 But at Birmingham, Oliphant's team had reached a strikingly different conclusion. Oliphant had delegated the task to two German refugee scientists, Rudolf Peierls and Otto Frisch, who could not work on Oliphant's radar project because they were enemy aliens and therefore lacked the necessary security clearance. Francis Perrin had calculated the critical mass of uranium to be about 40 tonnes (39 long tons; 44 short tons). He reckoned that if a Neutron reflector were placed around it, this might be reduced to 12 tonnes (12 long tons; 13 short tons). Peierls attempted to simplify the problem by using the fast neutrons produced by fission, thus omitting consideration of moderator. He too calculated the critical mass of a sphere of uranium in a theoretical paper written in 1939 to be "of the order of tons".
