[en] The fuel understanding of the kinetics of a subcritical assembly is a key issue for its online reactivity control. Point kinetics is not sufficient to determine the prompt reactivity of a subcritical assembly through the response to a Dirac pulse in particular in the cases of a large reflector, a small reactor, or a large subcriticality. Taking into account the distribution of inter-generation times, which appears as a robust characteristic of each type of reactor, helps to understand this behaviour. Eventually, a method is proposed for the determination of the prompt reactivity. It provides a decrease rate function depending on the prompt multiplication coefficient k_eff^p. Fitting a measured decrease rate with this function, calculated once for the reactor, gives the true value of k_eff^p. The robustness of the method is tested. (authors)