A large unstable nucleus can absorb a neutron and split into smaller nuclei. Fission releases energy, gamma radiation and neutrons that can initiate further fissions. A chain reaction requires emitted neutrons to cause subsequent fissions; control rods absorb neutrons to control the rate in a reactor. Example: Explain a chain reaction. Neutrons from one fission cause further fissions. Distinguish the observation from the explanation, and support any judgement with evidence. Check your reasoning.