An IND could be bought, or it could be built from the components of a stolen weapon or from scratch using nuclear material (plutonium or highly enriched uranium).
A successful detonation would result in catastrophic loss of life, destruction of infrastructure, and nuclear contamination of a very large area.
One type of an improvised nuclear device [IND] attack could be conducted by terrorists with radiological materials using a radiological dispersal device (RDD), or so-called “dirty bomb.”[5][6][7] An RDD causes no nuclear explosion but instead uses chemical explosive to spread harmful radioactivity.
The INDs when built from scratch would most likely be of gun-type[8] which is fairly easy to design and manufacture (however, requiring highly enriched uranium).
Improvised nuclear devices are likely to cause more nuclear fallout because of incomplete fission - due to overengineering the pit (making it heavier to ensure fission occurs at least in part of it); using the gun-type design; or as a result of a fizzle (yield lower than planned due to engineering/manufacturing errors) — and are likely to be exploded at ground level rather than at an altitude, which causes ground materials to be irradiated and drawn up into the mushroom cloud.