Page 259 - Dynamics and Control of Nuclear Reactors

260 APPENDIX C Basic reactor physics

Table C.1 Energy distribution for fission induced by thermal neutrons in
U-235.
Source Energy (MeV)
Fission product KE 168
Neutron KE 5
Energy in gamma radiation (instantaneous and delayed) 11
Energy in β-decay of fission products 7
• Total (available as thermal energy): 191
Energy not available as heat 11
• Total energy created in one fission reaction: 202

The neutron binding energies of U-235, U-233, and U-239 nuclei (odd number of
neutrons) are about I MeV higher than the nuclei of Th-232 and U-238 (even number
of neutrons). This additional binding energy is sufficient to exceed the critical energy
for fission, with low-energy neutrons. For example: for U-235, critical energy for
fission to occur is 5.5 MeV and the binding energy of an extra neutron is
6.6 MeV. The critical and binding energies for U-238 are 5.9 MeV and 4.9 MeV.
Out of two to three neutrons released per fission (depending on the fissioning iso-
tope involved), one of these is used to produce the next fission reaction in a steady-
state chain reaction. The remaining neutrons are consumed by
• Leakage from the core
• Capture by non-fuel reactor constituents (such as coolant, moderator and
structural materials)
• Non-fission capture in the fuel (radiative capture)
• Capture by the fertile nuclei (such as U-238, resonance capture).

The distribution of fission energy in various forms is shown in Table C.1.

C.5 Fast and thermal neutrons
Immediately following fission, the neutrons possess high kinetic energy, in the
million-eV range (0.1–15 MeV). Most current-generation reactors include a material
(called a moderator) whose purpose is to slow down neutrons while capturing few
neutrons. Fast neutrons lose their energy due to scattering collisions with various
nuclei in the medium (especially in the moderator, see Section C.2), and become
slow neutrons (energy <1 eV).
It should be noted that tabulated cross sections are usually for monoenergetic neu-
trons at an energy of 0.0253 eV or a speed of 2200 m/s. This energy corresponds to a
temperature of 20 °C or 293 K. Thermal neutrons are those whose kinetic energy
reaches equilibrium with the thermal energy of the moderator. Higher moderator
temperature means greater thermal motion of moderator atoms and a consequent