|
|
| Fig. 1: Example of Deuterium-Tritium hydrogen fusion. (Source: Wikimedia Commons) |
While many consider nuclear fusion to be the best long-term alternative to fossil fuels, a large number of people believe nuclear energy is unsafe. Others claim that harnessing nuclear energy is too expensive compared to other candidate energy sources. In an effort to overcome these issues, some researchers have shifted focus from more traditional hydrogen fusion to what is known as aneutronic fusion. [1]
Many designs for nuclear fusion technology, such as the reactors currently being built by the international nuclear research project, ITER, opt for the use of fuel composed of deuterium and tritium, two types of hydrogen nuclei. [2] This type of fusion releases energy in the form of fast-moving neutrons (see Fig. 1). The common deuterium-tritium hydrogen fusion can produce up to 17.6 MeV of energy in a single reaction. [3] Aneutronic fusion, on the other hand, uses nuclei of heavier elements such as helium, lithium, and boron. Two examples of this are proton-boron fusion, which releases up to 8.68 MeV of energy per reaction, and deuterium-helium fusion, which releases up to 18.3 MeV of energy per reaction. [1,3] This type of fusion releases the majority of its energy in the form of fast-moving, positively charged particles, as opposed to neutrons, which carry no charge (see Fig. 2). [2] This small but important difference makes the harnessing of nuclear energy both more efficient and safer.
Hydrogen fusion reactors need to implement a Rankine steam cycle in order to convert from nuclear energy to electrical energy. [4] During the steam cycle, water is heated and evaporated using the kinetic energy carried by the neutrons. The vaporized water then turns a turbine to generate electrical energy. While simple, this method of collecting energy is highly inefficient and a large fraction of the initial energy is lost during each step of the cycle. [5] Aneutronic fusion reactors do not require a steam cycle. Instead, these reactors can capture energy through direct electric conversion by making use of induction and other changes in the electromagnetic fields that result from the emission of charged particles. [6,7] This means that aneutronic fusion reactors can potentially be more efficient than hydrogen fusion reactors.
 |
| Fig. 2:Example of Aneutronic Fusion. (Source: Wikimedia Commons) |
Furthermore, aneutronic fusion reactors can be less dangerous than hydrogen fusion reactors. Hydrogen fusion reactions create a hazardous byproduct known as ionizing radiation. [1-8] The energetic neutrons radiated by hydrogen fusion reactors are capable of penetrating and damaging structures containing the plasma. [9] This radiation can also cause damage to living organisms by penetrating cells and burning tissue. [10] Therefore, because aneutronic fusion creates a relatively small amount of neutrons, it is said that aneutronic fusion produces little to no ionizing radiation. [11] Because of this, it is generally considered safer than deuterium-tritium fusion and other forms of hydrogen fusion.
In theory, aneutronic fusion can be better than hydrogen fusion as a source of nuclear energy. However, there are several limitations in our current technology that prevents aneutronic fusion from being a viable alternative energy option. First and foremost, nuclear fusion requires large amounts of energy in order to begin the fusion process. Nuclear fusion is only possible when plasmas are superheated to surpass extremely high temperatures. In the case of hydrogen fusion, this threshold temperature has to be high enough for the hydrogen particles to overcome the repulsive magnetic force created by a single pair of positively charged protons. [9] Because aneutronic fusion uses particles of elements with more than one proton, such as helium with two and lithium with three, the repulsive barrier between the fuel particles is greater. [11] This means the temperature required to create aneutronic fusion is also greater. To illustrate this, we can compare the fusion temperature of deuterium-tritium fusion with that of deuterium-helium fusion, the aneutronic reaction with only one extra