Revealed in 2002, the heavy water reactor in Arak remains under construction, according to the IAEA. Based on its design, Iran could eventually use the facility to produce weapons-grade plutonium - something it currently cannot do - which would be a major worry for Israel and the United States. But there are major hurdles the Iranians must first clear.
Iran was hoping to have the reactor finished as early as 2013, but, with ever-tightening international sanctions making it difficult for the Iranians to obtain materials from abroad, it's unclear how long it could take for the reactor to be made functional. Fitzpatrick, at the IISS, says 2014 now seems more likely, but still uncertain.
If and when it is completed, Arak could theoretically make enough plutonium by reprocessing spent fuel from the reactor to arm one, possibly even two nuclear weapons per year.
The reprocessing of spent plutonium fuel represents another hurdle. Iran currently has no reprocessing facilities to carry out that work. Fitzpatrick says there is no indication Iran is working to build such a facility, either, so if Iran does hope to produce weapons-grade plutonium, they are not making fast progress in that direction.
Regardless of how far down the road Arak's potential advent is, the specter of Iran creating its own supply of weapons-grade plutonium would likely be sufficient impetus for Israel to put the facility in its sights, should a preemptive strike be ordered.