[en] Inside a reactor, when the drift velocity of the neutron gas is small with respect to its velocity of thermal agitation, it is possible to introduce an approximation in the usual Grad expansion and to obtain a simple form of the neutron distribution as a function of the components of their individual velocities and of the usual moments. Carrying that distribution into Boltzmann's equation, and integrating conveniently, one obtains equations of the hydrodynamic type. The terms which express the influence of collisions contain noticeable functions which are characteristic of the nature of the nuclei and of the neutron temperature. In the case of a spherical fast-neutron reactor, the determination of the multiplication coefficient boils down to the integration of four differential equations. The results thus obtained for a certain number of configurations are compared with those obtained with the D.S.N. method