@mhmck Presuming that they have, they could set it off at any time, and will do so if there is any attempt to storm the plant, and blame the invading force for the catastrophe.

The only thing that might be preventing the Russians from doing this is the fact that much of the fallout would blow into Russia, and likely have the same effect on the regime there as the fallout from Chernobyl did on the former Soviet Union.