Denial is not just a river in Egypt.
Lol, actually there isn't. The first thread is terrorists who don't need to create a nuclear chain reaction to create the terrorist effects of a nuke. Blowing up a small amount of plutonium with a small conventional bomb, not even a truck bomb, will have nearly as much effect on a city as an actual nuke. It doesn't even need to be refined plutonium. A few pounds of highly radioactive waste spread around Wall St or near the White House would have an enormous effect. Every day thousands of cars drive within 500 feet of the White House. Any one of them could carry the bomb that would shake our capital to the core.
But if you wanted to make an atomic bomb, just assemble a small team of recent PhDs and stir for three years and out comes a viable design.