The diversity approach is commonly used to ensure dependability attributes of such critical computer-based systems as nuclear power plants information and control (NPPs I&C) systems, including the Reactor Trip System. There are two basic problems emerging in this approach -the choice of product-process diversity kinds and assessment of multi-version systems dependability. This paper presents a study for dependability assessment of two architectures of a typical NPP I&C system, the Reactor Trip System: a diverse two-channel system with three parallel tracks on voting logic "2-out-of-3" in each channel and a diverse three-channel system with two parallel tracks on voting logic "1-out-of-2" in each channel. The multi-fragmentation approach is proposed to provide a detailed description of RTS hardware-software interconnection. The resulting models are stiff, which poses a problem in case of achieving the accurate numerical solution. Thus, the models are solved using a number of approaches and tools to verify the results. Obtained results can help to make informed decision between the observed RTS architectures.