Answer
Naturally, a small scale chain reaction took place. Copper reflected neutrons from splitting $U^{285}$ atoms back into Uranium sphere to trigger a chain reaction. Note that a sphere has the most appropriate geometry for chain reactions. In fact, during the implosion process prior to an atomic explosion, fragments of $U^{235}$ are pressed roughly into a sphere for the chain reaction to occur.
Work Step by Step
Naturally, a small scale chain reaction took place. Copper reflected neutrons from splitting $U^{285}$ atoms back into Uranium sphere to trigger a chain reaction. Note that a sphere has the most appropriate geometry for chain reactions. In fact, during the implosion process prior to an atomic explosion, fragments of $U^{235}$ are pressed roughly into a sphere for the chain reaction to occur.
