The general structure and main principles of creating a multi-version post-severe NPP accident monitoring system, which consists of one wired and three drone-based wireless network subsystems, are proposed. Reliability block diagrams for the system are built. On the basis of the reliability block diagrams, reliability models of the system are developed. The dependence on probability of failure-free operation for different structures of the system on number of the redundant drones for the Wi-Fi drone-based communication section is obtained and analysed.