Answer:
Energy is always conserved. You can neither create nor destroy energy. The same goes for mass. They can only be moved from one frame of reference to another. Fission has nothing to do with it. Ditto for fusion.
One area of "confusion" for many people is Einstein's mass-energy equivalence equation e = mc2. Some people think it means that mass can be converted to energy and vice versa. Nope. Not even close. Mass is energy, and energy is mass. Think about that.