Basically its super hard to get a nuclear bomb to detonate all of its material, because the force of the explosion tends to just blow the nuclear material away before it can become part of the runaway chain reaction. From what I have read you need extreme precision and carefully placed essentially "counter explosions" around the core to keep the material close enough together in the chain reaction long enough to get high rate of conversion of the material into as much energy as possible
The "crude terrorist nuke" assumes that terrorists aren't going to be able to get that precision and technique and will only get a very small yield because the bomb will blow itself apart before the chain reaction can consume all the material. After all nation states took many, many tries to get it right
The "crude terrorist nuke" assumes that terrorists aren't going to be able to get that precision and technique and will only get a very small yield because the bomb will blow itself apart before the chain reaction can consume all the material. After all nation states took many, many tries to get it right