"Heavy nuclei" refers to atoms that contain a large number of protons and neutrons in their nucleus. These atoms are usually larger and more massive compared to lighter atoms. Full definition
Meitner's work in elucidating the process of nuclear fission in 1938 is well accepted by her fellow physicists — but Otto Hahn, who won the 1944 Nobel Prize in Chemistry «for his discovery of the fission of heavy nuclei,» barely acknowledged her contribution. (fortune.com)
In addition, said Aschenauer, «RHIC's unique ability to collide polarized protons with heavy nuclei provides the only opportunity in the world to get a first look at the orbital momentum of gluons inside the proton.» (sciencedaily.com)
This is the heaviest element that can be made in this way, because the balance of forces within nuclei is such that forming heavier nuclei, far from releasing energy, requires additional energy from outside. (newscientist.com)