What happens during nuclear fission that causes the release of large amounts of heat energy? a) Atoms of uranium chemically combine with water molecules to become radioactive. As these combined molecules collide, neutrons are released, and they split apart as the bond is broken. b) Chemical means are used to combine uranium atoms. The combined atoms are then bombarded with neutrons, causing them to physically split apart. c) Nuclei of lighter isotopes of uranium join together to create larger, heavier nuclei that then emit neutrons and split. d) A neutron of a radioactive isotope hits the nucleus of another and sticks to it, making this nucleus unstable and causing it to emit more neutrons. Each interaction in this chain reaction causes atoms to split.