decayed by leveraging geological and engineering barriers. However, due to the long half-life of high-level waste, there are still concerns about the leakage of radioactive material into the biosphere, for instance, through underground water streams. Additionally, finding locations for Deep Geological Repositories and building them is a long and costly process, with long-term nuclear waste disposal cost estimates ranging from $148,000 to $1,041,000 per Metric Ton of Heavy Metal (MTHM). [4,5] For instance, the Yucca Mountain nuclear waste repository in Nevada is projected to have an NPV of $298,866 to $488,391 per MTHM. The US alone would thus have to pay between $26 billion and $42 billion for existing waste. [5]
Considering the risks and costs involved in long-term storage, eliminating radioactive species from high-level waste through partitioning and transmutation (P and T) has become a research focus. Since 99.9% of long-term radiotoxicity comes from very few isotopes (less than 1% of the waste), the idea of transmutation is to extract these isotopes from the spent fuel and produce transmutation fuel directly from these elements after a cooling down period in the hopes of reducing the load and radioactivity of the waste. [3]
Currently, most designs for high-level nuclear waste transmutation envision breaking down the radioactive parts of the waste in an accelerator-driven system. In these designs, minor actinides and long-lived fission products would be added to the fuel of an Accelerator-driven System (ADS) for transmutation after being extracted from the spent fuel of other commercial reactors. An accelerator-driven system is a device formed by coupling a subcritical nuclear reactor core with a charged particle accelerator like a proton cyclotron or proton linear accelerator (LINAC). The reason why these elements could not be added to a thermal reactor is that the neutron flux would be too low, which would impede the transmutation of minor actinides. Similarly, most fast reactors, which have a significantly higher neutron flux compared to thermal reactors, would not be suitable for nuclear waste transmutation as the transmutation of transuranics in a critical system presents significant safety concerns. [3] Thus, an accelerator-driven subcritical reactor could provide a better environment for high- level waste transmutation.
In this system, energy is generated through multiplying nuclear cascades initiated by the accelerated protons hitting the spallation target (usually lead). [3] When the high-energy protons hit the spallation target, the nuclei in the lead become excited, leading to the ejection of neutrons. These neutrons are then directed into the subcritical core (keff<1), where they are absorbed by a fissile nucleus, leading to fission. Here, the minor actinides, which are mixed with the fuel, are exposed to the neutron flux in the reactor and can absorb neutrons through a process called neutron capture. Upon neutron capture, some of the minor actinides undergo fission, which leads to their transmutation. On the other hand, long-lived fission products do not undergo fission and are transmuted through neutron capture alone, transforming them into isotopes with shorter half-lives or stable isotopes. Thus, the ADS could potentially enable energy generation and waste transmutation simultaneously. [3] In that case, the transmutation process could significantly reduce the radioactivity and load of the minor actinides and long-lived fission products. However, even if transmutation were successful, interim storage would still be necessary, although the requirements of that type of storage would be less strict. [3]
So far, no fully functional ADS reactor for waste transmutation exists. The first large-scale attempt to build an ADS for both energy generation and waste transmutation is the MYRRHA subcritical fast reactor, which is being constructed at the Belgian Nuclear Research Center SCK CEN. The reactor is estimated to have a final capital cost of over $1.6 billion and an operational cost of around $74 million per year. [6]
High-level nuclear waste presents a large and growing problem with no current long-term solution. Due to its high levels of radioactivity and long half-live of several thousand years, it poses a serious threat to humans and the natural environment. Nuclear waste transmutation could help solve the problem of high-level waste storage by breaking down the minor actinides and long-lived fission products in high-level waste, decreasing its radioactivity level and volume. Most research focuses on using an accelerator-driven system to transmute high-level waste, and with MYRRHA, the first subcritical fast reactor to test HLW transmutation is currently being built in Belgium. However, it will still take time until waste transmutation can be more widely used, and even if scientists succeed in transmuting high-level waste, interim storage of waste will be necessary to guarantee the safe disposal of high-level waste after the transmutation process.
© Cosima Paul. 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] L. M. Krall et al., "Nuclear Waste From Small Modular Reactors," Proc. Natl. Acad. Sci. (USA) 119, e2111833119 (2022).
[2] A. Herrera-Marténez, Y. Kadi, and G. Parks, "Transmutation of Nuclear Waste in Accelerator-Driven Systems: Thermal Spectrum," Ann. Nucl. Energy 34, 564 (2007).
[3] B. Madres, "Storage and 'Disposal' of Nuclear Waste," Physics 241, Stanford University, Winter 2011.
[4] C. Paul, "High Level Nuclear Waste Management in the UK, Physics 241, Stanford University, Winter 2024.
[5] C. Cranmer, "Cost of Nuclear Waste Management in the US, Physics 241, Stanford University, Winter 2024.
[6] "ESFRI Roadmap 2018 - Part1, Part2, Part3," European Srategy Forum on Research Infrastructures, 2018.
[7] "2018 - 2019 Information Digest," U.S. Nuclear Regulatory Commission, NUREG-1350 Vol. 30, August 2018.
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