Lawmakers are picking their way through the massive and complicated nuclear deal with Iran in a series of hearings ahead of an expected vote in September whether to approve or disapprove of it.

The deal was reached July 14 in Vienna between Iran, the United States and five other world powers, and would basically put Iran's nuclear program on ice for 10 years. In exchange, nearly all of the international sanctions that have crippled Iran's economy and legitimacy for its Shiite Muslim theocracy would be lifted.

Under a law enacted May 22, Congress has 60 days to review the deal, though not much is at stake. The U.N. Security Council unanimously approved it Monday, making it binding under international law, and only U.S. sanctions would remain in place if lawmakers passed a resolution of disapproval over President Obama's veto.

Still, most lawmakers already don't like the agreement, and even those who do approve have serious concerns about how it will work. Here are three of the major ones:

Sunset provisions: The limits on Iran's nuclear program begin to expire after 10 years, with the ability to increase the number and sophistication of centrifuges used to enrich uranium. After 15 years, the limits on the level to which uranium is enriched, and in 25 years all special limitations under the deal will be lifted.

While after that Iran would continue to be bound by the provisions of the Nuclear Non-Proliferation Treaty, it would not be subject to any additional restrictions on its nuclear program that weren't ratified as part of that process — making it that much more difficult to determine if its leaders decide to build a nuclear weapon.

"I would like to hear, and I have never heard, a defense of the sunset clause," noted Ray Takeyh, an Iranian-born scholar at the Council on Foreign Relations, in a July 14 House Foreign Affairs Committee hearing. "If you defend this agreement you should defend why it should expire in 10 years."

Many critics of the deal consider this to be the single biggest concern, and even Democrats who like the deal worry about what happens when it ends.

"The deal, if fully implemented, is dramatically better than the status quo before the negotiations started — for the first 10 years. I have some questions about after year 10 — from years 10-25 some of the obligations fall off and there are some things that are permanent and many that are not," Sen. Tim Kaine, D-Va., told the Washington Examiner.

Even Obama has suggested this may be a problem, noting in an NPR interview in April that the one-year "breakout time" for Iran to produce enough material to build a nuclear weapon created by the limits in the deal would disappear after those limits start to expire.

"What is a more relevant fear would be that in year 13, 14, 15, they have advanced centrifuges that enrich uranium fairly rapidly, and at that point the breakout times would have shrunk almost down to zero," Obama said.

Sanctions relief: The agreement is intended to remove only international sanctions aimed at preventing Iran from building a nuclear weapon, though that category is broadly defined — too broadly, for many sanctions experts and lawmakers.

For example, it calls for the end of the U.N. arms embargo against Iran in five years — earlier if Iran's program is certified to be peaceful, something former undersecretary of state Nicholas Burns, who supports the deal, called a "painful tradeoff."

Lawmakers also are concerned about how the re-integration of Iranian banks into the international financial system would affect the tracking of efforts to fund terrorism, and more broadly how sanctions relief would enable Iran to better support groups like the Lebanese Shiite militia Hezbollah.

"Most Iranian banks, including some [Islamic Revolutionary Guards Corps]-controlled banks, will be permitted back onto the SWIFT financial messaging system, giving Iran's most dangerous actors access to the global financial system," Mark Dubowitz, a sanctions expert and executive director of the Foundation for the Defense of Democracies, told a congressional task force on terrorism financing on Wednesday.

"These delistings are a direct challenge to the conduct-based nature of the sanctions regime imposed by the Obama and Bush administrations. Those sanctions were designed to target the full range of Iran's illicit activities and not just Iran's nuclear program."

Rep. Robert Pittenger, R-N.C., vice chairman of the task force, said lawmakers are extremely concerned about how much harder it will be for U.S. officials to track the flow of potentially enormous sums of money in and out of Iranian banks to terrorists.

"It's going to be incredibly challenging," he told the Examiner. "We have compounded the problem of terrorism financing and tracking."

Size of Iran's nuclear program: The agreement allows Iran to maintain much of its nuclear infrastructure, and also to develop it, though at a much slower pace than it had been doing. This has led to concerns that even if Iran keeps its bargain to the letter, it will emerge at the end of the deal with an "industrial-scale" nuclear program in which resources could easily be diverted to produce a bomb.

"We've ended up in a situation where the deal that's on the table basically codifies the industrialization of their nuclear program. It's an amazing, amazing transition that has occurred," Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn., said Thursday. "And yet everyone here — not a person in this room, including our witnesses — everyone here knows there's not one practical need for the program that they're building — not one."

Kerry pushed back against those concerns in the hearing, challenging senators to suggest an alternative.

"So what's your plan? Knock out their entire capacity? Erase their memory of how to do a fuel cycle? Totally go to war? ... I just ask people to be reasonable," said Kerry.