Explain why a large spherical mass of uranium maybe able to sustain chain reactor while the same mass,couldn't

Answer 1

If there were things around the U that absorbed the neutrons emitted during the fission a chain reaction would not be sustained. For instance C and H2O can both absorb stray neutrons. Heavy water, D2O, was used in early U reactors until enriched U was perfected. The D2O did not absorb as many neutrons so the chain reaction could proceed.

Answer 2

I think you left out half of the question. I'll guess that the remainder was "...the same mass couldn't sustain a chain reaction when shaped as a flat sheet" (or something nonspherical). The answer to that is that the mean path length for a neutron emitted by the decay of an atom before it exits the uranium is maximum for a sphere. A chain reaction depends on emitted neutrons from the fission of atoms being reabsorbed to trigger other atoms' fission. The probability of this is greatest when the mean path is longest.