  Depending on mass and metallicity as well as evolutionary phase, stars
occasionally experience convective-reactive nucleosynthesis episodes. We
specifically investigate the situation when nucleosynthetically unprocessed,
H-rich material is convectively mixed with a He-burning zone, for example in
convectively unstable shell on top of electron-degenerate cores in AGB stars,
young white dwarfs or X-ray bursting neutron stars. Such episodes are
frequently encountered in stellar evolution models of stars of extremely low or
zero metal content [...] We focus on the convective-reactive episode in the
very-late thermal pulse star Sakurai's object (V4334 Sagittarii). Asplund etal.
(1999) determined the abundances of 28 elements, many of which are highly
non-solar, ranging from H, He and Li all the way to Ba and La, plus the C
isotopic ratio. Our simulations show that the mixing evolution according to
standard, one-dimensional stellar evolution models implies neutron densities in
the He that are too low to obtain a significant neutron capture nucleosynthesis
on the heavy elements. We have carried out 3D hydrodynamic He-shell flash
convection [...] we assume that the ingestion process of H into the He-shell
convection zone leads only after some delay time to a sufficient entropy
barrier that splits the convection zone [...] we obtain significantly higher
neutron densities (~few 10^15 1/cm^3) and reproduce the key observed abundance
trends found in Sakurai's object. These include an overproduction of Rb, Sr and
Y by about 2 orders of magnitude higher than the overproduction of Ba and La.
Such a peculiar nucleosynthesis signature is impossible to obtain with the
mixing predictions in our one-dimensional stellar evolution models. [...] We
determine how our results depend on uncertainties of nuclear reaction rates,
for example for the C13(\alpha, n)O16 reaction.
