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 $0.7leq M_mathrm{RAWD}/M_odotleq 0.75$ 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.
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