We present here some theoretical and experimental results about the application of the time-domain reflectometry (TDR) technique to a once-through steam generator (OTSG) bayonet-type, which is considered here, from a new point of view, as a coaxial transmission line and used as a level monitoring device for its own dynamics. This original approach leads to the design of a new kind of sensor, employable as a general-level measuring system in harsh conditions as well. This sensor may offer some interesting features for the design of control systems for nuclear facilities as well as for oil and gas plants. In order to experimentally verify the theoretical part, a mockup has been designed and built, so as to be used as a level control for a dedicated hydraulic loop system both in static and dynamic conditions. The purpose of the hydraulic loop system is to implement a short-/medium-term transient-level dynamics requiring fast sensor response, online measurement, and some nonlinearity features that are crucial for its control. The obtained results show that the TDR application on the OTSG is feasible at standard ambient temperature and pressure (SATP) conditions, paving the way for tests at the operating pressure and temperature ranges of a real plant.