By Albert S, Fall 2020.
As climate change and severe pollution are taking place in the 21st Century, nature seems to have shown us a premonition of the incoming energy crisis. Having relied on non-renewable energy sources like fossil fuels for decades, the whole human world could be at stake due to climate change, pollution, and all kinds of the negative consequences of these practices. Moreover, these nonrenewable resources could be depleted over this century. To remedy this, sustainable energy forms like solar energy and wind energy have been considered. However, the efficiency and practicality of these energy forms are still not up to par with fossil fuels. Nuclear energy, on the other hand, with proper waste management, generates abundant energy with limited carbon emissions.
Nuclear reactions are reactions on the atomic level that give extraordinary amounts of energy. They usually involve the forming of a bigger atom from smaller atoms or the breaking of bigger atoms into smaller atoms. Such reactions can produce millions of times more energy per unit mass than chemical reactions like burning coal. Nuclear energy could therefore be a potential solution to the sustainability problem, as it is more powerful than other energy sources.
There are mainly two types of nuclear reactions for generating energy: nuclear fission and nuclear fusion. Nuclear fission is the process of atoms breaking down into smaller atoms due to bombardment of neutrons. A canonical example of this process would be shooting a neutron at an atom of Uranium-235, which turns the atom into Uranium-236. As the bombardment brings more kinetic energy to the system, the atom can no longer be held together by the original force of the Uranium-235 atom, and thus it begins to split. Nuclear fission is usually carried out and amplified with what’s called a “chain reaction.” Each time an atom splits, neutrons are emitted, which can bombard other atoms, causing them to emit more neutrons and so on [1].
Nuclear fission is indeed powerful, but fusion can produce four times the energy fission produces with the same reactant mass. Moreover, catastrophes like meltdowns that can happen on fission reactors are theoretically impossible on fusion reactors. Physically, fusion is also very different from fission. The Sun is a good example of fusion occurring in nature. In the core of the Sun, the inner forces squeeze four hydrogen atoms into one helium atom, and in doing so a huge amount of repelling force (e.g., the electric force between two protons) has to be overcome. However, as fusion reactors are nowhere near as massive as the Sun, it would take extraordinary artificial efforts to overcome the repelling force [2]. Even despite these challenges, ever since the first nuclear fusion reaction done with a particle accelerator, scientists and engineers have been able to enable the reaction in multiple ways. Though the idea of nuclear fusion as a source of energy has been around for nearly a century, nuclear fusion reactors still have not been able to supply electricity to consumers. Why is that and how far are we from fusion energy?
Nuclear fission has been used for power generation for many decades. In a nuclear fission reactor, the area where the reactions occur is called the “core” of the reactor. The core is contained in a radiation protected structure, and usually immersed in coolants like water to ensure the temperature of the system stays low. The energy generated by the reaction is used to heat the coolant, which circulates between the core and a water tank to generate steam, and the rest of the reactor acts much like a steam engine.
Why then, do we still not have fusion reactors in a similar manner? In order for smaller atoms to stick together to form big atoms, the repulsion between them has to be overcome. Coulomb force is the force repelling two particles with the same sign of charge, and in order to overcome the repulsion, atoms must have huge kinetic energies. The first time humans achieved fusion was by a particle accelerator: shooting deuterium nuclei into a metal foil containing deuterium. However, using particle accelerators to produce nuclear fusion was proven to be inefficient. More particles deviate from the course than bind with atoms, and the net input of energy would be larger than the net output of energy, making the device useless. To overcome this barrier, researchers discovered that in order for the atoms to constantly collide at high speed, and ensure the net energy output is more than the net input, what we would need is a system where materials are heated to an extremely high temperature. As mentioned above, nuclear fusion is all about overcoming the repulsion. The smaller the atom, the less repulsion there is going to be since smaller particles carry fewer protons. However, even for particles as small as the hydrogen atom, the temperature requirement for overcoming the repulsion is as high as 100 million Kelvin.
How can we contain such a hot system? The International Thermonuclear Experimental Reactor (ITER) is making a step in achieving this by using magnetic force. The model they are working with is called Tokamak [3]. Tokamak means toroidal chamber with magnetic coils. It is a device that uses a powerful magnetic field to confine hot plasma in a torus shape. A magnetic field exerts a force on a traveling particle in a direction perpendicular to the direction of the velocity of the particle. Thus a particle traveling in a magnetic field usually goes around in a periodic trajectory (in fact, in an ideal case, a particle traveling with uniform velocity perpendicular to the magnetic field travels in a perfect circle). Theoretically, if the magnetic field is powerful enough, the hot plasma resulting from the heating of the system can be contained in this magnetic field. Once hot plasma is contained in the system, the atoms within rumble around at high velocities and fuse. However, there are many ongoin issues with this prototype, including particle balance and slow oscillations of the central temperature [3]. Another issue is that sometimes the system starts “chirping,” which means that the hot plasma is not entirely contained in where we want it to be, but diverges onto edges, which is not ideal for the reaction[4].
There is a great deal of effort to turn theories into prototypes, just as there is a great deal of effort to turn experimental prototypes into usable products. Luckily, not only have international governmental efforts come together, but also private companies are too working on sustainable energy for the next generation. Companies such as General Fusion and TerraPower are all dedicated to bringing fusion power plants into reality. While no one knows what the future will be like, nuclear energy will be almost certain to play a role in tomorrow’s world.
[1]“Nuclear Fission.” ANS. Accessed November 24, 2020. http://nuclearconnect.org/know-nuclear/science/nuclear-fission.
[2]“Nuclear Fusion.” ANS. Accessed November 24, 2020. http://nuclearconnect.org/know-nuclear/science/nuclear-fusion.
[3]“Machine.” Accessed November 24, 2020. https://www.iter.org/mach.
[4]“How to Keep the Superhot Plasma inside Tokamaks from Chirping.” Princeton Plasma Physics Lab. Accessed November 24, 2020. https://www.pppl.gov/news/2016/08/how-keep-superhot-plasma-inside-tokamaks-chirping.