  We study the enrichment and mixing of r-process elements in ultrafaint dwarf
galaxies (UFDs). We assume that r-process elements are produced by neutron-star
mergers (NSMs), and examine multiple models with different natal kick
velocities and explosion energies. To this end, we perform cosmological
simulations of galaxy formation to follow mixing of the dispersed r-process
elements driven by star formation and the associated stellar feedback in
progenitors of UFDs. We show that the observed europium abundance in Reticulum
II is reproduced by our inner explosion model where a NSM is triggered at the
centre of the galaxy, whereas the relatively low abundance in Tucana III is
reproduced if a NSM occurs near the virial radius of the progenitor galaxy. The
latter case is realised only if the neutron-star binary has a large natal kick
velocity and travels over a long distance of a kilo-parsec before merger. In
both the inner and outer explosion cases, it is necessary for the progenitor
galaxy to sustain prolonged star formation over a few hundred million years
after the NSM, so that the dispersed r-process elements are well mixed within
the inter-stellar medium. Short-duration star formation results in inefficient
mixing, and then a large variation is imprinted in the stellar europium
abundances, which is inconsistent with the observations of Reticulum II and
Tucana III.
