Adapted from an image by Department of Energy-Office of Public Affairs, restored by Yann (public domain), via Wikimedia Commons.
Enrico Fermi
Physicist who led the team that built the first nuclear reactor, and won the 1938 Nobel Prize in Physics.
Elements
Under the stands of a Chicago football field, Enrico Fermi's team ran the first self-sustaining chain reaction, in a pile of graphite blocks and uranium.
Fermium was found in 1953 in the debris of the first hydrogen bomb test, where uranium had captured many neutrons at once. Kept secret until 1955, it was named after Enrico Fermi.
Fermi's reactor kept uranium fission going as a chain reaction. Start one yourself.
Splitting the atom
A uranium-235 nucleus that catches a slow neutron can split in two, releasing energy and 2 or 3 new neutrons. If those neutrons split more nuclei, the process feeds itself: a chain reaction. It’s what powers nuclear power stations.
One split, step by step
1. A slow neutron approaches
A neutron, with no electric charge, drifts towards a uranium-235 nucleus: 92 protons (red) and 143 neutrons (grey).
Proton NeutronDrag to rotate
Where the energy comes from: E = mc²
Weigh the neutron and the uranium atom, then weigh everything that comes out. Masses are in atomic mass units (u), about the mass of one proton.
- Neutron1.008665 u
- Uranium-235235.043928 u
Every particle counted and weighed. Now split it and weigh the pieces.
Split every atom in 1 kg of uranium-235 this way and just 0.79 g of mass disappears, less than a paper clip. That’s about 71 trillion joules, as much as burning about 3,000 tonnes of coal.
The chain reaction
Fire a neutron into the fuel. Add control rods to soak up neutrons, or change how much of the fuel is uranium-235, and try to keep the reaction steady, like a reactor operator.
Fire a neutron to start.
- Splits
- 0
- Neutrons flying
- 0
- Uranium-235 left
- 0
A toy model: a real reactor holds billions of billions of atoms, and water or graphite slows the neutrons so uranium-235 can catch them. Natural uranium is only 0.7% uranium-235; power-station fuel is enriched to a few percent. Barium-141 and krypton-92 are one common pair of fragments among many. Atomic masses from the 2020 Atomic Mass Evaluation (IAEA). Averaged over all the ways it can split, and counting energy the fragments give off later, a fission releases about 200 MeV. Coal taken as 24 MJ/kg.
Keep exploring
Where the dates and stories on this page come from; each event on the timeline links to its own source.
- Enrico FermiEncyclopaedia Britannica
- CP-1 goes criticalUS Department of Energy
- FermiumRoyal Society of Chemistry
- Enrico FermiWikipedia