  We compute the structure of a Newtonian, multi-ion radiation-mediated shock
(RMS) for different compositions anticipated in various stellar explosions. We
use a multifluid RMS model that incorporates electrostatic coupling between the
different plasma constituents as well as Coulomb friction in a self-consistent
manner, and approximates the effect of pair creation and the presence of free
neutrons in the shock upstream on the shock structure. We find that under
certain conditions a significant velocity separation is developed between
different ions in the shock downstream and demonstrate that in fast enough
shocks ion-ion collisions may trigger fusion and fission events at a relatively
high rate. Our analysis ignores anomalous coupling through plasma
microturbulence, that might reduce the velocity spread downstream below the
activation energy for nuclear reactions. A rough estimate of the scale
separation in RMS suggests that for shocks propagating in BNS merger ejecta the
anomalous coupling length may exceed the radiation length, allowing a
considerable composition change behind the shock via inelastic collisions of
$\alpha$ particles with heavy elements at shock velocities
$\beta_u\gtrsim0.25$. A sufficient abundance of free neutrons in the shock
upstream, as expected during the first second after the merger, is also
expected to alter the ejecta composition through neutron capture downstream.
The resultant change in the composition profile may affect the properties of
the early kilonova emission. The generation of microturbulence due to velocity
separation can also give rise to particle acceleration that might alter the
breakout signal in supernovae and other systems.
