A nuclear power plant is essentially a massive concrete bunker. It's designed to contain a nuclear reaction, so I suspect you'd need a phenomenal amount of explosives to cause any damage, and if you have those, there are likely much more effective ways to use them for terrorism.
If instead you attack the facility to attempt to gain control and cause a meltdown, you'd need to gain access to all the various safety overrides, and then hold that position for as long as it takes the reactor to meltdown. And that's assuming there are no absolute fail-safes that can't be disabled.
And all this is assuming that the failure mode of the reactor is even bad enough to cause significant damage, which is less the case with newer reactors.