[en] This paper is associated with the centrifugation process for isotope separation, using the principle of a cylinder rotating at high speed in a vacuum casing. As in the most widely used configuration, the gas containing the isotope mixture is introduced by a fixed axial feed pipe and expands in the cylinder. It is subjected to high centrifugal acceleration, undergoes rigid body rotation and stratifies radially according to a barometric-type pressure law. By pressure diffusion, the heavier isotopes migrate to the cylinder wall and the lighter to the center. A temperature gradient on the wall and the presence of a scoop in the fluid, produce a vertical countercurrent which transforms the radial separation effect into an axial effect. The scoop extracts the gas depleted in light isotopes, called W, and another is used to recover the gas enriched in light isotopes, called P. Practically all the gas is governed by the Navier-Stokes equations in 2D axial symmetry. Due to the strong pressure stratification, continuous fluid equations are not valid in the whole cylinder, with or without linearization of the model. Consequently, an internal boundary separates the continuum domain from a rarefied domain in which the feed gas expands. The radial position of this cut-off then approaches the cylinder wall with increasing rotation speeds. In the rarefied domain, the Boltzmann equation is solved and a well suited numerical method is the Monte-Carlo method. A complete simulation of feed gas expansion and interaction with rotating gas, presented here with the DSMC (Direct Simulation Monte-Carlo) code, provides realistic boundary conditions for fluid flow calculations. The reference centrifuge is a hypothetical machine enabling the scientific community to compare results obtained for the optimization of separation performance. Its radius a is 6 cm, and its peripheral speed a is 600 m/s. The selected gas, containing the isotopes, is UF₆. The gas pressure p(a) at the cylinder wall is set at 100 Torr and the gas temperature is 310 K al the feed pipe outlet. The domain studied with DSMC extends radially up to rmax 5 cm, i.e., outside the estimated fluid limit of around 4,5 cm, and the domain is limited axially to xmaX = 5 cm. At the center, the radius of the feed pipe is taken at r = 0,5 cm. The axial extension h of the outlet orifice is 1 cm and there is a symmetry plane at x = 0. The outlet condition is assumed to be sonic. The rotating gas is simulated by an emission from the internal boundary, at r = rmax. Because of the rigid body rotation in the fluid region, the speed of the gas emitted over a half-Maxwellian is rmax. The domain, which is initially empty, is divided into uniform, 100*100 cells. The time step is 5.10^-7 second. At zero feed, the emitted gas from the rotating internal boundary has a barometric type pressure profile even up to the axis. In the feed expansion, rotation of the feed gas is induced by collisions with the revolving gas and its radial speed drops drastically. The influence of HF, present in the rotating UF6, on the rotation of the feed gas is also studied. From the technological standpoint, it is essential to know the mechanical power of the rotating cylinder and the heat flux dissipated on the two cylinders, as a function of the residual pressure in the casing. The Couette flow of the rarefied gas in the casing, between the rotating cylinder and the fixed outside cylinder, is so studied with DSMC. For small Knudsen numbers (Kn * 0.01), defined as the ratio of the mean free path - mean distance of a molecular collision - to the distance between the cylinders, steady Taylor-Couette vortices are identified by this kinetic description