One atom splits, three neutrons escape—each finds another, and suddenly you're producing millions

In an uncontrolled chain reaction, every neutron released has the potential to trigger another fission, causing the number of reactions to double with each generation. This exponential growth is what makes nuclear weapons so destructive—within microseconds, trillions of nuclei have split, releasing energy equivalent to thousands of tons of TNT. The critical mass is the minimum amount of fissile material needed for this self-sustaining reaction to occur; below it, too many neutrons escape without causing fissions. In practice, weapons use sophisticated compression techniques to achieve supercriticality, while reactor cores are kept deliberately subcritical in their base state.