Nuclear Fission
Nuclear fission is the splitting of a heavy atomic nucleus into two or more lighter nuclei, accompanied by the release of a large amount of energy, several neutrons, and gamma radiation. Discovered in 1938 by Otto Hahn and Fritz Strassmann (with theoretical explanation by Lise Meitner and Otto Frisch), fission fundamentally changed both energy production and geopolitics in the 20th century.
The Fission Process
When a heavy nucleus like uranium-235 absorbs a slow (thermal) neutron, it forms an excited compound nucleus (uranium-236) that is so unstable it rapidly deforms and splits. The nucleus elongates like a liquid drop, develops a neck, and then ruptures into two fission fragments of unequal mass, along with 2-3 free neutrons and roughly 200 MeV of energy per fission event.
The fission fragments are not unique -- uranium-235 can split in over 40 different ways. However, the fragment mass distribution is characteristically asymmetric, with peaks near mass numbers 95 and 140. Common fragment pairs include krypton-92 + barium-141 and strontium-94 + xenon-140. The fragments are highly neutron-rich and undergo multiple beta decays to reach stability, producing chains of radioactive daughter products.
Energy Release
The 200 MeV released per fission event may sound small, but on a per-mass basis it is extraordinary. Fissioning one kilogram of uranium-235 releases approximately 8.2 x 10^13 joules -- equivalent to burning about 2,700 metric tons of coal. This enormous energy density is what makes nuclear power practical.
The energy appears in several forms: roughly 80% as kinetic energy of the fission fragments (which quickly converts to heat), 5% as kinetic energy of the prompt neutrons, 5% as prompt gamma rays, and 10% as the energy of beta particles, neutrinos, and delayed gamma rays from fission product decay.
Chain Reactions and Critical Mass
The 2-3 neutrons released per fission event can each trigger additional fissions, creating a chain reaction. The behavior of this chain reaction depends on the neutron multiplication factor (k):
- k < 1 (subcritical): The reaction dies out. Each generation produces fewer fissions than the last.
- k = 1 (critical): The reaction sustains itself at a constant rate. This is the operating condition for nuclear power reactors.
- k > 1 (supercritical): The reaction accelerates exponentially. This is the condition exploited in nuclear weapons.
The minimum amount of fissile material needed to sustain a chain reaction is called the critical mass. For a bare sphere of weapons-grade uranium-235 (93% enrichment), the critical mass is about 52 kg. For plutonium-239, it is only about 10 kg. Surrounding the material with a neutron reflector (such as beryllium) significantly reduces the critical mass.
Fissile and Fissionable Materials
Not all heavy nuclei undergo fission equally well:
- Fissile isotopes (U-235, Pu-239, U-233) can be fissioned by neutrons of any energy, including slow thermal neutrons. These are the primary fuels for reactors and weapons.
- Fissionable isotopes (U-238, Th-232) require fast, high-energy neutrons to fission. U-238 constitutes 99.3% of natural uranium but is not fissile with thermal neutrons. However, U-238 absorbs neutrons to eventually produce Pu-239, which is fissile.
Nuclear Reactors: Controlled Fission
A nuclear power reactor maintains a controlled chain reaction at k = 1. Key components include:
- Fuel: Enriched uranium (typically 3-5% U-235) in the form of ceramic UO2 pellets stacked in metal fuel rods.
- Moderator: A material (water, heavy water, or graphite) that slows neutrons to thermal energies, where U-235's fission cross-section is highest.
- Control rods: Neutron-absorbing materials (boron, cadmium, hafnium) inserted into the reactor core to fine-tune the reaction rate.
- Coolant: A fluid (water, gas, or liquid metal) that carries heat from the core to steam generators.
Delayed Neutrons and Reactor Control
Reactor control would be nearly impossible if all fission neutrons were released instantaneously. Fortunately, about 0.65% of neutrons from U-235 fission are delayed neutrons, released seconds to minutes after fission from the decay of certain fission products. These delayed neutrons slow the reactor's response time enough to allow mechanical control systems to maintain k = 1. Reactors are designed to be critical only when delayed neutrons are included (delayed critical), never on prompt neutrons alone.