  A fraction of neutrino emission from GRB accretion disks annihilates above
the disk, creating e+- plasma that can drive GRB explosions. We calculate the
efficiency of this annihilation using the recent detailed model of
hyper-accretion disks around Kerr black holes. Our calculation is fully
relativistic and based on a geodesic-tracing method. We find that the
efficiency is a well-defined function of (1) accretion rate and (2) spin of the
black hole. It is practically independent of the details of neutrino transport
in the opaque zone of the disk. The results help identify the accretion disks
whose neutrino emission can power GRBs.
