You can't convert mass to energy like that in this context. A megaton just means a million tons (of TNT in an energy context). You can say: (the energy released when detonating) a megaton of TNT is equivalent to 46.55 g of pure mass to energy conversion (E = mc**2 type), but that would be irrelevant in this context because no one is planning converting this mass to energy, nor would it be possible.