Abstract DYNSUB is a novel two-way pin-based coupling of the simplified transport ( SP 3 ) version of DYN3D with the sub-channel code SUBCHANFLOW. The new coupled code system allows for a more realistic description of the core behaviour under steady state and transient conditions. The details of the developed internal coupling approach of both codes together with its implementation are presented and discussed. Comparisons of the results predicted by DYNSUB with the ones of coarser coupled solutions have shown very good agreement in the global parameters ( k eff and power distribution at steady state and position and magnitude of the power peak in the transient cases) validating the correctness of the coupling strategy. At local level however, important (and expected) deviations in the local safety parameters (maximal clad, fuel and moderator temperatures) have arisen. Differences up to 150 K in the centreline fuel rod temperature were found. It demonstrates the novel capabilities of the developed coupled system DYNSUB. The more detailed coupling solution had also an important impact on the convergence process, mainly in the neutronics internal convergence due to a smoother gradient on the thermal-hydraulics feedback parameters between neighbour sub-channels, compared to the gradient between assembly level channels. DYNSUB has successfully been applied to analyze the behaviour of one eight of a PWR core during a REA transient by a pin-by-pin simulation consisting of a huge amount of nodes.