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| Fig. 1: Modelled cost of Co-Ex Reprocessing [9] (Courtesy of the DOE.) |
Developing sustainable and politically viable management strategies for spent nuclear fuel is critical to the viability of nuclear energy in decades to come. Two overarching options exist for spent nuclear fuel: disposal (a once-through fuel cycle) or reprocessing (in pursuit of a closed fuel cycle).
Spent nuclear fuel from a typical light-water reactor contains around 95.5% uranium-oxide (~99% of which is U-238, a fertile isotope), up to 1% plutonium (70% of which is fissile), 3.5% fission products and 0.1% other actinides. This means that ~96.4% of spent fuel is not in fact waste, but an energy resource: U-238 and Pu can both be reprocessed into MOX (Mixed- Oxide) fuel that is suitable for light water reactors. [1]
Reprocessing is therefore attractive for several reasons. [2]
It allows for more efficient use of mined uranium.
It reduces the requirement for enriched uranium (U-235), which is costly to produce.
It aids in waste management by reducing the volume of radioactive materials sent to geological storage by ~96%.
It also reduces the volume of decay heat production sources in stored waste.
As of the time of writing, all commercial reprocessing plants use some variation of a mature technology called the PUREX (plutonium uranium extraction) process. [3] There are also several novel reprocessing technologies under development that aim to solve the challenges of PUREX. The technology closest to commercialization is COEX (see Fig. 1), with differences to PUREX outlined in Table 1.
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| Table 1: Comparison of PUREX and COEX processes. [4] |
In the PUREX process, spent fuel is removed from the cladding using a chemical solution (nitric acid). The resulting nitrate solution is then mixed with other solvents to separate the uranium and plutonium from the fission products. [3]
The uranium/plutonium solution then undergoes several further cycles of extraction to separate and purify the uranium and plutonium. The outputs of the PUREX process are solidified uranium (primarily non-fissile U-238) and plutonium, as well as the waste solution containing the fission products. Thie plutonium oxide product, sometimes in combination with enriched uranium, is recycled into Mixed Oxide (MOX) fuel, which can be used in light water reactors. The waste solution is vitrified to form glass waste that can be sent to long-term storage. [4]
The PUREX process has several benefits: it is a mature technology, and improvements over time have improved economic viability and helped to minimize the volume of radioactive waste.
However, the process is dangerous, as the combination of chemicals used (nitric acid and TBP) have a high of explosion if not carefully managed, and result in the corrosion of equipment. A significant amount of energy is required, particularly during the first separation cycle. [5] Additionally, there is no separation of Americium and Curium from the fission products. [6]
Further, as one of the output streams is high-purity plutonium, this type of reprocessing carries an inherent proliferation risk that requires safeguards.
It is also commonly argued that the cost of a closed fuel cycle (reprocessing) is significantly higher than that of a once-through fuel cycle. A 2014 study examined the levelized cost of each type of fuel cycle and found that with a uranium price of $51/lb, the cost per kWh of the cycles were approximately equal. Above this price, a closed fuel cycle had the potential to be more economic. Notably, their results were sensitive to (a) the discount rate used, (b) the cost of the PUREX process, and (c) the fabrication cost of MOX fuel. At their high estimate of PUREX costs, the breakeven price of uranium rises to $90/lb. [7] Per the St. Louis Fed, the price of uranium rose to $70/lb in December 2023, its highest level since early 2008. If this study is accurate and pressures on the uranium supply chain continue, it suggests that PUREX reprocessing may become more financially attractive.
Research on advanced reprocessing is ongoing in France, Japan and the US (and likely China and Russia). Most of these new reprocessing technologies attempt to manage one of the following issues:
Concerns over proliferation caused by separating plutonium.
The volume of high-level waste present at the end of the process.
The economics of the process.
The safety of the process.
The needs of next generation reactors (which can burn different types of fuel).
