DESIGING THE BOMB CHAPTER 3

A separate and remarkable story lies behind the fact that the scheduled deliveries were made on time. The story being told here will limit itself primarily to what happened at Los Alamos.
The Los Alamos task was to discover means of making the desired explosive liberation of energy take place efficiently and at the right time. A verit¬able mountain of difficulties stood in the way.
No one knew how much fissionable material had to be put together to support an explosive chain reaction, but it was known that the reaction could not occur if the amount were insufficient. The burning of conventional explosives is a chain reac¬tion of a different kind; a tiny quantity of TNT burns as readily as a larger amount. Fission chains cannot occur in the saline way, because the neutrons on which they depend must remain within the fuel until they encounter fissionable nuclei. If the sur¬face area of the fuel mass is large compared to the volume (i.e., if the fuel mass is too small or too much flattened out), then the neutron escape area is too large compared to the neutron source volume, and too many neutrons will find their way out with¬out causing fission.