* 83 senators say Iran has no right to enrich uranium
* Majority of House also writes letter to Obama
* Arms control expert says lawmakers goals are not realistic (Adds letter from House lawmakers, arms control expert paragraphs 9-11)
By Timothy Gardner
WASHINGTON, March 18 As talks on a nuclear deal for Iran resumed in Vienna Tuesday, a wide majority of U.S. senators urged President Barack Obama to insist that any final agreement state that Iran "has no inherent right to enrichment under the Nuclear Non-Proliferation Treaty."
That lack of entitlement was one of several principles the 83 senators outlined in the letter. They urged Obama to "insist upon their realization in a final agreement" that six world powers and Iran are hoping to hammer out by late July. The senators also want to prevent Iran from ever having the capacity to build nuclear weapons.
The initiative in the 100-member chamber was spearheaded by Robert Menendez, a New Jersey Democrat and the chairman of the foreign relations committee, and Lindsey Graham, a South Carolina Republican.
Whether Iran should be able to enrich low-level uranium for use in nuclear power plants is one of many issues expected to be addressed in this week's talks on a comprehensive agreement over its nuclear program. Such uranium can be further enriched to be used in nuclear weapons.
Iran, a signatory of the 1970 NPT, insists it does have the right to enrich low-level uranium for nuclear power plants. Other countries that signed the treaty, such as Germany and Japan, enrich uranium for their power plants.
The U.S. Congress has long taken a harder line on Iran than the White House. Menendez has sponsored a bill to impose new sanctions on Iran and to prevent it from enriching any uranium, which Obama has threatened a veto if it were to pass. The bill is stalled in the Senate, after it did not get enough support to overcome a veto.
The senators also wrote in the letter that any final agreement must dismantle Iran's nuclear weapons program and prevent it from ever having a uranium or plutonium path to a nuclear bomb.
Western powers fear that Iran's Arak planned research reactor, once operational, could provide a supply of plutonium, one of two materials including highly enriched uranium that can trigger a nuclear explosion.
How to deal with Arak is another of the thorny issues expected to be debated in the talks intended to work out a final deal in the decade-old nuclear dispute by late July.
In the House of Representatives, 395 lawmakers in the 435-member chamber also sent a letter to Obama, asking him to push for a deal in which Iran would not be able to build or buy a nuclear weapon.
An arms control expert said Congress was sending a message that could harm the talks because Iran's nuclear weapons capacity can be significantly reduced but not eliminated altogether.
"If Congress insists on unattainable outcomes ... the chances for a diplomatic resolution will decrease, Iran's nuclear capabilities may grow, and the chances of a conflict will increase," said Daryl Kimball, head of the Arms Control Association.
TOO MUCH RELIEF?
The senators also said Iran must not be allowed to circumvent sanctions during the six-month temporary deal implemented on Jan. 20.
Under that deal, which can be renewed, Iran agreed to curb its nuclear program in return for gaining access to more than $4 billion in oil revenues that had been frozen by Western sanctions.
Backers of strong sanctions have complained that data showing Iran's oil exports increased in February reveals the temporary deal is allowing Iran to get more economic relief than originally thought.
The Obama administration believes Iran's oil shipments will fall in coming months and will be held to 1 million barrels per day on average from February to July.
The senators are not convinced. The months during talks on a final deal are "fraught with the danger of companies and countries looking to improve their commercial position in Tehran," they wrote. (Reporting by Timothy Gardner; Editing by Stephen Powell and Prudence Crowther)