  IRAS 16293-2422 is a well studied low-mass protostar characterized by a
strong level of deuterium fractionation. In the line of sight of the
protostellar envelope, an additional absorption layer, rich in singly and
doubly deuterated water has been discovered by a detailed multiline analysis of
HDO. To model the chemistry in this source, the gas-grain chemical code
Nautilus has been used with an extended deuterium network. For the protostellar
envelope, we solve the chemical reaction network in infalling fluid parcels in
a protostellar core model. For the foreground cloud, we explored several
physical conditions (density, cosmic ionization rate, C/O ratio). The main
results of the paper are that gas-phase abundances of H2O, HDO and D2O observed
in the inner regions of IRAS16293-2422 are lower than those predicted by a 1D
dynamical/chemical (hot corino) model in which the ices are fully evaporated.
The abundance in the outer part of the envelope present chaotic profiles due to
adsorption/evaporation competition, very different from the constant abundance
assumed for the analysis of the observations. We also found that the large
abundances of gas-phase H2O, HDO and D2O observed in the absorption layer are
more likely explained by exothermic surface reactions rather than
photodesorption processes.
