Three-group diffusion theory has been applied to a computation of the 231Pa mass required to sustain a fast-neutron chain reaction. The method was tested by preliminary calculation of several 235U-238U systems and of a single 239Pu-240Pu system. The best agreement between predictions and measurements was found for high-enrichment 235U-238U systems. In the case of 231Pa, where many of the necessary data do not exist, use was made of the characteristics of 237Np as a substitute. The predicted critical radius for 231Pa is 22.67 ± 1.81 cm, and the corresponding critical mass is 750 ± 180 kg.