In a nuclear weapon the radio active material is placed in the weapon but it needs an explosive charge to force the material to compress together in order to create a chain-reaction. By blowing up the facility the nuclear material is most likely scattered and not compressed together. If the material has been scattered within the cavern, it would be very dangerous for someone to enter without suffering from radiation overexposure.

Firstly, you would need to have already produced enough highly enriched uranium to make a bomb with and it seems that Iran is not there yet.

Secondly, just blowing it up is not sufficient to set it off. Instead of blowing it apart you need to blow it together! You need more than a certain amount, called the critical mass, and then you use explosives to blow it together, to try and compress it.

The material will emit neutrons because it is radioactive. Those neutrons will produce two or three more so the process accelerates rapidly - chain reaction.

When that happens, there is a tiny change in mass and that creates energy. That mass needs to be multiplied by a huge number to work out the energy produced and that is where E = mc² comes in.

Energy produced = Lost Mass times about 90,000,000,000,000,000.

If it would blow up easily it would be easy to make a bomb. However, there should have been some radioactive material blown about somewhere.

fission != explosion