  Most of fast radio bursts (FRB) do not show evidence for repetition, and such
non-repeating FRBs may be produced at the time of a merger of binary neutron
stars (BNS), provided that the BNS merger rate is close to the high end of the
currently possible range. However, the merger environment is polluted by
dynamical ejecta, which may prohibit the radio signal to propagate. We examine
this by using a general-relativistic simulation of a BNS merger, and show that
the ejecta appears about 1 ms after the rotation speed of the merged star
becomes the maximum. Therefore there is a time window in which an FRB signal
can reach outside, and the short duration of non-repeating FRBs can be
explained by screening after ejecta formation. A fraction of BNS mergers may
leave a rapidly rotating and stable neutron star, and such objects may be the
origin of repeating FRBs like FRB 121102. We show that a merger remnant would
appear as a repeating FRB in a time scale of about 1-10 yrs, and expected
properties are consistent with the observations of FRB 121102. We construct an
FRB rate evolution model including these two populations of repeating and
non-repeating FRBs from BNS mergers, and show that the detection rate of
repeating FRBs relative to non-repeating ones rapidly increases with improving
search sensitivity. This may explain that the only repeating FRB 121102 was
discovered by the most sensitive FRB search with Arecibo. Several predictions
are made, including appearance of a repeating FRB 1-10 years after a BNS merger
that is localized by gravitational wave and subsequent electromagnetic
radiation.
