  The impact of unstable Majorana tau neutrinos on primordial nucleosynthesis
is considered. The mass and lifetime of nu_tau are taken in the intervals
0.1-20 MeV and 0.001-400 sec respectively. The studied decay modes are nu_tau
-> nu_mu + phi and nu_tau -> nu_e + phi, where phi is a massless (or light)
scalar. Integro-differential kinetic equations are solved numerically without
any simplifying assumptions. Our results deviate rather strongly from earlier
calculations. Depending on mass, lifetime, and decay channels of the nu_tau,
the number of effective neutrino species (found from He4), in addition to the 3
standard ones, varies from -2 to +2.5. The abundances of H2 and Li7 are also
calculated.
