  Context. Interstellar ice is the main form of metal species in dark molecular
clouds. Experiments and observations have shown that the ice is significantly
processed after the freeze-out of molecules onto grains. The processing is
caused by cosmic-ray particles and cosmic-ray-induced UV photons. These
transformations are included in current astrochemical models only to a very
limited degree. Aims. We aim to establish a model of the "cold" chemistry in
interstellar ices and to evaluate its general impact on the composition of
interstellar ices. Methods. The ice was treated as consisting of two layers -
the surface and the mantle (or subsurface) layer. Subsurface chemical processes
are described with photodissociation of ice species and binary reactions on the
surfaces of cavities inside the mantle. Hydrogen atoms and molecules can
diffuse between the layers. We also included deuterium chemistry. Results. The
modeling results show that the content of chemically bound H is reduced in
subsurface molecules by about 30 % on average. This promotes the formation of
more hydrogen-poor species in the ice. The enrichment of ice molecules with
deuterium is significantly reduced by the subsurface processes. On average, it
follows the gas-phase atomic D-to-H abundance ratio, with a delay. The delay
produced by the model is on the order of several Myr. Conclusions. The
processing of ice may place new constraints on the production of deuterated
species on grains. In a mantle with a two-layer structure the upper layer (CO)
should be processed substantially more intensively than the lower layer (H2O).
Chemical explosions in interstellar ice might not be an important process. They
destroy the structure of the mantle, which forms over long timescales. Besides,
ices may lack the high radical content needed for the explosions.
