  We present radiative transfer simulations for blue kilonovae hours after
neutron star (NS) mergers by performing detailed opacity calculations for the
first time. We calculate atomic structures and opacities of highly ionized
elements (up to the tenth ionization) with atomic number Z = 20 - 56. We find
that the bound-bound transitions of heavy elements are the dominant source of
the opacities in the early phase (t < 1 day after the merger), and that the
ions with a half-closed electron shell provide the highest contributions. The
Planck mean opacity for lanthanide-free ejecta (with electron fraction of Ye =
0.30 - 0.40) can only reach around kappa ~ 0.5 - 1 cm^2 g^-1 at t = 0.1 day,
whereas that increases up to kappa ~ 5 - 10 cm^2 g^-1 at t = 1 day. The
spherical ejecta model with an ejecta mass of Mej = 0.05Msun gives the
bolometric luminosity of ~ 2 x 10^42 erg s^-1 at t ~ 0.1 day. We confirm that
the existing bolometric and multi-color data of GW170817 can be naturally
explained by the purely radioactive model. The expected early UV signals reach
20.5 mag at t ~ 4.3 hours for sources even at 200 Mpc, which is detectable by
the facilities such as Swift and the Ultraviolet Transient Astronomy Satellite
(ULTRASAT). The early-phase luminosity is sensitive to the structure of the
outer ejecta, as also pointed out by Kasen et al. (2017). Therefore, the early
UV observations give strong constraints on the structure of the outer ejecta as
well as the presence of a heating source besides r-process nuclei.
