A drawing of the first nuclear reactor constructed at the University of Chicago, where a group of scientists succeeded in initiating and maintaining the first controlled, self-sustaining nuclear fission chain reaction.

The first controlled, self-sustaining nuclear fission chain reaction was initiated on 2 December 1942 at 15:25 beneath Stagg Field at the University of Chicago. Fermi and his collaborators had built a graphite-moderated nuclear reactor known as Chicago Pile-1. It was a pile about four metres high and 7.5 metres wide, constructed of graphite blocks containing natural uranium metal and uranium oxide. The reactor core was shaped approximately as a flattened ellipsoid. The original intention was to construct a spherical reactor core, but according to Fermi’s calculations, a smaller structure was sufficient. The pile contained about 350 tonnes of graphite, 36 tonnes of uranium oxide and 5.6 tonnes of uranium metal. Its construction cost about one million US dollars.

The reactor power was controlled manually by inserting or withdrawing cadmium control rods, which absorbed neutrons. Several independent emergency measures were available, including safety rods that could be rapidly inserted into the pile. As a last resort, a team nicknamed the “suicide squad” stood ready to pour a cadmium salt solution over the reactor to stop the chain reaction. However, there was no need to intervene: the reactor functioned as expected.

The historic experiment was witnessed by several dozen scientists using relatively simple measuring instruments. At 15:25 the reactor became self-sustaining and remained critical for about half an hour before Fermi ordered it to be shut down. Its power during the first critical operation was only about 0.5 W, although it was later operated at substantially higher power.

The first usable electricity generated from nuclear energy was produced on 20 December 1951, when the EBR-I reactor supplied electricity to four light bulbs.

The first nuclear reactor to generate usable electricity was Experimental Breeder Reactor I (EBR-I) in Idaho. On 20 December 1951, electricity generated from nuclear fission powered four 200-watt light bulbs. The following day, the reactor produced enough electricity to power the entire building. The primary goal of its creators was not electricity generation, but to demonstrate whether a nuclear reactor could produce more fissile fuel than it consumed. EBR-I was a fast breeder reactor, in which fission is sustained by fast, unmoderated neutrons and plutonium-239 can be produced from uranium-238. This plutonium can then be used as nuclear fuel. The world’s first nuclear power plant to supply electricity to a public grid began operation on 27 June 1954 near Obninsk in the Soviet Union. Its AM-1 reactor had an electrical output of 5 MW and a thermal output of about 30 MW. It was a graphite-moderated, water-cooled channel-type reactor and represented an important step in the development of Soviet nuclear power technology.