I 1938 fandt et par tyske forskere ud af, at hvis man bombarderer uranatomer med neutroner, kan deres kerne skilles ad (deles/splittes), og neutroner derfra kan så bombardere de næste atomer etc. Derved kan man skabe en selvopholdende kædereaktion (fission).
“At 8:16 a.m. on August 6, 1945, a fission weapon containing sixty-four kilograms of uranium detonated 580 meters abovce the Japanese city of Hiroshima and Einstein’s equation [E=mc2] proved mercilessly accurate. The bomb itself was extremely inefficient: just one kilogram of the uranium underwent fission, and only seven hundred milligrams of mass – the weight of a butterfly – was converted into energy. But it was enough to obliterate an entire city in a fraction of a second.”‘Midnight in Chernobyl’ af Adam Higginbotham, s. 25-26