Boiling helium natural circulation loops are used as cooling system of some large superconducting magnets. Their thermalhydraulic behavior has already been studied quite thoroughly in steady state operation. However, the transient response of these systems remains up to date poorly studied, and transient boiling experiments in other fluids or heating configurations are difficult to extrapolate. In this work, an experimental study of helium natural circulation loops is conducted in which the system is excited by a step-pulse power wave function from a static initial condition (no power applied). The measurements of wall temperature on the heated section allowed the detection of boiling crisis onset during the transient. Different boiling crisis onset mechanisms could be determined from the dependence of the nucleate boiling lifetime on applied heat flux. The simultaneous analysis of thermal aspects and mass flow rate evolution allowed determining a semi-empirical local criterion based on the evolution of vapor fraction along the section for the prediction of transient boiling crisis. (C) 2016 Elsevier Ltd. All rights reserved.