  Dark photons are well motivated hypothetical dark sector particles that could
account for observations that cannot be explained by the standard model of
particle physics. A search for dark photons that are produced by an electron
beam striking a thick tungsten target and subsequently interact in a 3
kiloton-scale neutrino detector in Yemilab, a new underground lab in Korea, is
proposed. Dark photons can be produced by "darkstrahlung" or by oscillations
from ordinary photons produced in the target and detected by their visible
decays, "absorption" or by their oscillation to ordinary photons. By detecting
the absorption process or the oscillation-produced photons, a world's best
sensitivity for measurements of the dark-photon kinetic mixing parameter of
$\epsilon^2 > 1.5 \times 10^{-13} (6.1 \times 10^{-13})$ at the 95\% confidence
level (C.L.) could be obtained for dark photon masses between 80 eV and 1 MeV
in a year-long exposure to a 100 MeV-100 kW electron beam with zero ($10^3$)
background events. In parallel, the detection of $e^+e^-$ pairs from decays of
dark photons with mass between 1 MeV and $\sim$86 MeV would have sensitivities
of $\epsilon^2 > \mathcal{O}(10^{-17}) (\mathcal{O}(10^{-16}))$ at the 95% C.L.
with zero ($10^3$) background events.
