The Organization for Economic Cooperation and Development/Nuclear Energy Agency Main Steam Line Break (MSLB) benchmark provides a comparison of state-of-the-art and best-estimate models used to compute reactivity accidents. A comprehensive study has been carried out by Commissariat à l'Energie Atomique and Institut de Radioprotection et de Sûreté Nucléaire with the CATHARE, CRONOS2, and FLICA4 codes. The effect of mixing between primary loops in the core vessel is analyzed, then zero-dimensional and three-dimensional (3-D) kinetics are compared, and finally, the effect of the core thermal-hydraulic model is presented. The aim of this analysis is to assess the 3-D effects in the MSLB accident and to explain the return-to-power occurrence.