BOX 2-1
What Is a “Nuclear Weapon”?

The terms “nuclear warhead” and “nuclear weapon” have not been defined with much precision in existing treaties. The Nuclear Non-Proliferation Treaty (NPT) restricts the transfer or acquisition of “nuclear weapons or other nuclear explosive devices” but offers no definition for this term, nor do the treaties that prohibit the deployment of nuclear weapons in space or on the seabed. In the START I Treaty, the term “warhead” is defined simply as “a unit of account used for counting,” and earlier U.S.-Soviet arms control treaties used the term without definition.

The most complete definition is given in the 1985 South Pacific Nuclear Free Zone Treaty (Treaty of Rarotonga) and the 1996 African Nuclear-Weapon-Free Zone Treaty (Treaty of Pelindaba): “‘nuclear explosive device’ means any nuclear weapon or other explosive device capable of releasing nuclear energy, irrespective of the purpose for which it could be used. The term includes such a weapon or device in unassembled and partly assembled forms, but does not include the means of transport or delivery of such a weapon or device if separable from and not an indivisible part of it.” This definition is not entirely satisfactory, inasmuch as “capable of releasing nuclear energy” remains undefined, but it under-scores the importance of understanding at what point a weapon is considered dismantled and no longer counted as a “nuclear weapon.”

Like past treaties, agreements dealing with weapon transparency measures could simply refer to “nuclear weapons or other nuclear-explosive devices” without adopting a more detailed definition. As discussed in this chapter, there are several approaches for confirming, with varying degrees of confidence, whether or not an object is a weapon. In this way, the term ”weapon” would be defined operationally by the objects that are declared to be weapons and the techniques used to confirm that an object is a weapon.

All nuclear weapons include a fission explosive device, which creates a divergent fission chain reaction by rapidly assembling a supercritical mass of nuclear-explosive material (NEM)—plutonium or highly enriched uranium (HEU). (See Chapter 3 and

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