The United Nations atomic watchdog has said that Iran has exceeded a "soft limit" on sensitive material that was agreed upon under its 2015 nuclear agreement with six world powers,for a second time this year. In a report released yesterday, the International Atomic Energy Agency (IAEA) said Tehran has again surpassed the 130-metric-ton threshold for heavy water since the deal was put in place.<br/><br/>Heavy water is used to cool reactors, which can produce substantial amounts of plutonium, an element that is used in producing the fissile core of nuclear warheads. The watchdog said Tehran had stockpiled 130.1 tons of the material, which is used at reactors such as ones at Iran's unfinished facility at Arak.