A new formulation for the importance function in the kinetics of subcritical reactors

► In this paper we propose a new formulation for the importance function in the kinetics of subcritical systems. ► We analyze the relevance of an external neutron source for the subcritical interval 0.95 < keff < 1. ► We obtain a new system of the point kinetic equations that is rather precise in the subcriticality range considered.