  A series of optical and one near-infrared nebular spectra covering the first
year of the Type Ia supernova SN 2011fe are presented and modelled. The density
profile that proved best for the early optical/ultraviolet spectra, "rho-11fe",
was extended to lower velocities to include the regions that emit at nebular
epochs. Model rho-11fe is intermediate between the fast deflagration model W7
and a low-energy delayed-detonation. Good fits to the nebular spectra are
obtained if the innermost ejecta are dominated by neutron-rich, stable Fe-group
species, which contribute to cooling but not to heating. The correct thermal
balance can thus be reached for the strongest [FeII] and [FeIII] lines to be
reproduced with the observed ratio. The 56Ni mass thus obtained is 0.47 +/-
0.05 Mo. The bulk of 56Ni has an outermost velocity of ~8500 km/s. The mass of
stable iron is 0.23 +/- 0.03 Mo. Stable Ni has low abundance, ~10^{-2} Mo. This
is sufficient to reproduce an observed emission line near 7400 A. A
sub-Chandrasekhar explosion model with mass 1.02 Mo and no central stable Fe
does not reproduce the observed line ratios. A mock model where neutron-rich
Fe-group species are located above 56Ni following recent suggestions is also
shown to yield spectra that are less compatible with the observations. The
densities and abundances in the inner layers obtained from the nebular
analysis, combined with those of the outer layers previously obtained, are used
to compute a synthetic bolometric light curve, which compares favourably with
the light curve of SN 2011fe.
