Although many methods have been proposed to localize a robot using onboard sensors in GPS-denied environments, achieving robust localization in geometrically degenerated tunnels remains a challenging problem in robot-based inspection tasks. In this work, we first present a novel model to analyze the localizability of the prior map at a given location. Then we propose the utilization of a single Ultra-Wideband (UWB) ranging device to compensate for the degeneration of LiDAR-based localization inside tunnels. A probabilistic sensor fusion method is developed and demonstrated to achieve real-time robust localization inside a geometrically degenerated tunnel.