answers.input_text: short
answers.span_text: Startup neutron source
questions: where do the neutrons come from in nuclear fission
name: Startup neutron source
contexts: Startup neutron source is a neutron source used for stable and reliable initiation of nuclear chain reaction in nuclear reactors , when they are loaded with fresh nuclear fuel , whose neutron flux from spontaneous fission is insufficient for a reliable startup , or after prolonged shutdown periods . Neutron sources ensure a constant minimal population of neutrons in the reactor core , sufficient for a smooth startup . Without them , the reactor could suffer fast power excursions during startup from state with too few self - generated neutrons ( new core or after extended shutdown ) .