A chain reaction can occur if enough fissionable material, in the proper geometry, is present. A sphere has the minimum possible surface area for a given mass, and hence minimizes the leakage of neutrons. This allows more neutrons to be available for the fission process.
To build a bomb, one must have a mass larger than the critical mass (supercritical mass) and must hold it together long enough for the desired energy release to occur. The critical mass for a given chain reaction depends on the isotope involved, the geometry of the device being used,
