The Office of Second Line of Defense (SLD) works to prevent illicit trafficking in nuclear and radiological materials by securing international land borders, seaports and airports that may be used as smuggling routes for materials needed for a nuclear device or a radiological dispersal device.
Since the fall of the Soviet Union in the early 1990s, NNSA has focused on securing nuclear materials and weapons at well over one hundred research, storage and manufacturing facilities in Russia and other states of the Former Soviet Union. The Second Line of Defense Program is a natural complement to these activities. The deployment of radiation detection systems at high-risk land border crossings, airports and seaports provide a backstop to the nuclear site security systems, increasing the likelihood that nuclear materials, if stolen from protected facilities, will be detected and interdicted.
The SLD Program has two main components: the Core Program  and the Megaports Initiative .
For more information about the Office of Second Line of Defense (SLD), read the NNSA's Second Line of Defense Fact Sheet .