PROMPT AND DELAYED NEUTRONS
DOE-HDBK-1019/1-93
Reactor Theory (Neutron Characteristics)
NP-02
Page 30
Rev. 0
For most applications, it is convenient to combine the known precursors into groups with
appropriately averaged properties. These groups vary somewhat depending on the fissile
material in use. Table 3 lists the characteristics for the six precursor groups resulting from
thermal fission of uranium-235. The fraction of all neutrons that are produced by each of these
precursors is called the delayed neutron fraction for that precursor. The total fraction of all
neutrons born as delayed neutrons is called the delayed neutron fraction ( ). The fraction of
delayed neutrons produced varies depending on the predominant fissile nuclide in use. The
delayed neutron fractions ( ) for the fissile nuclides of most interest are as follows: uranium-233
(0.0026), uranium-235 (0.0065), uranium-238 (0.0148), and plutonium-239 (0.0021).
TABLE 3
Delayed Neutron Precursor Groups
for Thermal Fission in Uranium-235
Group
Half-Life
Delayed Neutron
Average Energy
(sec)
Fraction
(MeV)
1
55.7
0.00021
0.25
2
22.7
0.00142
0.46
3
6.2
0.00127
0.41
4
2.3
0.00257
0.45
5
0.61
0.00075
0.41
6
0.23
0.00027
-
Total
-
0.0065
-
Neutron Generation Time
The neutron generation time is the time required for neutrons from one generation to cause the
fissions that produce the next generation of neutrons. The generation time for prompt neutrons
(# * - pronounced "ell-star") is the total time from birth to rebirth. Three time intervals are
involved: (a) the time it takes a fast neutron to slow down to thermal energy, (b) the time the
now thermal neutron exists prior to absorption in fuel, and (c) the time required for a fissionable
nucleus to emit a fast neutron after neutron absorption.
Fast neutrons slow to thermal energies or leak out of the reactor in 10 seconds to 10 seconds,
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depending on the moderator. In water moderated reactors, thermal neutrons tend to exist for
about 10 seconds before they are absorbed. Fission and fast neutron production following
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neutron absorption in a fissionable nucleus occurs in about 10 seconds. Thus, fast reactors
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have an # * of about 10 seconds, while thermal reactors have an # * of about 10 seconds + 10
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seconds, which is about 10 seconds to 10 seconds.
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