To expand, the 'mass' lost in nuclear reaction is not actually 'matter' (meaning particles) it is just energy stuck in the bonds between particles, not unlike chemical bonds. Of course we observe it as 'mass' because e=mc^2, and there is enough bonding energy to effect the mass we observe, but e=mc^2 does not mean we can obliterate particles into pure energy. The only way to do that is with anti-matter, which we can only make by expending a vast amount of energy (more than we would get back out)