  New ${\bar{\nu}_e},e$ scattering experiments aimed for sensitive searches of
the ${\nu}_e$ magnetic moment and projects to explore small mixing angle
oscillations at reactors call for a better understanding of the reactor
antineutrino spectrum. Here we consider six components, which contribute to the
total ${\bar{\nu}_e}$ spectrum generated in nuclear reactor. They are: beta
decay of the fission fragments of $^{235}$U, $^{239}$Pu, $^{238}$U and
$^{241}$Pu, decay of beta-emitters produced as a result of neutron capture in
$^{238}$U and also due to neutron capture in accumulated fission fragments
which perturbs the spectrum. For antineutrino energies less than 3.5 MeV we
tabulate evolution of ${\bar{\nu}_e}$ spectra corresponding to each of the four
fissile isotopes vs fuel irradiation time and their decay after the irradiation
is stopped and also estimate relevant uncertainties. Small corrections to the
ILL spectra are considered.
