the energy you spoke of, rineholdr, is binding energy (BE). this also explained as mass defect(dM), the mass equivalent of (binding energy), the energy used to bind the nucleons together(overcoming the EMF, i think). you may be aware of the "energy well" analogy used with electrons. it works similarly with nucleons. we add energy to "fissionable" nuclei, usually by throwing a neutron at it. this raises their nucleons to an excited state or makes the atom's nucleus unstable(protons' charges repelling each other overcomes the strong force holding them together) to a point where fission is probable.
so what is left after fission? usually two fission products(smaller, usually radioactive atoms), some neutrons, and some gammas(high energy photons). remember that the energy released here(neutron KE, photons, and fission product KE is all due to the binding energy being released. kinda like letting out a fart after holding it for a half an hour. well, not really.
dont't know how entropy work's into this. i guess it goes up like to always does.
do you need more fissionable material? yes, unless you want to go chasing around some fission products i'd suggest finding more uranium/plutonium.
™ Version 4.2