This experimental study investigates the thermal hydraulic behavior and boiling mechanisms present in a natural circulation loop experiencing boil-off. Extended duration tests with inventory loss were conducted to examine the transient behavior of a test loop under constant heat load. Five different stages of natural circulation flow were identified: single-phase heating, transitional nucleate boiling, hydrostatic head fluctuations, stable two-phase flow, and geysering. The water level in the storage tank was observed to play an important role in the stability of the system as it directly influences the hydrostatic pressures and, therefore local saturation temperatures. It would found that changes in inventory can shift the boiling boundary to different segments of the loop, whose geometry and orientation dictate the flow regime and ultimately the system behavior. With continued loss of inventory, a break in the loop occurs and induces flow stagnation with geysering in the heated region. Further operation results in severe degradation of heat removal, with a potential for burn out and possible structural damage. For safety-related applications, the venting of steam can extend the long-term cooling time window for heat removal and is of significant practical interest. Published by Elsevier Ltd.