Energy Production From Nuclear Fusion Has A Timeline
Sun provides more than 99% of the energy on Earth and it derives its energy from nuclear fusion which is the fundamental source of energy in Universe. Humans want to use the same process for energy generation on the Earth.
Each nucleus has a certain binding energy pre nucleon i.e. proton or neutron, which is the energy released when the nucleus is formed. If a nucleus is formed by fusing lighter elements, the excess binding energy is released. This is the basis of nuclear fusion reactions.
Benefits
In theory, nuclear fusion offers a permanent solution to mankind’s energy needs. The most commonly used reactions fuse Deuterium, an isotope of Hydrogen with itself (D-D). Or with Tritium, another isotope of Hydrogen (D-T). Deuterium is abundant in nature and can be extracted from sea water (30 grams in every cubic meter). Tritium is rare but can be collected from nuclear fission reactors or created in nuclear fusion reactors using Lithium. There is no supply issue for the amount of Lithium needed for this purpose. In contrast, the fossil fuels e.g. Coal, Oil and Gas etc. will last not more a few hundred years.
Nuclear fusion delivers more energy per unit mass as compared to nuclear fission e.g. D-T reaction generates 4 times energy as generated in Uranium based fission reaction. It also creates less radioactive wastes, a major issue with nuclear fission reactors. Tritium is radioactive but its half life time is 12.3 years as compared to some radioactive wastes of nuclear fission reactors that are toxic for thousands of years. It is also expected to be less accident prone as compared to nuclear fission e.g. Chernobyl disaster (1986) and the Fukushima Daiichi nuclear disaster (2011).
Nuclear energy, through fission or fusion has an advantage over renewable sources of energy. The 2 commercially viable ones, i.e. solar and wind have strong dependency on weather. This requires additional requirement for storage of electricity. However nuclear energy can be generated as per needs.
Challenges
For nuclei to fuse, their Columbic repulsion of protons has to be overcome. This requires onerous conditions of density and temperature for a significant amount of time to generate net output of power. The temperature requirements is about 100 million 0C and exceeds that of stars as Gravitation aids in confinement in stars. Density is a critical factor for energy production to be viable. E.g. nuclear fusion generates only 276.5 watts of energy per cubic meter in Sun. Since Columbic repulsion increases with atomic number, only the lightest elements qualify for being used as fuels.
Neutrons are produced in many nuclear fusion reactions. The neutrons produced in D-D and D-T reactions carry 2.45 MeV and 14.1 MeV of energies respectively. Such high energy neutrons can induce radioactivity in the surrounding materials. D-D reaction itself produces radioactive Tritium.
The enormous amount of energy that a nuclear fusion reactor generates would create another challenge, i.e. its conversion into electricity in a controlled way.
These enormous technological challenges have led to a statement, “Fusion power is only 30 years away, and always will be”. While this is changing, fusion energy will need to commercially compete with renewable sources.
Approach
Magnetic confinement of plasma
This is the most common approach and uses a combination of powerful electromagnets to confine the super heated fuel in form of plasma into a toroid (doughnut) shape container called Tokamak. Electric coils surround the ring to produce the toroidal magnetic field. Other magnets provide poloidal (along poles) field and one of them induces current in the plasma heating it up. Further heating is provided by electromagnetic radiation and particle beams. Eventually more energy must be generated than what is fed in as represented by a symbol, Q.
The current record of Q is 0.67 and is held by JET (Joint European Torus). The largest experimental reactor in progress, ITER (International Thermonuclear Experimental Reactor) aims to have a Q = 10. By 2035, it will generate 500 MW of energy using 50 MW to heat the fuel. A commercial reactor will need Q > 10 if all energy needed to heat as well lost during conversion to electricity is taken into account. ITER will help test a number of critical fusion technologies, including blankets to slow down and absorb neutrons and later for Tritium breeding, plasma control, super cooled superconducting magnets, cooling systems, divertors to remove waste etc. Separately, EAST(Experimental Advanced Superconducting Tokamak) at China has achieved a plasma temperature of 120 million 0C for 100 seconds while KSTAR at Korea has achieved 100 million 0C for 20 seconds. These times are comparable to plasma confinement times needed for commercial needs.
Though ITER will not generate electricity, its successors i.e. DEMO reactors will generate electricity latest by 2050s. Many government and private initiatives have started to achieve the targets faster.
CFETR (Chinese Fusion Engineering Test Reactor) uses ITER research and will be commercially operational by 2050 though a prototype will be ready by 2035. K-DEMO of Korea will also use ITER data and will start electricity generation in 2050s.
UK government has started the STEP project (Spherical Tokamak for Electricity Production), that uses spherical tokomaks that have a smaller hole in the middle than toroidal ones. They use smaller magnets and the project aims to deliver electricity by 2040. A related company, Tokomak Energy uses spherical tokomaks and high temperature superconducting magnets, to reduce energy consumption. It aims to deliver electricity by 2030.
TAE Technologies aims to fuse Hydrogen and Boron, a reaction that does not directly produce neutrons but requires temperature of 1 billion 0C. It uses a different method called field-reversed configuration that does not requires toroidal magnetic field and hence simplifies construction and maintenance. It aims to deliver electricity by 2030.
Commonwealth Fusion Systems working with MIT is also using advances in electromagnets to use smaller electromagnet. It aims to deliver a net energy positive reactor by 2025.
Technological challenges may cause these dates to slip as has happened with ITER, but they still represent a time horizon.
Inertial confinement of plasma
This approach uses laser beams to heat up small pellets of fuels about a few millimetres in thickness which burns the outer surface of the pellets and generates an inward compression wave. This wave raises density of the fuel above 1000 times its liquid density and creates conditions of nuclear fusion. The energy released will induce further fusion reactions in surrounding fuel. As powerful lasers were not available initially, this approach had a late start.
But now the largest facility at National Ignition Facility (NIF), USA has achieved the Q value of 0.7, exceeding results of JET.
Conclusion
In the next 30 years, nuclear fusion reactors will start generating electricity. This will be a giant technological leap in mankind’s quest to produce energy. However, its commercial viability is unclear as of now.
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