Question

what happens when the amount of fissionable material exceeds the critical mass?
the chain reaction accelerates as more neutrons are generated with each round of reaction.
the chain reaction is not sustained and eventually stops.
the density of the reacting particles decreases, slowing the reaction.
the amount of material needed to establish a new critical mass increases.

Explanation:

In a nuclear fission chain - reaction, critical mass is the minimum amount of fissile material required to sustain the reaction. When the amount of fissionable material exceeds the critical mass, more neutrons are available to cause further fissions, accelerating the chain - reaction.

Answer:

The chain reaction accelerates as more neutrons are generated with each round of reaction.