

Nuclear weapons require very precise detonators to explode, unlike conventional exposives which generally require only heat (and can blow up in the way you describe).
It’s unclear, but most international experts agree that Iran has not yet actually put the nuclear material into any detonators. The problem is that Iran has been refining and stockpiling nuclear payloads, which could fairly easily be put into a bomb. That’s what most of the world wants to prevent.
Iran has consistently hid nuclear refinement facilities from international inspection, and there have been numerous reports of materials and facilities that serve no civilian purpose over the past 20 years, when such activity is in violation of international agreements.