The facility that exploded does nuclear research. It is same facility that international Nuclear Inspectors were blocked by iran.

June 5th: "The UN nuclear watchdog has expressed serious concern at Iran's continuing failure to allow access to sites the agency wants to visit.

The International Atomic Energy Agency (IAEA) says that for more than four months Iran has been blocking inspections of two suspect locations.

The IAEA says Iran's enriched uranium stockpile now exceeds the agreed limit."

[link to www.bbc.co.uk (secure)]