sentence: fission product yields by mass for thermal neutron fission of u-235 , pu-239 , a combination of the two typical of current nuclear power reactors , and u-233 used in the thorium cycle if a graph of the mass or mole yield of fission products against the atomic number of the fragments is drawn then it has two peaks , one in the area zirconium through to palladium and one at xenon through to neodymium .