Abstract <p>The MULTIMSR computational complex (CC) created at the Kurchatov Institute for modeling molten salt reactor (MSR) for various purposes consists of several software tools operating in a single structure and having the ability to exchange computational information. The MULTIMSR CC uses the SERPENT software to perform neutronic calculations using the Monte Carlo method. In addition, the CC includes a number of software tools designed to solve thermal hydraulics equations, neutron transport in the diffusion approximation, and auxiliary problems implemented within the OpenFOAM computational operations library, a database of physical properties of fuel salts, intermediate coolants, and structural materials, and also a unifying subroutine necessary for the coordinated operation of the CC elements. The description and block diagram of the CC, as well as the results of the analysis of the influence of the number of groups used in the diffusion approximation on the correctness of the results obtained with respect to the MSR burner (MSR-B) of transuranic elements from SNF of reactors of the VVER-1000/1200 type with fuel salt of Li, Be, and A_<i>n</i>/F_<i>m</i> composition, have been provided. Multiphysical modeling of the development of an accident in the event of loss of forced circulation of fuel salt and failure of protection operation in MSR-B with 2.4-GW thermal power with a core of cavity type has been performed. Some conclusions on reactor stability in a number of transient processes have been presented.</p>