It's certainly not impossible for such a thing to happen, but it requires enough coincidences to make it unlikely. For a group of terrorists to make a nuclear

It's certainly not impossible for such a thing to happen, but it requires enough coincidences to make it unlikely. For a group of terrorists to make a nuclear weapon, they need the scientific background, materials, and engineering skill to build it. The scientific knowledge isn't too hard to get, but acquiring enough resources to make a sizable bomb without attracting too much attention is difficult. Add to that the fact that while the theory of the bomb is relatively simple, I'd imagine that actually engineering it would be the hardest part. One would need to fit the bomb into a relatively small space (no larger than a car) and testing the thing would be well near impossible (a successful test would give you away completely). So, the only people capable of pulling this off would have to be educated, engineering geniuses, in a convenient position in which they can obtain materials without attracting undue attention, and have an intense desire to blow as many people as possible off the face of the planet. It doesn't surprise me that the intersection of these sets approaches the null set.