Obama just hinted at the huge trade-off at the heart of the Iran deal

President Barack Obama says the landmark nuclear deal with Iran might still leave Tehran with the ability to accumulate a weapon's worth of nuclear fuel within a matter of months 15 years down the road.

In an interview with NPR, Obama said that Iran's "breakout time" - or the time needed to produce enough weapons-grade uranium for a single nuclear device - would plunge to "a matter of months" 15 years into the deal.

Obama added that this 15-year delay in Iran's capabilities was one of the virtues of the agreement that US and the five permanent members of the United Nations Security Council and Germany (the P5+1) signed with Iran in July.

"If in fact the breakout times now are a few months, and we're able to push that breakout time out to a year so that we have more time and space to see whether or not Iran is cheating on an agreement, kicking out inspectors, going for a nuclear weapon; if the breakout time is extended for 15 years and then it goes back to where it is right now, why is that a bad deal?" Obama said.

The acknowledgment of Iran's future capabilities hints at a trade-off that lies at the heart of the nuclear deal's logic. The deal controls Iran's stockpile of fissile material, while leaving it with the infrastructure needed to rapidly accumulate bomb fuel even within the life of the deal - something that puts an intense amount of pressure on international monitors and future US leaders.

Going by administration statements since 2013, the US didn't always want the deal to turn out this way, and intended for a final agreement with Iran to curtail aspects of Iran's nuclear infrastructure. At times in 2013, chief US Iran negotiator Wendy Sherman and Secretary of State John Kerry said that a strong deal would include the closure of the Fordow enrichment facility and the Arak heavy water reactor, respectively. Under the Joint Comprehensive Plan of Action (JCPOA), all of the country's nuclear facilities remain open, including those two.

The deal forces Iran to take roughly half of its currently operating centrifuges offline, prohibits Iran from operating advanced centrifuges for a period of 8 1/2 years, imposes centrifuge research and development restrictions for 10 years, and limits uranium enrichment and heavy water reactor development for 15 years. But it doesn't actually require Iran to export or destroy any of its nuclear infrastructure.

All of its nuclear facilities will remain open. Iran will be allowed to operate hundreds of centrifuges for enriching non-fissile placeholder elements at Fordow, a facility inside of a mountain on an Iranian Revolutionary Guards Corps base that was only discovered by Western intelligence agencies in 2009. And Iran can enrich uranium at Fordow 15 years into the deal.

For a period of 15 years, Iran will have to modify its heavy-water reactor at Arak so as to make make it impossible to produce bomb-grade plutonium. But it will still get to keep a reactor which has no conceivable civilian purpose - and, along with it, a possible future plutonium path to a nuclear weapon.

The administration's early statements about their negotiating objectives suggest that the US wanted a nuclear deal predicated on infrastructure rollback and on denying Iran the physical capability of quickly producing a weapon. Instead, the current deal is based largely around fastidious stockpile management.

It prohibits Iran from possessing more than 300 kilograms of low-enriched uranium. And it reaches a compromise on centrifuge development and numbers: It allows Iran to mothball rather than export or completely destroy its centrifuges, giving Tehran the ability to rapidly narrow its breakout time if it ever decided to take them back online. But experts believe this would take months to accomplish and would definitively tip off international inspectors on activities that the JCPOA explicitly disallows.

In the words of administration officials, the deal "cuts off all pathways to a nuclear weapon." It still doesn't remove the means of reaching a nuclear weapon within a short time span if Iran ever decided to scrap the deal. And after 15 years, enrichment and stockpile limits disappear, even if end-use monitoring for fissile materials remains in place.

It's possible the US negotiators believed that stockpile controls obviated the need for Iran to export its centrifuges, close its illicit facilities, and shutter its heavy-water reactor. After all, the deal itself suggests that infrastructure control wasn't the negotiators' primary objective, as the JCPOA actually includes provisions that expand the range of Iran's nuclear hardware and expertise. For instance, the deal obligates signatories to assist Iran in the development of its fuel fabrication capabilities, something that would wean Tehran off of the need to import fuel assemblies for its nuclear reactors.

But it's also possible that Iran negotiated successfully enough to force the P5+1 off of its initial demands. A deal that even administration officials said would be based on infrastructure rollback instead had to depend on the net-best option: Stringent stockpile controls that still allowed Iran to keep nearly all of its nuclear hardware in some form and to bring that hardware online within the life of the deal.

This puts a huge amount of pressure on international monitors and on the future P5+1 leaders who must interpret and enforce the deal. And it leaves Iran with the option of rapidly accumulating weapons fuel if it ever believed the deal was no longer working to its advantage.

Obama motions towards this tradeoff in the NPR interview. The president describes a "situation where 15 years from now, that breakout time is approximately where it is now, but we now have an entire infrastructure that's been built to keep track of exactly what Iran's doing, and we had the entire international community behind us."

Obama says the agreement displaces Iran's current breakout capabilities by 15 years, but makes up for it through unprecedented stockpile monitoring and control. Weeks before a decisive Congressional vote, the deal's most forceful public advocate has been forced to speak frankly about what his negotiating outcome buys the US and its allies.