As is the case with the NPT, the TPNW does not define “nuclear weapons” or “other nuclear explosive devices”. There is, though, a settled understanding among states of what these terms mean. A nuclear explosive device is an explosive device whose effects are derived primarily from nuclear chain reactions. A nuclear weapon is a nuclear explosive device that has been weaponised, meaning that it is contained in and delivered by, for example, a missile, rocket, or bomb. Thus, all nuclear weapons are a form of nuclear explosive device but not all nuclear explosive devices are nuclear weapons.
The key components of a nuclear explosive device are fissile material (typically highly enriched uranium or reprocessed plutonium) and the means of triggering the nuclear chain reaction. Also key components are the precursors to fissile material, which are termed source material (e.g. naturally occurring uranium). As set out in the Statute of the International Atomic Energy Agency (IAEA), fissile material (termed special fissionable material in the Statute) “means plutonium-239; uranium-233; uranium enriched in the isotopes 235 or 233….” Source material “means uranium containing the mixture of isotopes occurring in nature; uranium depleted in the isotope 235….”
In a nuclear weapon, additional key components are the missile, rocket, or other munition, including both the container and any means of propulsion. Delivery platforms such as bombers and submarines are not key components of nuclear weapons as such, but they may be integral to a nuclear-weapon system and, in certain circumstances, investment in such a system, or the transfer of nuclear-capable bombers or submarines, could amount to prohibited assistance.
 Article XX, 1956 Statute of the IAEA (as amended).