The CABRI facility is an experimental pulse nuclear reactor funded by the French Nuclear Safety and Radioprotection Institute and operated by the French Atomic Energy Commission. It is designed to study the behavior of fuel rods at high burnup under reactivity-initiated accident (RIA) conditions, such as a control rod ejection. The distinctive feature of this reactor is its reactivity injection system. The fast depressurization into a discharge tank of 3He (strong neutron absorber) previously introduced inside 96 tubes (so-called transient rods) located among the fuel rods allows us to create a power burst from 100 kW to 20 GW with a full-width at half-maximum of 10 to 80 ms. The total energy deposit in the tested rod is adjusted by dropping the control and safety rods after the power transient. The neutron flux is measured online by compensated boron chambers located outside the reactor and operated in the current mode. These neutron detectors are calibrated during a commissioning phase thanks to standards given by a conventional heat balance. To assess the energy released into the test rod, we had to integrate the driver core power signal measured online. Thus, the beginning of the transient, called transient overpower (TOP) onset, has to be estimated. The TOP onset of a transient test is defined as the instant of the beginning of the test. It is determined by experimentalists during the processing phase. It corresponds to the beginning of the increase of the neutron detector signal, measured by the compensated boron chamber devices sufficiently sensitive at low current levels. So far, the choice of this instant has been realized by a visual choice zooming in the zone of interest, which may induce some shift according to experimentalists. In an attempt to overcome this issue, we put forward in this paper a theoretical method of determination to calculate the TOP onset. The main asset of the method is to formalize the TOP onset determination. Furthermore, it provides the possibility of associating an uncertainty, which is impossible by the manual process. The methodology relies on the fact that at the beginning of the RIA transient, the neutron flux at any point of the reactor core undergoes an exponential evolution as a function of the time. Then, a logarithmic transform allows us to show that the search for the TOP onset is equivalent to solving a nonlinear regression. The methodology has been validated in the last 14 experiments. Moreover, the reactor restarted in October 2015 and now gives us the opportunity to apply this methodology on signals recently acquired and pertaining to the power commissioning phase with a view to preparing the experiment foreseen next year.