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, -4 -6 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 -4 neutron  absorption in a fissionable nucleus occurs in about 10     seconds.  Thus, fast reactors -13 have an # * of about 10   seconds, while thermal reactors have an # * of about 10    seconds + 10 -6 -6 -4 seconds, which is about 10    seconds to 10    seconds. -4 -5