Suppose three neutrons are released when an atom in a sample of fissionable nuclei undergoes fission. Each of these neutrons has enough energy to cause another fission reaction in another nucleus of the material. If the reaction is not controlled, how many atoms will have undergone fission after 2 more stages of the chain reaction?

13 nuclei

Explanation:

From the question, the fission of one nucleus produces three neutrons which causes  more nuclei to undergo fission.

This implies that, after the first fission, three neutrons cause three nuclei to undergo fission. The three nuclei that underwent fission produces nine neutrons which causes the fission of nine nuclei.

All together we the number of nuclei that underwent fission as;

1 + 3 + 9 = 13 nuclei.

they are all solids.