When 10 Commandos Stopped Hitler

https://thumbs.dreamstime.com/z/linear-hydro-electric-station-hydroelectric-power-plant-vector-illustration-engineering-waterpower-creative-76433121.jpg

Nuclear fission is the reaction that causes radioactive material to split into two or more lighter nuclei when it is bombarded with neutrons. The reaction produces a tremendous amount of energy, and unless a moderator is present, it can lead to a catastrophic situation.

Graphite and heavy water are the preferred moderators.