The 2015 deal restricts Iran from accumulating a large stock of heavy water, which is used in scientific research as well as in nuclear reactors that produce plutonium.

Iran has been producing heavy water in large quantities and has already exceeded a prescribed 130-ton limit on its stock twice.

Tehran defused a standoff with Washington over the issue in December by shipping excess water to Oman, where it is being stored until a buyer can be found.

In a letter to the United Nations nuclear watchdog made public on March 17, Iran is arguing that the deal does not require it to ship excess water out of the country if it can't immediately find buyers.

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