These technologies can be split into aqueous reprocessing (where nuclear fuel is dissolved into an acidic solution, as in PUREX), and pyroprocessing (separation is accomplished through high-temperature electrorefining). Pyroprocessing has not yet moved beyond laboratory-scale demonstrations, nor have most advanced aqueous reprocessing technologies. However, one advanced aqueous reprocessing technology is closer to commercial viability: COEX. [4] For the purpose of this paper, we will therefore focus on this as the primary alternative to PUREX.
The COEX (co-extraction) process is being developed in France by AREVA and CEA.
It is an adjustment of the PUREX process, meaning that it is still an aqueous reprocessing technology and uses chemical solutions to extract and separate the spent fuel components. However in this process, uranium and plutonium are extracted together. The end product is a combined uranium/plutonium product in a ratio that is adequate for MOX fuel fabrication, as well as a pure uranium stream. There is no separation of plutonium, which in theory should improve proliferation resistance as compared to PUREX. [8]
There is limited information about the cost of the COEX process as compared to PUREX. A 2020 DOE study suggested a range for COEX of $1,300-1,900/kgHM (mean of $1,633/kgHM). [9] This compares to the PUREX cost of $800-1,800/kgHM (mean of $1,100/kgHM) modelled by the authors in the 2014 fuel cycle cost study. [7] The cost of COEX could therefore be economically viable if the price of uranium remains high.
Of the advanced aqueous reprocessing technologies, COEX may be the most promising.
However, many of the issues of the PUREX process remain: nitric acid is still explosive and risks equipment corrosion, and energy usage is high. Further, while plutonium is not separated, it is still present in relatively concentrated quantities and so needs to be kept secure.
Additionally, co-extraction of plutonium and uranium does not negate the risk of proliferation as the PUREX process demonstrates, these can be separated into their components given time and resources.
The COEX process seeks to resolve the key issue with PUREX reprocessing by ensuring that there is no separation of plutonium during reprocessing. As discussed, a bad actor could still recover the plutonium; however, it removes some of the concern around a country generating and then needing to manage a plutonium stockpile.
© Caitlin Cranmer. 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] O. H.Zabunoǧlu and L. Özdemir, "Purex Co-Processing of Spent LWR Fuels: Flow Sheet," Ann. Nucl. Energy 32, 151 (2005).
[2] S. . Woo, S. S. Chirayath, and M. Fuhrmann, "Nuclear Fuel Reprocessing: Can Pyro-Processing Reduce Nuclear Proliferation Risk?" Energy Policy 144, 111601 (2020).
[3] G. Senentz, "The Current Industrial Mono-Recycling of U and Pu: Reprocessing Step," in Encyclopedia of Nuclear Energy, ed. by E. Greenspan (Elsevier, 2021), p. 482.
[4] M. F. Simpson and J. D. Law, "Nuclear Fuel Reprocessing," Idaho National Laboratory, INL/EXT-10-1753, February 2010.
[5] S. I. Stepanov and A. V. Boyarintsev, "Reprocessing of Spent Nuclear Fuel in Carbonate Media: Problems, Achievements, and Prospects," Nucl. Eng. Technol. 54, 2339 (2022).
[6] "Spent Fuel Reprocessing Options," International Atomic Energy Agency, IAEA-TECDOC-1587, August 2008.
[7] C. Zhou et al., "Economic Analysis of Two Nuclear Fuel Cycle Options," Annals of Nuclear Energy 71, 230 (2014).
[8] D. Bascone, P. Angeli, and E. S. Fraga, "Optimal Design of a COEX Process For Spent Nuclear Fuel Reprocessing Using Small Channels," Comput. Aided Chem. Eng. 44, 2365 (2018).
[9] "Advanced Fuel Cycle Cost Basis Report: Module F1 Spent Nuclear Fuel Aqueous Reprocessing Facility," Idaho National Laboratory, INL/EXT-21-62313, April 2021.