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.
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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.
