I came across this today…
News that Russia will load nuclear fuel rods into an Iranian reactor has touched off a countdown to a point of no return, a deadline by which Israel would have to launch an attack on Iran’s Bushehr reactor before it becomes effectively "immune" to any assault, says former Bush administration U.N. Ambassador John R. Bolton.
Once the fuel rods are loaded, Bolton told Fox News on Friday afternoon, "it makes it essentially immune from attack by Israel. Because once the rods are in the reactor an attack on the reactor risks spreading radiation in the air, and perhaps into the water of the Persian Gulf."
Russian Prime Minister Vladimir Putin declared in March that Russia would start the Bushehr reactor this summer. But the announcement from a spokesman for Russia’s state atomic agency to Reuters Friday sent international diplomats scrambling to head off a crisis. [source]