PROMPT AND DELAYED NEUTRONSDOE-HDBK-1019/1-93Reactor Theory (Neutron Characteristics)NP-02Page 32Rev. 0Prompt and Delayed Neutrons SummaryPrompt neutrons are released directly from fission within 10 seconds of the-13fission event.Delayed neutrons are released from the decay of fission products that are calleddelayed neutron precursors. Delayed neutron precursors are grouped according tohalf-life. Half-lives vary from fractions of a second to almost a minute.The fraction of neutrons born as delayed neutrons is different for different fuelmaterials. Following are values for some common fuel materials.Uranium-2350.0065Plutonium-2390.0021Delayed neutrons are produced by a classification of fission products known asdelayed neutron precursors. When a delayed neutron precursor undergoes a decay, it results in an excited daughter nucleus which immediately ejects a-neutron. Therefore, these delayed neutrons appear with a half-life of the delayedneutron precursor.The delayed neutron generation time is the total time from the birth of the fastneutron to the emission of the delayed neutron in the next generation. Delayedneutron generation times are dominated by the half-life of the delayed neutronprecursor. The average delayed neutron generation time is about 12.5 seconds.A prompt neutron generation time is the sum of the amount of time it takes a fastneutron to thermalize, the amount of time the neutron exists as a thermal neutronbefore it is absorbed, and the amount of time between a fissionable nuclideabsorbing a neutron and fission neutrons being released. Prompt neutrongeneration time is about 5 x 10 seconds.-5The average neutron generation time can be calculated from the prompt anddelayed neutron generation times and the delayed neutron fraction usingEquation (2-12).Delayed neutrons are responsible for the ability to control the rate at which powercan rise in a reactor. If only prompt neutrons existed, reactor control would not bepossible due to the rapid power changes.
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