VIENNA, Dec 23 (Reuters) - Iran and the major powers with which it reached a landmark nuclear accord in 2015 have agreed on clarifications that diplomats say will reduce the amount of enriched uranium that counts towards a limit set by the deal.
The U.N. nuclear watchdog circulated the clarifications, laid out in eight documents, to its member states on Friday after receiving them from the office of European Union foreign policy chief Federica Mogherini, who coordinates the main forum for discussions created by the deal, the Joint Commission.
"These documents are merely providing clarifications, developed by the Joint Commission, for the implementation of Iran's nuclear-related measures as set out in the JCPOA," Mogherini's office said in an attached letter, using the deal's full name, the Joint Comprehensive Plan of Action.
The International Atomic Energy Agency (IAEA) is policing the deal's restrictions on Iranian atomic activities to help ensure they are not put to developing a nuclear weapon. The accord also lifted sanctions against the Islamic Republic.
The IAEA has already expressed concern to Iran about its repeated testing of one of the deal's less strictly defined limits - its stock of heavy water, a substance used as a moderator in reactors like Iran's unfinished one at Arak, which has had its core removed to make it unusable.
Diplomats have said Iran has come close to exceeding other limits under the deal, particularly the 300 kg (661 pound) cap on its stock of enriched uranium, a breach of which would most likely be far more damaging than excess heavy water.
There was concern during a visit to Iran this week by IAEA chief Yukiya Amano that Iran was about to go over that threshold, diplomats said. There is also great uncertainty about how U.S. President-elect Donald Trump, a vocal critic of the deal, will handle any future difficulties that arise with Iran.
"It was close," one diplomat who follows Iran closely said, without giving a figure. "If they had gone over the limit while he was there, that would have been embarrassing."
The 45 pages of often highly technical documents circulated on Friday specify, among other things, items that do not count towards Iran's uranium stockpile, such as some unrecoverable material left in pipes.
"All current low-level solid waste contaminated with low-enriched uranium (LEU) ..., which is deemed unrecoverable, is not part of Iran's enriched uranium stockpile as specified in the JCPOA provided that Iran does not build or operate any facility or part of a facility capable of recovering LEU from solid waste for 15 years," one typically worded clause said.
How much difference exactly the clarifications will make to the size of Iran's uranium stockpile is not clear. The IAEA does not give estimates in its reports on Iran to member states, which has drawn criticism from some countries, though it liaises with major powers that reached the deal with Tehran. (Reporting by Francois Murphy; editing by Mark Heinrich)