It said that after a period between 11 to 13 years, Iran could replace its 5,060 inefficient centrifuges with up to 3,500 advanced machines.
Centrifuges churn out uranium to levels that can range from use as reactor fuel and for medical and research purposes to much higher levels for the core of a nuclear warhead.
The document, obtained by The Associated Press in Vienna, outlines Tehran's plans to expand its uranium enrichment program after the first 10 years of the nuclear deal. Like the diplomats, the official demanded anonymity in exchange for discussing the document.
The document does not say what would happen with Iran's enrichment process after year 13, which means that an end to all restrictions could be on the cards.
He said it was created by Iran's "negotiators and industry experts" and that even foreign media have noted Iran is likely to strive for restoring its full enrichment after 10 years.
But because the newer models are up to five-times as efficient, they will allow Iran to enrich at more than twice the rate it is doing now, meaning the time Iran would need to make a weapon would drop from a year to six months.
The U.N. said it had communicated with Iran throughout the investigation of the report and shared its findings with the Islamic Republic. As well, the stockpile will remain restricted to a level used for reactor fuel that is well below weapons grade.
Presenting the Secretary-General's first report on implementation of the resolution, Mr. Feltman today told the Council that since 16 January of this year, the Secretary-General "has not received any report, nor is he aware of any open source information, regarding the supply, sale, transfer or export to Iran of nuclear-related items undertaken contrary to the provisions of the JCPOA".
While the secret document hasn't been made public, USA officials say members of Congress have been able to see it. Iran and the six countries that negotiated the deal, including the United States, approved the add-on. But opposition from U.S. Republicans could increase with the revelation that Iran's potential breakout time would be more than halved over the last few years of the pact.
Sen. Tom Cotton (R., Ark.) said Monday that he is "truly shocked" the Obama administration misled the American people on the Iran nuclear deal.
Cotton said he could not provide a response on a breaking news report, but said he would not be surprised to see those constraints removed in 10 years or see Iran violate the restrictions.