proton. The threshold temperature for the fusion of deuterium and tritium around 30 million Kelvin is while the threshold temperature for the fusion of deuterium and helium is around 10 times greater. [3] Fusion with larger elements such as lithium and boron requires even higher temperatures. Therefore, aneutronic fusion requires higher input energy than hydrogen fusion. Moreover, because aneutronic fusion has a higher threshold temperature, it is extremely difficult to sustain. In order to ignite a self-sustaining fusion reaction, the fusion plasma has to meet what is known as the Lawson Criterion. [2] The Lawson Criterion graphs the relationship between the temperature of fusion plasmas and the amount of energy lost at those temperatures. [12] In Fig. 3, we can see that for the aneutronic deuterium-helium, the ignition temperature for self-sustaining, low-loss fusion is more than 2 times higher than that of deuterium-tritium hydrogen fusion. As follows, the ignition temperature for plasmas of higher larger element fuel is even greater. In addition, the structures built to contain the superheated plasma would also need to be sturdier. [8]
 |
| Fig. 3: Lawson Criterion of Lowest threshold aneutronic fusion compared to hydrogen fusion, created from derivations by J. D. Lawson. [15] (Source: Wikimedia Commons) |
Today, aneutronic fusion is far from being a viable source of commercial energy. However, as the search for the best alternative energy source continues to lead many toward nuclear energy, aneutronic fusion will remain a potential substitute for the more traditional hydrogen fusion. Currently, researchers in China and Russia are experimenting with various types of fuel for aneutronic fusion, such as proton-boron, deuterium-lithium, and helium-helium fuels. [13] Efforts are also being made to create better confinement technology through the use of magnetic fields. Future developments in low-temperature fusion can also advance the possibility of using aneutronic fusion as an energy source. [14]
© Gabriel Ruiz. 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] C. Baccou et al., "New Scheme to Produce Aneutronic Fusion Reactions by Laser-Accelerated Ions," Laser Part. Beams 33, 117 (2015).
[2] S. M. Motevalli and R. Fadaei, "A Comparison Between the Burn Condition of Deuterium-Tritium and Deuterium-Helium-3 Reaction and Stability Limits," Z. Naturforsch. A, 70, 79 (2015).
[3] R. F. Post, "Controlled Fusion Research - An Application of the Physics of High Temperature Plasmas," Rev. Mod. Phys. 28, 338 (1956).
[4] N. Lior, "Advanced Energy Conversion to Power," Energy Convers. Manage. 38, 941 (1997).
[5] F. Carlson et al. "Model of the Impact of Use of Thermal Energy Storage on Operation of a Nuclear Power Plant Rankine Cycle," Energy Convers. Manage. 181, 36 (2019).
[6] T. Weaver, J. Nuckolls, amd L. Wood, "Fusion Microexplosions, Exotic Fusion Fuels, Direct Conversion: Advanced Technology Pptions for CTR," Lawrence Livermore Laboratory UCID-16309, April 1973.
[7] G. H. Miley et al., "Inertial Electrostatic Confinement as a Power Source for Electric Propulsion," in Vision-21: Interdisciplinary Science and Engineering in the Era of Cybespace, U.S. National Aeronautics and Space Administratin, NASA Conf. Pub. 10129, March 1993, p. 185.
[8] G. Pajer et al., "Modular Aneutronic Fusion Engine," Princeton Plasma Physics Laboratory, PPPL-4761, May 2012.
[9] S. Mirhosseini, M. A. Polak, and M. Pandey, "Nuclear Radiation Effect on the Behavior of Reinforced Concrete Elements," Nucl. Eng. Des. 269, 57 (2014).
[10] J. L. Ryan, "Ionizing Radiation: the Good, the Bad, and the Ugly," J. Invest. Dermatol. 132, 985 (2012).
[11] S. Son, and N. J. Fisch, "Aneutronic Fusion in a Degenerate Plasma," Phys. Lett. A 329, 76 (2004).
[12] R. G. Mills, "Lawson Criteria," IEEE Trans. Nucl. Sci. 18, 205 (1971).
[13] B. Nayak, "Reactivities of Neutronic and Aneutronic Fusion Fuels," Ann. Nucl. Energy 60, 73 (2013).
[14] J. Gruenwald, "Proposal for a Novel Type of Small Scale Aneutronic Fusion Reactor," Plasma Phys. Control. Fusion 59 025011 (2016).
[15] J. D. Lawson, "Some Criteria For a Power Producing Thermonuclear Reactor," Proc. Phys. Soc. B, 70, 6 (1957).
