Possible Criticality of Protactinium-231

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.