By Hiranya Bose 10 O (AIS NOIDA)
To understand fusion, we need to understand nuclear energy, which can simply be defined as the energy released by the nucleus of an atom. As of now, the only two processes known to man through which nuclear energy is produced are fission, and of course, fusion. Splitting the atomic nucleus in 1938 was big news, now we’ll see why joining atomic nuclei is even bigger news.
Current Energy Production
Currently, humans have only been able to produce nuclear energy for commercial use through nuclear fission – which is the splitting of a heavier atomic nuclei into lighter nuclei. A neutron is forced to collide with a heavy atom, like that of Uranium-235. Collision takes place and the nucleus is split into lighter nuclei – it’s exothermic – and this releases nuclear energy. More neutrons boil off from the split nucleus and collide with more heavy nuclei, leading to a chain reaction. The minimum amount of fissile material required for such a self-sustained chain reaction is called critical mass, but a mass larger than this, where there’s a higher rate of fission, is called supercritical mass. If not controlled with control rods at this stage, an intense wave of heat, light, air pressure, and radiation is liberated – causing a nuclear explosion. An uncontrolled reaction like this is produced by atomic weapons.
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| Source: https://www.atomicarchive.com/science/fission/index.html |
Although a reliable source of clean and safe (if controlled) energy, the heavy atoms which split in nuclear fission are getting exhausted and used up. These elements are not renewable in nature, which means neither is fission energy. That’s why the need to explore fusion is essential.
Nuclear Fusion
Although a reliable source of clean and safe (if controlled) energy, the heavy atoms which split in nuclear fission are getting exhausted and used up. These elements are not renewable in nature, which means neither is fission energy. That’s why the need to explore fusion is essential.
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| Source: https://www.atomicarchive.com/science/fusion/index.html |
Fusion in a star
A natural example of fusion can be seen in the bright sky, the Sun...or any star for that matter. Although the amount of fuel present in a star is finite, nuclear fusion keeps a star active for billions of years before it succumbs to its own gravity and collapses. In the star’s core, four hydrogen nuclei fuse to make a heavy helium nucleus. Such a phenomenon, when elements are created from fusion in a star, is called stellar nucleosynthesis. In the core, hydrogen atoms get heated to such a high temperature that they gain sufficient kinetic energy to overcome the forces of electric repulsion and, ultimately, fuse. In this case, a large amount of energy is released due to the change in mass, abiding by Einstein’s famous equation proposed in his Theory of Special Relativity:
E=mc2
Where ‘E’ is energy, ‘m’ is mass, and ‘c’ is the speed of light in vacuum (approximately 299,792,458 m/s)
Fusion in a thermonuclear bomb
Nuclear scientists and engineers are still struggling to produce commercial energy through fusion. We’ll get to that soon. However, fusion has satisfied humans in one instance, ironically, with weapons of mass destruction – Teller and Ulam’s thermonuclear bombs. Edward Teller was a Hungarian American physicist who worked alongside J. Robert Oppenheimer in the Manhattan Project as the Director of the Theoretical Division, while Ulam was a nuclear physicist and mathematician who also contributed to the project. Now, these weapons are also known as ‘hydrogen-bombs’ because light nuclei of hydrogen are used in the fusion process since they carry weaker positive charges - which means less resistance. These weapons are approximately one thousand times more powerful than atomic bombs because its explosive process includes both fission and fusion.
The primary stage: The detonation of conventional explosives brings together sufficient uranium to fission, causing a massive explosion with large amounts of heat released. However, this explosion is reflected inwards by a uranium casing, not causing widespread destruction yet. The force and heat of the explosion are channeled to the fusion fuel – lithium-6 deuteride. The bomb has now entered the secondary stage. Lithium-6 deuteride is the bomb’s fusion fuel, which converts into tritium – a hydrogen isotope used in fusion. As the fuel gets exposed to heat, fusion occurs and the tritium collides with another hydrogen isotope used in fusion, deuterium. Nuclear fusion also releases neutrons as a ‘by-product’since electrons and protons fuse under great heat, and these neutrons collide with the surrounding uranium to increase the rate of fission, which will put pressure on the deuteride to produce more fusion reactions. A massive explosion occurs when enough energy is obtained, which contaminates water, air, and soil, and can destroy buildings and cause third degree burns from several kilometers away. The most destructive thermonuclear weapon ever created is the Russian ‘Tsar Bomba’, which could cause severe burns even from 100 kilometers away.
Binding Energy
Fission and fusion release binding energy. Binding energy is the measure of the bonds between nucleons of an atom. Binding energy is a key requirement to calculate the net release of energy from nuclear reactions. The formula to calculate binding energy is:
B = (Zmp + Nmn – M)c 2
Where ‘Z’ is the number of protons, ‘N’ is the number of neutrons, ‘mp’ is the proton mass, ‘mn’ is the neutron mass, and ‘c’ is the speed of light.
Deuterium and Tritium Fusion
Nuclear fusion has the potential to produce clean, safe, and renewable energy for commercial use. That’s the goal, but obstacles lie ahead. To sustain fusion, we would require a viable fuel. All elements lighter than Iron can fuse. However, most of these fusion reactions can only take place under the intense heat and conditions of plasma, like that of a star. A fuel which has the potential to satisfy commercial requirements on Earth is ‘D-T fuel’ or ‘Deuterium – Tritium fuel’. Not only can these isotopes of hydrogen reach the fusion stage under lower temperatures, but it also releases more energy than other fusion reactions. 1 gram of deuterium-tritium fuel can produce as much energy as 9084 liters of oil.
