I don't remember clearly, due to it being an old character, but don't power armor suits already take plutonium cells? Isn't that kinda...fusion core-y?
Oh, actually, if a proper fusion core type thing is added, maybe the plutonium could be used as a base component in crafting? Put the recipe in some lab journal, and there, a way to craft fuel for the murderfucker armor. I mean, so long as you have the rare components.