PROMPT AND DELAYED NEUTRONSDOE-HDBK-1019/1-93Reactor Theory (Neutron Characteristics)NP-02Page 30Rev. 0For most applications, it is convenient to combine the known precursors into groups withappropriately averaged properties. These groups vary somewhat depending on the fissilematerial in use. Table 3 lists the characteristics for the six precursor groups resulting fromthermal fission of uranium-235. The fraction of all neutrons that are produced by each of theseprecursors is called the delayed neutron fraction for that precursor. The total fraction of allneutrons born as delayed neutrons is called the delayed neutron fraction(). The fraction ofdelayed neutrons produced varies depending on the predominant fissile nuclide in use. Thedelayed 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 3Delayed Neutron Precursor Groupsfor ThermalFission in Uranium-235GroupHalf-LifeDelayed NeutronAverage Energy(sec)Fraction(MeV)155.70.000210.25222.70.001420.4636.20.001270.4142.30.002570.4550.610.000750.4160.230.00027-Total-0.0065-NeutronGenerationTimeThe neutron generation time is the time required for neutrons from one generation to cause thefissions 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 areinvolved: (a) the time it takes a fast neutron to slow down to thermal energy, (b) the time thenow thermal neutron exists prior to absorption in fuel, and (c) the time required for a fissionablenucleus 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 -6depending on the moderator. In water moderated reactors, thermal neutrons tend to exist forabout 10 seconds before they are absorbed. Fission and fast neutron production following-4neutron absorption in a fissionable nucleus occurs in about 10 seconds. Thus, fast reactors-13have an # * of about 10 seconds, while thermal reactors have an # * of about 10 seconds + 10-6-6 -4seconds, which is about 10 seconds to 10 seconds.-4 -5
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