Can Terrorists Build the Bomb? provides “an examination of each step a terrorist organization would need to take to pull off a nuclear attack, and what is being done to raise the hurdles.” Step One is acquiring the raw materials:
All the next Mohammed Atta would need to make a bomb big enough to instantly obliterate everything within a third of a mile is about 100 pounds of uranium enriched to 90 percent: a lump about the size of a bowling ball, or a bigger lump if the enrichment level is lower. It takes even less plutonium, which is far more fissile than uranium, to build an equally destructive bomb: about 35 pounds, a grapefruit-size hunk.