  Observations of deuterated species are useful in probing the temperature,
ionization level, evolutionary stage, chemistry, and thermal history of
astrophysical environments. The analysis of data from ALMA and other new
telescopes requires an elaborate model of deuterium fractionation. This paper
presents a publicly available chemical network with multi-deuterated species
and an extended, up-to-date set of gas-phase and surface reactions. To test
this network, we simulate deuterium fractionation in diverse interstellar
sources. Two cases of initial abundances are considered: i) atomic except for
H2 and HD, and ii) molecular from a prestellar core. We reproduce the observed
D/H ratios of many deuterated molecules, and sort the species according to
their sensitivity to temperature gradients and initial abundances. We find that
many multiply-deuterated species produced at 10 K retain enhanced D/H ratios at
temperatures $\la 100$ K. We study how recent updates to reaction rates affect
calculated D/H ratios, and perform a detailed sensitivity analysis of the
uncertainties of the gas-phase reaction rates in the network. We find that
uncertainties are generally lower in dark cloud environments than in warm IRDCs
and that uncertainties increase with the size of the molecule and number of
D-atoms. A set of the most problematic reactions is presented. We list
potentially observable deuterated species predicted to be abundant in low- and
high-mass star-formation regions.
