  The COSINE-100 dark matter search experiment is an array of NaI(Tl) crystal
detectors located in the Yangyang Underground Laboratory (Y2L). To understand
measured backgrounds in the NaI(Tl) crystals we have performed Monte Carlo
simulations using the Geant4 toolkit and developed background models for each
crystal that consider contributions from both internal and external sources,
including cosmogenic nuclides. The background models are based on comparisons
of measurement data with Monte Carlo simulations that are guided by a campaign
of material assays and are used to evaluate backgrounds and identify their
sources. The average background level for the six crystals (70 kg total mass)
that are studied is 3.5 counts/day/keV/kg in the (2-6) keV energy interval. The
dominant contributors in this energy region are found to be $^{210}$Pb and
$^3$H.
