In their research titled “Nuclear Security Briefing Book”, Wyn Bowen and Matthew Cottee state that nuclear terrorism involves the acquisition and detonation of an intact nuclear weapon from any state arsenal.
The world has not experienced any act of nuclear terrorism yet, but it is not for a lack of trying. The International Atomic Energy Agency (IAEA) has observed many incidents of lost, theft and unauthorized control of nuclear material. The increased use of nuclear technology for peaceful purposes has intensified the threat that terrorist can target these places for acquiring nuclear materials. They cannot build a nuclear weapon because production would require a technological infrastructure. However, even if they struggle to make an actual nuclear bomb; they can build a dirty bomb instead.
Source: Daily Times