  The primordial abundance of long-lived heavy Majorana neutrinos is calculated
from the full Boltzmann equation. Inclusion of scattering reactions drastically
change the predicted abundance of a heavy neutrino species. This loosens the
well known mass constraint on MeV neutrinos from Big Bang nucleosynthesis, and
allows for the existence of a Majorana tau neutrino with mass m > 11 MeV.
Further experimental efforts are therefore needed to investigate the range 11
MeV < m < 24 MeV. Some interesting cosmological consequences of an MeV nu_tau
are also pointed out.
