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By Ali Arouzi and Alastair Jamieson

As Iran and world powers reach a historic deal over its nuclear program, how will the agreement work and what happens next?

Is the deal finally fixed?

Not quite. Congress has 60 days to pick over the terms, giving opponents plenty of time to challenge the Obama administration with hearings in both the House and the Senate from officials and likely some outside experts.

The deal has already faced strong criticism from Republicans. If Congress passes a resolution of disapproval and sends it to President Barack Obama for his signature, he has 12 days to veto it. Obama said Tuesday that he would take such action, if necessary.

The White House is also facing intense opposition from Israel, which is lobbying hard.

The agreement will also need the endorsement of Iran's supreme leader, Ayatollah Ali Khamenei, who may face opposition from hardliners.

It will then have to be ratified by U.N. Security Council.

Will sanctions be lifted immediately?

It is unlikely significant sanctions will be lifted before 2016, giving U.N. experts enough time to clear Iran of remaining suspicions about the scope of its nuclear program.

The U.N. International Atomic Energy Agency will give its verdict by Dec. 15, Director-General Yukiya Amano said Tuesday.

Access to the Parchin military site — previously a stumbling block in talks — is part of a separate "arrangement," Amano added. "By December 15, 2015, the director-general will provide the final assessment on the resolution of all past and present outstanding issues," he told reporters.

Diplomats told Reuters that a U.N. arms embargo would remain in place for five years and that U.N. missile sanctions would stay in place for eight years — though either could end earlier if the IAEA concludes "that all nuclear material in Iran remains for peaceful activities."

The release of frozen overseas assets, worth more than $100 billion, could come later — as well as the lifting of restrictions on Iranian institutions such as its banks and national airline.

What if Iran goes back on its word?

Iran has accepted a so-called "snapback" plan that will restore sanctions in 65 days if it violates the deal, diplomats told Reuters.

How long would the deal stay in place?

Tuesday's agreement will be terminated 10 years from the date of its adoption "provided no U.N. sanctions have been reinstated" — a move that would need another U.N. Security Council resolution.

Will America resume normal relations with Iran?

Obama said Tuesday's deal is "an opportunity to move in a new direction," but that doesn't mean a normalizing of relations is on the agenda — even if Washington wanted it.

The U.S. remains a hate object for conservative hard-liners in Iran, and Tehran's vehement anti-Israel stance would likely prevent any meaningful relationship.

"However, an improved and more open economy and a less isolated Iran could facilitate long-term changes in Iranian society and politics," wrote Alireza Nader, Senior International Policy Analyst with the RAND Corporation.

Reuters and Frank Thorp V contributed.