As I understand it, the overestimation was (at least partly) about a failure to understand how cascades worked. In particular, to develop a proper model of a 3

As I understand it, the overestimation was (at least partly) about a failure to understand how cascades worked. In particular, to develop a proper model of a 3D cascade in a sphere. The result was a consistent underestimation of how quickly fission would advance, implying the need for a larger reacting body than is actually required to sustain a chain reaction.