Even so present-day generation nuclear weapons produce considerable levels of fallout in the form of highly radioactive fission and this has been one of the main factors limiting their military application. Yet, the lethality of the weapons due to neutron radiation as well as explosive force would still be great. Moreover, thermonuclear reactions do not require a minimum critical mass to be assembled. Hence pure fusion weapons could range in size from a few kilograms to multi-megatons of TNT equivalent. Pure fusion weapons have been unattainable so far because it is very difficult to create the conditions that enable a large enough number of nuclear fusion reactions to occur in a small enough space and short enough time.
Fusion reactions involve the fusion of light nuclei, which results in a release of nuclear energy.
These forces must be overcome if the nuclei are to be brought close enough together so that the probability of fusion reactions may be high. This allows the kinetic energy of the nuclei to be large enough to overcome the repulsive force. The even greater attractive force between nuclear particles, which operates to a significant degree only at very close range, is then able to overcome the repulsive electrical force and produce nuclear fusion. Keeping together a dense enough mass of nuclear material for long enough to produce a large number of nuclear reactions, but short enough to produce an explosion, has proved possible by only one means. That has been to create the extremely high temperatures and pressures by a fission explosion.