nuclear nonproliferation
Formal term used in international relations and security contexts, referring to the prevention of the spread of nuclear weapons.
The Nuclear Non-Proliferation Treaty is an important framework for international security.
核拡散 means 'nuclear proliferation' (the spread of nuclear weapons), while 核不拡散 is its opposite, 'nuclear nonproliferation'.
Compound of 核 (kaku, 'nuclear') + 不 (fu, 'non-') + 拡散 (kakusan, 'proliferation').