What would happen if Russia attacked these power plants directly? Are they built to "fail safe" even when hit by a missile, or is there a big risk of nuclear meltdown?
Russians again shell themselves on the NPP they control since 2022.
There is zero need to attack a NPP directly.
If you really need to shutdown it then just target the power distribution system - a reactor can't run without providing the energy and if there is no load then you need to shutdown it.