At Bushehr there are approximately 1000+ Russian specialists. When one hears about them being evacuated or leaving, then one could expect something to happen. On a side note, Bushehr is not Iran's most important nuclear plant, Natanz is. Its the one place that needs to be protected with everything that Iran has. Getting Bushehr done is of political importance. Getting Natanz complete (it already functions but with 6k centrifuges, needs to reach 50k) is of important to Iran's deterrence capability (assuming a "bomb" is in the works). Assuming IR1's are only used for the full 50k, it would be a nuke every nine days. Though Iran wants to install it all with IR6 (popularly referred to as IR3).