Official Iranian news reports that the core of the country's Arak heavy water nuclear reactor has been removed, and the space filled with cement.
The Arak reactor was the subject of major disagreements during last year's negotiations between the Islamic Republic and the P5+1. Ultimately Iran agreed to reconfigure the site so that it would not be able to produce fissile plutonium, which would be used in a nuclear weapon.
Removing the core is a mandatory step leading up to the sanctions on Iran being lifted.