From Live Science:
It took only a matter of hours last week for the United Nations’ nuclear watchdog agency to shoot down a news report that its experts had drafted a secret document warning that Iran has the expertise to build a nuclear bomb.
"With respect to a recent media report, the IAEA [International Atomic Energy Agency] reiterates that it has no concrete proof that there is or has been a nuclear weapon program in Iran," the European-based agency said in statement.
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My Comment: An easy to read and comprehensive description on why it is difficult to make the bomb.