Title is self-explanatory. Can I do it? And take two mortal wounds of course.
To the extent of my knowledge, when you use the Nova Reactor you get to pick an ability for the duration of the turn. Think of it like turning a switch on and off- you can't turn it on again without first turning it off. Likewise with the Nova Reactor, once it's "on" you can't turn it "on" again. At least until your next turn.