Nuclear fusion is a type of nuclear reaction where two or more atomic nuclei combine and form one or more heavier nuclei. The process of fusion forms many of the elements of the periodic table, plus it offers an opportunity for limitless energy production.
- Fusion combines two or more nuclei, forming one or more heavier nuclei.
- When light nuclei undergo fusion, such as deuterium and tritium, the reaction releases energy. However, combining heavy nuclei actually requires more energy than is released.
- Fusion occurs naturally in stars. The hydrogen bomb is an example of artificial fusion. Controlled artificial fusion holds promise as a useful energy source.
Nuclear Fusion vs Nuclear Fission (Examples)
Nuclear fusion and nuclear fission are both nuclear reactions, but they are opposite processes of one another. While fusion combines nuclei, fission splits them. For example:
- Nuclear fusion: Combining the hydrogen isotopes deuterium (H2) and tritium (H3) forms helium (H4). The reaction releases a neutron and energy. Each deuterium and tritium nucleus contains one proton. Deuterium has one neutron, while tritium has two. The helium nucleus has two protons and two neutrons.
- Nuclear fission: When an energetic neutron interacts with a uranium-235 (U235) nucleus (92 protons and 143 neutrons), the uranium atom splits apart. One possible outcome is a kypton-91 nucleus (36 protons and 55 neutrons), a barium-142 nucleus (56 protons and 86 neutrons), three neutrons, and energy.
In both fusion and fission, the number of protons and neutrons is the same on both sides of the reaction. The energy that is released in these reactions comes from the nuclear binding energy that holds the protons and neutrons together in the atomic nucleus. An atomic nucleus has more mass than the sum of its protons and neutrons on their own. This is because the binding energy has apparent mass. There is conservation of mass and energy, but remember from Einstein’s famous equation E=mc2 that energy and mass can be converted into one another. So, fusion releases energy when light atomic nuclei combine. On the other hand, fission releases energy when a heavy atomic nucleus splits. Fusion requires more energy than it releases when heavy nuclei combine, while fission takes more energy than it frees when light nuclei split.
How Nuclear Fusion Works
Fusion only occurs when two nuclei come together closely enough to overcome the repulsion between the positive electrical charges of the protons in their nuclei. When the distance between the nuclei is small enough, the strong nuclear force sticks the nucleons (protons and neutrons) together, forming a new, larger nucleus. This works because the strong force is (as you might guess from its name) stronger than electrostatic repulsion. But, it only acts over a very short distance.
Natural Fusion in Stars
Fusion occurs in stars because they are so massive that gravity brings nuclei close together. Mostly these nuclei are hydrogen and helium, although stars also form other elements via nucleosynthesis. Electrons don’t come into play because the extreme pressure and temperature within a star ionizes atoms into plasma.
Artificial Fusion
On Earth, fusion is quite a bit harder to achieve, or at least control. In lieu of tremendous mass and gravity, scientists apply extreme temperature and pressure differently than in stars. Mankind’s first successful fusion device was a boosted-fission device in the 1951 Greenhouse Item atomic test. Here, fission provided the compression and heat for fusion. The first true fusion device was the 1952 Ivy Mike test. The fuel for Ivy Mike was cryogenic liquid deuterium. The bombs dropped on Hiroshima and Nagasaki were atomic fission bombs. Much more powerful thermonuclear weapons combine fission and fusion.
Challenges for Artificial Fusion: Fuel and Confinement
Harnessing fusion for energy is tricky it requires the right fuel and a means of containment.
Fuel
There are relatively few reactions with suitable cross-sections for use as fuel:
- H2 + H3 → He4 + n0
- H2 + H2 → H3 + p+
- H2 + H2 → He3 + n0
- H2 + He3 → He4 + p+
- He3 + He3 → He4 + 2p+
- He3 + H3 → He4 + H2
- H2 + Li6 → 2 He4 or He3 +He4 + n0 or Li7 + p+ or Be7 + n0
- Li6 + p+ → He4 + He3
- Li6 + He3 → 2 He4 + p+
- B11 + p+ → 3 He4
In all cases, the reactions involve two reactants. While fusion occurs with three reactants, the probability of getting the nuclei together without the density found within a star just isn’t high enough. The reactant nuclei are small because the ease of forcing the nuclei together is directly proportional to the number of protons involved (the atomic number of the atoms).
Confinement
Confinement is the method of bringing the reactants together. The plasma is so hot that it can’t touch a container wall and needs to be in a vacuum. The high temperatures and high pressures make confinement challenging. There are four main methods of confinement:
- Gravitational confinement: This is how stars perform fusion. At present, we can’t replicate this method of forcing nuclei together.
- Magnetic confinement: Magnetic confinement traps nuclei because charged particles follow magnetic field lines. A tokamak uses magnets for confining plasma within a ring or torus.
- Inertial confinement: Inertial confinement pulses energy into fusion fuel, instantaneously heating and pressurizing it. A hydrogen bombs uses x-rays released by fission for inertial confinement that initiates fusion. Alternatives to x-rays include explosions, lasers, or ion beams.
- Electrostatic confinement: Electrostatic confinement traps ions within electrostatic fields. For example, a fusor contains a cathode within a wire anode cage. The negatively-charged cage attracts positive ions. If they miss the cage, they can collide with each other and fuse.
References
- Bethe, Hans A. (1950). “The Hydrogen Bomb”. Bulletin of the Atomic Scientists. 6 (4): 99–104. doi:10.1080/00963402.1950.11461231
- Eddington, A.S. (1920). “The internal constitution of the stars”. Nature. 106 (2653): 14–20. doi:10.1038/106014a0
- Janev, R.K. (ed.) (1995). Atomic and Molecular Processes in Fusion Edge Plasmas. Springer US. ISBN 978-1-4757-9319-2.
- Kikuchi, M.; Lackner, K.; Tran, M. Q. (2012). Fusion Physics. International Atomic Energy Agency. ISBN 9789201304100.
- Moses, E. I. (2009). “The National Ignition Facility: Ushering in a new age for high energy density science”. Physics of Plasmas. 16 (4): 041006. doi:10.1063/1.3116505