Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
The easiest approximation of prompt neutron spectra is a Maxwellian approximation
(evaporation spectrum) characterized by a temperature (which depends in this case upon
the fissioning system and upon incident neutron energy).
with the following relationship between average energy and the temperature:
= 1.5 T M
Nevertheless, the Maxwell approximation neglects some physical effects (motion of
fission fragments emitting neutrons…) A better approximation is the Watt spectrum,
which is in fact a centre-of-mass Maxwellian spectrum, transformed to the laboratory
system, assuming an average kinetic energy per nucleon Ef. The Maxwellian spectrum is
the limit of the Watt spectrum when Ef tends to 0 (immobile fragment).
with the following relationship between average energy and the temperature:
= �f + 1.5TM.
Better (and more complicated) approximations, like the Madland and Nix model, are used
in the evaluated files to describe the fission spectrum of the most important fissile
nuclides ( 235
U, 239
Pu).
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