The finding was made in a report prepared for presentation to the 35-nation IAEA board when it meets, starting Thursday, on whether to refer Iran to the U.N. Security Council, which has the power to impose economic and political sanctions on Iran.
The report was made available in full to The Associated Press.
First mention of the documents was made late last year in a longer IAEA report. At that time, the agency said only that the papers showed how to cast "enriched, natural and depleted uranium metal into hemispherical forms."
The agency refused to make a judgment on what possible uses such casts would have. But diplomats familiar with the probe into Iran's nuclear program said then that the papers apparently were instructions on how to mold highly enriched grade uranium into the core of warheads.
In the brief report obtained Tuesday, however, the agency said bluntly that the 15-page document showing how to cast fissile uranium into metal was "related to the fabrication of nuclear weapon components."
Asked about the finding, a senior diplomat close to the IAEA declined to elaborate but emphasized that the documents had no other use.
The report said the document was under agency seal, meaning that IAEA experts were able in theory to re-examine it, but "Iran has declined a request to provide the agency with a copy."
Diplomats familiar with the IAEA investigation of Iran said earlier Tuesday that part of the document recently was given to the agency in an effort to deflect building international momentum to report Iran to the Security Council. But the report did not mention Tehran handing over any papers.
The document was given to Iran by members of the nuclear black market network, the IAEA said. Iran has claimed it did not ask for the document but was given it anyway as part of other black market purchases.
The same network provided Libya with drawings of a crude nuclear bomb which that country handed over to the IAEA as part of its 2003 decision to scrap its atomic weapons program.