Nuclear fusion reactors, if they can be made to work, promise virtually unlimited power for the indefinite future. This is because the fuel, isotopes of hydrogen, are essentially unlimited on Earth. Efforts to control the fusion process and harness it to produce power have been underway in the United States and abroad for more than forty years.

Nuclear fusion is the source of energy in the sun and stars where high temperatures and densities allow the positively-charged nuclei to get close enough to each other for the (attractive) nuclear force to overcome the (repulsive) electrical force and allow fusion to occur. Fig. 3 shows one fusion reaction. The most promising fusion reaction,

³H + ²H Æ ⁴He + n + 17.6 MeV

involves the radioactive nuclide tritium ( ³H), available from the nuclear production reaction

⁶Li + n Æ ³H + ⁴He.

To produce energy using this reaction, both the magnetic confinement reactor with a high temperature plasma (a gas that has been completely ionized) and the inertial confinement reactor (which utilizes laser implosion technologies) have been investigated. Extremely high plasma temperatures are required in the magnetic confinement reactor and difficult laser implosion techniques are required for the inertial confinement reactor. Although significant progress has been made in these investigations, no working reactor that produces more energy than it consumes has been built. Unfortunately, the funding for continuing this work has declined, and the work is proceeding at a slower pace.

Although these types of reactors would not have the fission product waste disposal problem of fission reactors, fusion reactors generate large number of fast neutrons, leading to large quantities of radioactive byproducts.

Another approach to nuclear fusion–an approach that could lead to aneutronic power (power without neutrons) and non-radioactive nuclear energy–uses the concept of colliding-beam fusion (CBF). One aneutronic method features the ²H + ³He reaction leading to the products ¹H + ⁴He. However, this requires ³He as fuel and terrestrial sources of this are limited. The Moon is a potential source of ³He produced by cosmic-ray protons hitting the Moon directly and not being absorbed by an atmosphere as on Earth. Another potential approach for colliding beam fusion is the ¹¹B + ¹H reaction leading to the three ⁴He nuclei. The energy release is in the form of charged particles whose kinetic energy can be converted to electricity with a very high efficiency. Current research predicts that this energy source has an extremely high degree of cleanness and efficiency. In all current energy sources, approximately two-thirds of the energy is lost in the form of waste heat or thermal pollution. In the CBF approach, there is virtually no waste. This design favors small size for the greatest efficiency (100 MW_e or less), and would lead to either power plants with several reactors or decentralization of energy production.