Isotopes are atoms of the same element that have the same number of protons and electrons but a different number of neutrons. Some common isotopes in fusion are:
- Protium is a hydrogen isotope with one proton and no neutrons. It is the most common form of hydrogen and the most common element in the universe.
- Deuterium is a hydrogen isotope with one proton and one neutron. It is not radioactive and can be extracted from seawater.
- Tritium is a hydrogen isotope with one proton and two neutrons. It is radioactive, with a half-life of about 10 years. Tritium does not occur naturally but can be made by bombarding lithium with neutrons.
- Helium-3 is a helium isotope with two protons and one neutron.
- Helium-4 is the most common, naturally occurring form of helium, with two protons and two neutrons.
Physics of Nuclear Fusion: Reactions
Current nuclear reactors use nuclear fission to generate power. In nuclear fission, you get energy from splitting one atom into two atoms. In a conventional nuclear reactor, high-energy neutrons split heavy atoms of uranium, yielding large amounts of energy, radiation and radioactive wastes that last for long periods of time (see How Nuclear Power Works).
In nuclear fusion, you get energy when two atoms join together to form one. In a fusion reactor, hydrogen atoms come together to form helium atoms, neutrons and vast amounts of energy. It's the same type of reaction that powers hydrogen bombs and the sun. This would be a cleaner, safer, more efficient and more abundant source of power than nuclear fission.
There are several types of fusion reactions. Most involve the isotopes of hydrogen called deuterium and tritium:
- Proton-proton chain - This sequence is the predominant fusion reaction scheme used by stars such as the sun. Two pairs of protons form to make two deuterium atoms. Each deuterium atom combines with a proton to form a helium-3 atom. Two helium-3 atoms combine to form beryllium-6, which is unstable. Beryllium-6 decays into two helium-4 atoms. These reactions produce high energy particles (protons, electrons, neutrinos, positrons) and radiation (light, gamma rays)
- Deuterium-deuterium reactions - Two deuterium atoms combine to form a helium-3 atom and a neutron.
- Deuterium-tritium reactions - One atom of deuterium and one atom of tritium combine to form a helium-4 atom and a neutron. Most of the energy released is in the form of the high-energy neutron.
Conceptually, harnessing nuclear fusion in a reactor is a no-brainer. But it has been extremely difficult for scientists to come up with a controllable, non-destructive way of doing it. To understand why, we need to look at the necessary conditions for nuclear fusion.