A
pure fusion weapon is a
hypothetical hydrogen bomb design that does not need a
fission "primary" explosive to ignite the
fusion of
deuterium and
tritium, two heavy
isotopes of
hydrogen (see
thermonuclear weapon for more information about fission-fusion weapons). Such a weapon would require no fissile material and would therefore be much easier to build in secret than existing weapons. The necessity of separating high-quality fissile material requires a substantial industrial investment, and blocking the sale and transfer of the needed machinery has been the primary mechanism to control
nuclear proliferation to date.