The McSAFER project addresses an expanding interest in Small Modular Reactors (SMRs), in particular the demand for validated computational tools for simulating new key thermal hydraulic phenomena. In the McSAFER project, work package 4 assesses the simulation of the three-dimensional thermal-hydraulic phenomena inside the reactor pressure vessel using multiscale thermal-hydraulic tools in combination with traditional one-dimensional system thermal hydraulic codes. In work package 4, the focus is on the RPV behavior whereas the other systems are modelled as boundary conditions. This paper summarizes a description of the SMART reactor, the ATWS transient scenario and the simulation models. Two simulation models are developed with the main difference between both models being the 1D or 3D course mesh. A steady-state and ATWS transient analysis are performed and the results of the two models are compared. The ATWS transient is characterized by a sudden closure of the feedwater isolation valves causing an increase in the reactor coolant average temperature. The total negative neutronic feedback effects ensure that the core power decreases, and the passive residual heat removal system establishes a sufficient cooling of the core power. It is concluded that the potential improvements in the flow solution using the 3D approach do not have a substantial impact on the results.