Until now, U.S. and Western governments had been focused primarily on Iran’s vast program to enrich uranium, one path to creating the fissile materials needed for nuclear weapons. Now, the West is increasingly concerned Iran also could use the development of a heavy water nuclear reactor to produce plutonium for a bomb. A heavy-water reactor is an easier target to hit than the underground facilities that house Iran’s uranium-enrichment facilities…

The IAEA has been monitoring Arak since its construction began. But following Iran’s latest timeline, the site’s importance has vastly shot up for Washington and Brussels, said U.S. and European officials. “It really crept up on us,” said an official based at the IAEA’s Vienna headquarters…

“There’s no question that the reactor and its heavy water are more vulnerable targets than the enrichment plants,” said Gary Samore, who served as President Barack Obama’s top adviser on nuclear issues during his first term. “This could be another factor in Netanyahu’s calculations in deciding how long to wait before launching military operations.”

Any Israeli strike on the reactor complex, said current and former U.S. officials, would likely have to take place before Tehran introduces nuclear materials into the facility, because of the potential for a vast environmental disaster a strike could cause.