  We perform the first multidimensional fluid simulations of thermonuclear
helium ignition underneath a hydrogen-rich shell. This situation is relevant to
Type I X-ray bursts on neutron stars which accrete from a hydrogen-rich
companion. Using the low Mach number fluid code MAESTROeX, we investigate the
growth of the convection zone due to nuclear burning, and the evolution of the
chemical abundances in the atmosphere of the star. We also examine the
convective boundary mixing processes which cause the evolution to differ
significantly from previous one-dimensional simulations that rely on
mixing-length theory. We find that the convection zone grows outwards as
penetrating fluid elements cool the overlying radiative layer, rather than
directly from the increasing entropy of the convection zone itself.
Simultaneously, these flows efficiently mix composition, carrying carbon out
of, and protons into the convection zone even before contact with the hydrogen
shell. We discuss the implications of these effects for future modeling of
these events and observations.
