A solid sphere of of mass can sustain a fission chain reaction

PHYSICS · Subjective · A Level · Physics · Nuclear physics · Nuclear physics

Problem

A solid sphere of

^{235}_{92}\text{U}

of mass

5\,\text{kg}

can sustain a fission chain reaction, whereas two separate pieces of the same material, each of mass

2.5\,\text{kg}

and placed far apart, cannot. Explain why this happens.