Validation and verification of the coupled neutron kinetic/thermal hydraulic system code DYN3D/ATHLET

•General description of the coupled code DYN3D/ATHLET developed in HZDR (Helmholtz-Zentrum Dresden-Rossendorf) and used to simulate steady-state and transient behaviour of LWRs is given.•Nearly 20 real plant transients and dynamic benchmarks for 5 LWR designs were simulated to evaluate the coupling interface between two integrated codes and interaction of DYN3D model of 3-D neutron kinetics with plant components modelled by ATHLET.•An overview of DYN3D/ATHLET validation and verification activities is given with references to published results.•The results of two most recently performed validation (Kalinin-3) and verification (AER-7) tasks are presented in a separate chapter in more detail.•The validation/verification status of DYN3D/ATHLET was noticeably increased.