VIENNA (AP) -- Key restrictions on Iran's nuclear program imposed under an internationally negotiated deal will start to ease years before the 15-year accord expires, advancing Tehran's ability to build a bomb even before the end of the pact, according to a document obtained Monday by The Associated Press.
The confidential document is the only text linked to last year's deal between Iran and six foreign powers that hasn't been made public, although U.S. officials say members of Congress who expressed interest were briefed on its substance. It was given to the AP by a diplomat whose work has focused on Iran's nuclear program for more than a decade, and its authenticity was confirmed by another diplomat who possesses the same document.
Both demanded anonymity because they were not authorized to share or discuss the document.
The diplomat who shared the text with the AP described it as an add-on agreement to the nuclear deal in the form of a document submitted by Iran to the International Atomic Energy Agency outlining its plans to expand its uranium enrichment program after the first 10 years of the nuclear deal.
But while formally separate from the bigger nuclear accord, he said that it was in effect an integral part of that pact and had been approved by the U.S., Russia, China, Britain, France and Germany, the six powers that negotiated the deal with Tehran.
Details published earlier outline most restraints on Iran's nuclear program meant to reduce the threat that Tehran will turn nuclear activities it says are peaceful to making weapons.
But although some of the constraints extend for 15 years, documents in the public domain are short on details of what happens with Iran's most proliferation-prone nuclear activity - its uranium enrichment - beyond the first 10 years of the agreement.
The document obtained by the AP fills in the gap. It says that as of January 2027 - 11 years after the deal was implemented - Iran will start replacing its mainstay centrifuges with thousands of 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. From year 11 to 13, says the document, Iran will install centrifuges up to five times as efficient as the 5,060 machines it is now restricted to using.
Those new models will number less than those being used now, ranging between 2,500 and 3,500, depending on their efficiency, according to the document. But because they are more effective, they will allow Iran to enrich at more than twice the rate it is doing now.
Components other than centrifuge numbers and efficiency also go into the mix of how quickly a nation can make a nuclear weapon. They include how much enriched uranium it has to work with, and restrictions on Iran's stockpile extend until the end of the deal, crimping its full enrichment program.
But a comparison of outputs between the old and newer machines shows the newer ones work at double the enrichment rate. That means they would reduce the time Iran could make enough weapons grade uranium to six months or less from present estimates of one year.
And that time frame could shrink even more. While the document doesn't say what happens with centrifuge numbers and types past year 13, U.S. Energy Secretary Ernest Moniz told The AP that Iran will be free to install any number of advanced centrifuges beyond that point, even though the nuclear deal extends two additional years..
That will give Iran a huge potential boost in enrichment capacity, including bomb making should it choose to do so. But it can be put to use only after the deal expires.
Moniz noted that the limit on the amount of low-enriched enriched uranium Iran will be allowed to store will remain at 300 kilograms (660 pounds) for the full 15 years, significantly below the amount needed for further enrichment into a bomb. As well, the stockpile will remain restricted to a level used for reactor fuel that is well below weapons grade.
These restrictions translate into "serious constraints on ... (Iran's) nuclear program for 15 years," Moniz said.
In selling the deal to skeptics, the U.S. administration said it is tailored to ensure that Iran would need at least 12 months to "break out" and make enough weapons grade uranium for at least one weapon. Moniz said the document obtained by the AP posed no contradiction to that claim because "we made it very clear that we were focused on 10 years on the minimum one-year breakout time."
The document also notes that Iran will greatly expand its work with centrifuges that are even more advanced, including large-scale testing in preparation for the deal's expiry 15 years after its implementation on Jan. 18.
But State Department spokesman Mark Toner said that "the prohibition on Iran's pursuit of a nuclear weapon - and our ability to monitor the peaceful nature of its nuclear program - remains in effect indefinitely.
"The breakout time does not go off a cliff nor do we believe that it would be cut in half, to six months, by year 11," he said.
Iran insists it is not interested in nuclear weapons, and the pact is being closely monitored by the International Atomic Energy Agency. The IAEA says Tehran has essentially kept to its commitments since the agreement was implemented, a little more than six months after Iran and the six powers finalized it on July 14, 2015.
Marking the agreement's anniversary Thursday, President Barack Obama said it has succeeded in rolling back Iran's nuclear program, "avoiding further conflict and making us safer." 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.
Also opposed is Israel, which in the past has threatened to strike Iran if it deems that Tehran is close to making a nuclear weapon. Alluding to that possibility, David Albright, whose Washington-based Institute for Science and International Security is a U.S. government go-to resource on Iran's nuclear program, said the plan outlined in the document "will create a great deal of instability and possibly even lead to war, if regional tensions have not subsided."
The bigger nuclear deal provides Iran with sanctions relief in exchange for its nuclear constraints. But before going into recess, the U.S. Congress last week approved a bill to impose new sanctions for Tehran's continuing development and testing of ballistic missiles, a program the White House says is meant to carry atomic warheads even if it is not part of the nuclear agreement.
It also approved a measure that calls for prohibiting the Obama administration from buying more of Iran's heavy water, a key component in certain nuclear reactors.
The White House has said removing the country's surplus heavy water denies Tehran access to a material that may be stored for potential nuclear weapons production. But critics note that the purchase was made only after Iran exceeded heavy water limits proscribed by the nuclear deal and assert it rewarded Tehran for violating the agreement.