kendor said:you still need to get the Uranium 234 into a certain state (235) for it to be useful for weapons and it's that instability that is extremely complex and too hard for your average joe terrorist.
Critical Mass is what you need before triggering the chain reaction and this state is very hard to produce unless you have the equipment and knowhow
you get the u235 as a by product of domestic nuclear fuel use. it's only unstable if it gets to near other bits of u235 ie it approches critical mass which is the point where an uncontolled rection starts
All the bomb does is violently combines the two bits of U235 each of which is less than a critical mass by firing one into the other. Hey presto loud bang
The only difficult bits getting the U235 in the first place
have a look at this
http://www.fas.org/nuke/intro/nuke/design.htm