The simulation of the units for hydrogen isotope separation by cryogenic distillation involves very particular characteristics from both the modelling and the numerical points of view. Because tritium is radioactive, the distillations have to be simulated using a model of a reactive column. The presence of tritium traces, high purity products and repeated recycling necessitate the use of advanced numerical methods and convergence criteria that can ensure a very high degree of calculational accuracy. A reactive column model was integrated into the ProSim simulator for the conceptual design of this type of unit through parametric and structural optimization. The simulation results of the Laue-Langevin Institutes' tritium extraction plant are presented here.