  We have modelled the multicycle evolution of rapidly-accreting CO white
dwarfs (RAWDs) with stable H burning intermittent with strong He-shell flashes
on their surfaces for $0.7\leq M_\mathrm{RAWD}/M_\odot\leq 0.75$ and [Fe/H]
ranging from $0$ to $-2.6$. We have also computed the i-process nucleosynthesis
yields for these models. The i process occurs when convection driven by the
He-shell flash ingests protons from the accreted H-rich surface layer, which
results in maximum neutron densities $N_\mathrm{n,max}\approx
10^{13}$-$10^{15}\ \mathrm{cm}^{-3}$. The H-ingestion rate and the convective
boundary mixing (CBM) parameter $f_\mathrm{top}$ adopted in the one-dimensional
nucleosynthesis and stellar evolution models are constrained through 3D
hydrodynamic simulations. The mass ingestion rate and, for the first time, the
scaling laws for the CBM parameter $f_\mathrm{top}$ have been determined from
3D hydrodynamic simulations. We confirm our previous result that the
high-metallicity RAWDs have a low mass retention efficiency ($\eta < 10\%$). A
new result is that RAWDs with [Fe/H]$< -2$ have $\eta > 20\%$, therefore their
masses may reach the Chandrasekhar limit and they may eventually explode as
SNeIa. This result and the good fits of the i-process yields from the
metal-poor RAWDs to the observed chemical composition of the CEMP-r/s stars
suggest that some of the present-day CEMP-r/s stars could be former distant
members of triple systems, orbiting close binary systems with RAWDs that may
have later exploded as SNeIa.
