  A compact accretion disk may be formed in the merger of two neutron stars or
of a neutron star and a stellar-mass black hole. Outflows from such accretion
disks have been identified as a major site of rapid neutron-capture (r-process)
nucleosynthesis and as the source of 'red' kilonova emission following the
first observed neutron-star merger GW170817. We present long-term
general-relativistic radiation magnetohydrodynamic simulations of a typical
post-merger accretion disk at initial accretion rates of $\dot{M}\sim
1\,M_\odot\,\text{s}^{-1}$ over 400ms post-merger. We include neutrino
radiation transport that accounts for effects of neutrino fast flavor
conversions dynamically. We find ubiquitous flavor oscillations that result in
a significantly more neutron-rich outflow, providing lanthanide and 3rd-peak
r-process abundances similar to solar abundances. This provides strong evidence
that post-merger accretion disks are a major production site of heavy r-process
elements. A similar flavor effect may allow for increased lanthanide production
in collapsars. The formalism presented here may also be used in simulations of
core-collapse supernovae to explore whether fast conversions strengthen or
weaken the explosion.
