CHERNOBYL

The Chain Reaction

Nuclear power is based on a chain reaction where atomic nuclei decay and release neutrons, which in turn cause new decays. To maintain a stable state, each decay must produce exactly one new one. Fewer and the reaction stops; more and the process runs away. Control is crucial—a lump of uranium isn't enough. The neutrons released during decay travel far too fast to cause enough new decays. Something must slow the neutrons down: the moderator. Graphite or water are common moderators.