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| Fig. 1: The three isotopes of Hydrogen and Helium-4. (Image Source: H. Moise) |
If realized, nuclear fusion energy has the potential to supply clean and relatively limitless energy. Many scientific and engineering challenges remain before this technology can be realized, one of which is identifying a viable fuel to sustain fusion reactors like tokamaks and stellarators. Fusion reactions demand tremendous temperatures to proceed and sustain their burning plasmas, requiring a fuel feedstock that is abundant, processable, and can be stored easily. Deuterium-tritium fuel is touted as a promising feedstock for these nuclear fusion reactors as the fuel reaches reaction conditions at lower temperatures and releases more energy than other fusion reactions. Only a few grams of this fuel is required to produce a terajoule of energy enough energy to sustain a single individual in a developing country (~5000 kWh/yr) for about 60 years. [1] The robust supply of these two hydrogen isotopes will, however, require innovative process designs of breeder reactors capable of producing this fuel on-site and cost effectively. Common amongst most chemical processes, a critical and expensive step found throughout this process will be the separation of tritium and helium.
Hydrogen exists naturally as three different isotopes: protium, deuterium, and tritium (Fig. 1). All three isotopes have on proton with varying numbers of neutrons. Protium, which is the most abundant state of this element, contains no neutrons while deuterium and tritium have one and two, respectively. The ion masses of the three increase with the number of neutrons. Deuterium and tritium can fuse to produce a helium atom, consisting of two protons and two neutrons, and a highly energetic (14.1 MeV) neutron co-product. When uncontrolled, the simultaneous presence of tritium and helium also brings about several operational challenges.
This undesired composition can occur at several points throughout this process. Any unreacted tritium found in the reactor effluent must be separated and recirculated back into the process. As helium is the product of deuterium-tritium fusion, it is vital that it is not present in this recirculation feed. Helium also needs to be separated from the breeding blanket reactors which use high energy neutrons to split lithium into the tritium fuel and a helium co-product. Some designs call for the use of helium as a coolant to circulate through the reactor core and remove heat from the nuclear fuel, which would require a purge of any tritium co-products. Finally, tritium decays with a half-life of 12.3 years, meaning that He-3 would be produced at rates significant for the time-scales of chemical processes. A tritium storage bed that uses metal hydrides would become 5% He-3 after one year in use. As He-3 does not form a solid metal hydride as hydrogen does, the accumulation of He-3 gas in the bed leads to a pressure buildup as well as a dilution of the fuel, requiring a need to separate and bleed off this excess helium. [2]
It is challenging to separate tritium or helium from a gas mixture, both elements are similar in size and helium is completely inert. The only off-the-shelf technology that can separate tritium from helium at commercial scales is cryogenic distillation. The refrigeration unit required to separate these two species will need to reach -248°C, which can require more than 3 kWh/kg hydrogen assuming the gas is already at 20°C. [3] Below this temperature, tritium liquifies and the single phase feed becomes a two phase product stream with a gas phase composed mostly of helium. [4,5] An emerging field of research is focused on developing Pd based membranes to overcome this energy intense process. Membranes work in more moderate temperature ranges (300-600°C) and utilize Palladium's ability to activate diatomic hydrogen into atoms allowing them to diffuse and migrate through the thin film of the metal. [6] Aside from the high cost of Pd, which is oresently greater than $30,000/kg, these membrane units are not yet used at the scales required to supply global energy. Another field of developing research for tritium separation includes ZrCo getter beds operated similarly to pressure-swing adsorption (PSA) units, which forms a metal hydride at elevated pressures. After bed saturation, the tritium can be desorbed and released when heated to temperatures of 400°C. Current developments of this technology are hindered by slow reaction kinetics for the absorption and desorption of the hydrogen as well as a tendency to form nonreversible compounds in the presence of hydrogen pressure at high temperatures, called disproportionation. [7]
In conclusion, the separation of tritium and helium in nuclear fusion processes presents a complex and critical challenge, crucial for the viability and success of future fusion-based energy generation. The deuterium-tritium fusion process, while promising in terms of energy yield, necessitates the meticulous management of byproducts - primarily tritium and helium - to maintain a closed-loop fuel cycle and mitigate operational complexities. Current separation solutions, such as cryogenic distillation are effective, but are also energy-intensive and not without limitations, underscoring the need for new and innovative approaches. The exploration of Pd-based membrane technology and ZrCo getter beds signals a move towards more energy-efficient and potentially cost-effective methods. However, these emerging technologies are still low in technology readiness levels (TRLs) and while promising, face their own set of challenges including high material costs, scalability issues, and operational constraints.
As the quest for a sustainable and clean energy future continues, the development of efficient tritium-helium separation technologies remains a key area of research and development. Addressing the challenges associated with these technologies is not just a matter of scientific and engineering advancement but also a step towards realizing the immense potential of nuclear fusion as a major energy source. The journey towards harnessing fusion energy mirrors humanity's broader pursuit of technological solutions for a sustainable future, and the successful separation of tritium and helium will be a significant milestone in this journey.
© Henry Moise. 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] I. Chatzis and M. Barbarino, "What Is Fusion, and Why Is It So Difficult to Achieve?" IAEA Bulletin, International Atomic Energy Agency, May 2021, p. 4.
[2] "DOE Handbook: Tritium Handling and Safe Storage," U.S. Department of Energy, DOE-HDBK-1129-99, March 1999.
[3] M. T. Syed et al., "An Economic Analysis of Three Hydrogen Liquefaction Systems," Int. J. Hydrog. Energy 23, 565 (1998).
[4] D. Park et al., "Dynamic Optimization of Cryogenic Distillation Operation For Hydrogen Isotope Separation in Fusion Power Plant," Int. J. Hydrog. Energy 46, 24135 (2021).
[5] M.Enoeda et al., "Hydrogen Isotope Separation Characteristics of Cryogenic Distillation Column," Fusion Eng. Des. 10, 319 (1989).
[6] N. Pal et al., "A Review on Tpes, Fabrication and Support Material of Hydrogen Separation Membrane," Mater. Today: Proc. 28, 1386 (2020).
[7] L. Yue et al., "Hydrogen Separation From Hydrogen/Helium Mixtures Under Different Pressures By Using U-Shaped ZrCo Getter Beds," Fusion Eng. Des. 169, 112435 (2021).