  A possible mechanism for the formation of heavy-mass elements in supernovae
is the rapid neutron-capture-mechanism (r-process). It depends upon the
electron-fraction $Y_e$, a quantity which is determined by beta-decay-rates. In
this paper, we focus on the calculation of electroweak decay-rates in presence
of massive neutrinos. The resulting expressions are then used to calculate
nuclear reactions entering the rapid-neutron capture. We fix the astrophysical
parameters to the case of a core-collapse supernova. The neutrino sector
includes a mass scheme and mixing angles for active neutrinos, and also by
including the mixing between active and sterile neutrinos. The results of the
calculations show that the predicted abundances of heavy-mass nuclei are indeed
affected by the neutrino mixing.
