The risk of nuclear terrorism remains very real. Measures to secure nuclear weapons and the materials needed to make them are the most effective tools for reducing this risk. Terrorist threats are constantly changing—as the dramatic rise of the Islamic State in 2014 makes clear. The job of improving security for nuclear weapons and weapons-usable nuclear materials is never “done”—security must constantly evolve as the threat changes, technologies shift, and new vulnerabilities are revealed. In the two years since the last nuclear security summit, security for nuclear materials has improved modestly—but the capabilities of some terrorist groups, particularly the Islamic State, have grown dramatically, suggesting that in the net, the risk of nuclear terrorism may be higher than it was two years ago.
Bunn, Matthew, Martin B. Malin, Nickolas Roth, and William H. Tobey. "Preventing Nuclear Terrorism: Continuous Improvement or Dangerous Decline?" Project on Managing the Atom, Belfer Center for Science and International Affairs, Harvard Kennedy School, March 21, 2016.