Nuclear Fission
Nuclear fission is the process in which a heavy atomic nucleus — typically uranium-235 or plutonium-239 in practical applications — splits into two or more lighter nuclei (fission fragments), releasing a large amount of energy and typically 2–3 free neutrons, which can go on to trigger further fission events in a self-sustaining chain reaction.
Curious
Recall that binding energy per nucleon peaks around iron and generally decreases for heavier elements. This means very heavy nuclei, like uranium, are less tightly bound per nucleon than medium-mass nuclei — so if a heavy nucleus can be persuaded to split into two medium-mass fragments, the resulting fragments are more tightly bound overall, and the difference in binding energy is released as energy, following E = mc².
In practice, fission is typically triggered by a slow ("thermal") neutron striking a fissile nucleus like uranium-235, causing it to briefly deform and then split apart into two unequal fragments, releasing energy, gamma radiation, and — crucially — several additional free neutrons, which can go on to trigger further fission events in other nearby fissile nuclei.
Exploring
| Concept | Meaning |
|---|---|
| Critical mass | Minimum amount of fissile material needed to sustain a chain reaction (on average, one neutron per fission triggers exactly one more) |
| Moderator | Material (e.g. water, graphite) that slows fast neutrons to the lower speeds most effective at triggering further fission |
| Control rods | Neutron-absorbing material used to regulate reaction rate in a reactor |
Deep Dive
Nuclear fission was discovered in December 1938 by Otto Hahn and Fritz Strassmann, who found that bombarding uranium with neutrons unexpectedly produced barium — a much lighter element — rather than the heavier transuranic elements they had originally been hoping to find. The theoretical explanation — that the uranium nucleus had actually split into two roughly equal, lighter fragments — was worked out within weeks by Lise Meitner and her nephew Otto Frisch, who were in exile from Nazi Germany at the time and coined the term "fission," borrowing the biological term for cell division. Hahn alone received the 1944 Nobel Prize in Chemistry for the discovery, a decision now widely regarded by historians of science as having unjustly overlooked Meitner's essential theoretical contribution.
The liquid drop model and fission: The liquid drop model of the nucleus (also used to explain binding energy trends generally) provides an intuitive picture of why fission occurs: a heavy nucleus, modelled as a charged liquid drop, can be induced to oscillate and elongate when struck by a neutron; if the resulting deformation is severe enough, the electrostatic repulsion between the two halves of the elongating drop overcomes the short-range strong force trying to hold it together, and the nucleus splits — releasing the binding energy difference as kinetic energy of the fragments plus additional neutrons and gamma radiation.
The discovery of fission, combined with the discovery that each fission event releases multiple additional neutrons capable of triggering further fissions, immediately suggested the possibility of a self-sustaining chain reaction — a realisation that directly led, within just a few years and against the backdrop of the Second World War, to the Manhattan Project and the first controlled nuclear chain reaction (Chicago Pile-1, led by Enrico Fermi, December 1942).
Concept Relationship Map
Requires: nuclear binding energy
Enables: nuclear reactor design, Q-value [PENDING FULL ENTRY]
Commonly confused with: nuclear fusion (fission splits heavy nuclei; fusion combines light nuclei — opposite processes, same underlying energy source)
Sources
- Hahn, O., Strassmann, F. (1939). Naturwissenschaften, 27(1), 11–15.
- Meitner, L., Frisch, O.R. (1939). "Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction." Nature, 143(3615), 239–240.
- Krane, K.S. (1987). Introductory Nuclear Physics. Wiley, Ch. 13.