Tritium may not be in abundance, but that doesn’t mean it cannot be synthesized. Tritium can easily be made in the lab by exposing lithium-6, the reserves of which could last up to a million years, to neutrons emitted by nuclear fission. This process can be described by the equation:
Fusion devices, in theory, can also produce tritium by exposing lithium-6 to fusion neutrons. This is called tritium breeding.
Conditions for Fusion
The fuel can only undergo fusion in the presence of extreme heat – more than 100 million degrees Celsius. This is because the atoms can only collide into one another when they gain the kinetic energy required to overcome the forces of repulsion between their positively charged nuclei, and a fusion reaction would likely occur when the nuclei of both hydrogen isotopes are at least ~10-13 m from each other. The only way to get atoms this hot is through plasma – a state of matter, like an energized gas, where electrons separate from their nuclei to leave positively charged ions and negatively charged free moving electrons. The fusion reactions in stars and thermonuclear bombs also require this state of matter, with stars essentially being a ball of plasma and hydrogen bombs containing polystyrene foam which converts to plasma under heat.
Plasma generation for fusion requires two conditions:
1. Enough particle density within it to increase the likelihood of collision and fusion of the fuels.
2. Enough time while confining the plasma as it expands rapidly within a confined space.
Tokamaks
A tokamak is a big and expensive science experiment. The word is derived from the Russian acronym ‘тороидальная камера с магнитными катушками’ which translates to ‘toroidal chamber with magnetic coils.’ It’s a donut shaped machine in which plasma is generated and confined to conduct fusion. In tokamaks, plasma is confined with the help of a magnetic field due to which electrons and ions don’t seep out and attack the walls of the vessel. This method is called magnetic confinement fusion or MCF, which is the leading way to study fusion. In stars, plasma doesn’t need to be confined by a magnetic field, for the necessary conditions are sustained by its own gravity. At the start, air is removed from the vessel’s vacuum chamber. The magnets are then charged, and the gaseous fuel is brought in, which breaks down into plasma when electricity is passed through the ‘torus’. Fusion commences. The energy released by fusion reactions in the plasma is absorbed by the walls of the tokamak as heat which is then used to produce steam and generate electricity through the kinetic energy of turbines like conventional power plants.
ITER (initially the International Thermonuclear Experimental Reactor), which is the world’s largest fusion experiment in southern France, aims to build the world’s largest Tokamak with the help of the thirty-three nations involved in the project. Many tokamaks around the world have achieved fusion through the MCF method, but only for a matter of minutes. EAST in China managed to maintain plasma for approximately 17.77 minutes while WEST, in southern France, broke the world record with 22 minutes of magnetic confinement. There are certainly more records to be achieved and beat, but the problem regarding long term MCF lingers.
MCF Challenges
The major issue lies in the materials used in tokamaks. These materials are subjected to temperatures hotter than that of the Sun. The vessels components must maintain structural integrity whilst enduring not only the temperature, but energized neutrons, vacuum and radiation to make long term fusion a reality. Another challenge is the lack of human involvement in tokamaks due to the extreme conditions, work in these machines can only be executed by automated systems. Although this sounds efficient, it isn’t. This is once again due to the harsh environment created by the plasma, making it an engineering hurdle to ensure reliability in these robots. Other limiting factors include heat exhaust management and heat removal. Exhaust divertors are, unfortunately, a work in progress due to the power density in regions its placed. Heat removal is also a major challenge because tokamaks aren’t designed to expel heat well and the placement of such removal systems can interrupt the flow of plasma, ending the fusion reaction.
Inertial Confinement
Another method, though not as advanced or well researched about, is inertial confinement fusion or ICF. Here, fusion occurs due to swift heating and compression of pellets containing the fusion fuel. In fact, the earlier example of the hydrogen bomb was dependent not on MCF, but on ICF. Remember, the fission explosions caused the deuterium and tritium to fuse, and this took place through rapid compression and implosion. The Sandia National Laboratories’ Magnetized Liner Inertial Fusion (MagLIF) experiment vividly demonstrates how ICF works.
Although the ICF method sounds both interesting and promising, it is not as well explored compared to MCF. The National Ignition Facility makes use of an ICF strategy with lasers. These lasers generate the same conditions inside a star to produce fusion reactions. However, this mechanism is not designed for commercial energy production because the laser cannot be fired repetitively due to energy requirements – making it low yield. Moreover, the fusion conditions achieved in inertial confinement result in high pressure of the fusion fuel. This causes the fuel to disassemble rapidly, and fusion should begin before this happens. Lastly, along with many other concerns, ICF would require careful attention to materials because the pulses in inertial confinement would need the walls of the target to withstand the energy equal to kilograms of explosives.
Long-term fusion is necessary, not for now but for the coming generations. Before all our resources are consumed, it is crucial that we harness the power of the core of a star. Uranium will deplete, but hydrogenous isotopes will remain, perhaps outlasting humanity. More than quarter of a century ago, the culmination of physics was fission energy and fission weaponry. Unfortunately, fission will become history due to its exhaustible resources. Fusion, if achieved in the way we need it, could produce four million times the amount of energy produced by fossil fuels, and according to ITER, fusion reactors would last for more than a thousand years (not accounting for the massive lithium-6 reserves underwater).