Prompt and Delayed Neutrons Summary

PROMPT AND DELAYED NEUTRONS DOE-HDBK-1019/1-93 Reactor Theory (Neutron Characteristics) NP-02 Page 32 Rev. 0 Prompt and Delayed Neutrons Summary Prompt neutrons are released directly from fission within 10 seconds of the -13 fission event. Delayed neutrons are released from the decay of fission products that are called delayed neutron precursors. Delayed neutron precursors are grouped according to half-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 fuel materials. Following are values for some common fuel materials. Uranium-235 0.0065 Plutonium-239 0.0021 Delayed neutrons are produced by a classification of fission products known as delayed 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 delayed neutron precursor. The delayed neutron generation time is the total time from the birth of the fast neutron to the emission of the delayed neutron in the next generation. Delayed neutron generation times are dominated by the half-life of the delayed neutron precursor. 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 fast neutron to thermalize, the amount of time the neutron exists as a thermal neutron before it is absorbed, and the amount of time between a fissionable nuclide absorbing a neutron and fission neutrons being released. Prompt neutron generation time is about 5 x 10 seconds. -5 The average neutron generation time can be calculated from the prompt and delayed neutron generation times and the delayed neutron fraction using Equation (2-12). Delayed neutrons are responsible for the ability to control the rate at which power can rise in a reactor. If only prompt neutrons existed, reactor control would not be possible due to the rapid power changes.