The process known as “splitting an atom” is formally called nuclear fission. This reaction taps into the powerful forces holding atomic nuclei together by deliberately breaking a heavy nucleus into two or more smaller fragments. Fission releases a tremendous amount of energy, far exceeding the energy liberated by conventional chemical reactions like burning coal or oil. Chemical reactions rearrange atoms by dealing with orbiting electrons, but nuclear fission fundamentally changes the structure of the atom itself by altering the nucleus. This change unlocks a dense store of energy, making fission a potent source of heat for power generation.
The Core Process of Fission
The mechanism of nuclear fission begins with a heavy, somewhat unstable atomic nucleus, such as Uranium-235 (U-235). This target nucleus is split by introducing a projectile particle: a single neutron. When the slow-moving, or “thermal,” neutron approaches the U-235 nucleus, it is absorbed into the nuclear structure.
The absorption of this extra neutron transforms the U-235 into a highly unstable, short-lived compound nucleus, Uranium-236. This momentary addition of mass and energy causes the nucleus to become highly excited and deformed, similar to a wobbly drop of liquid. The intense repulsive forces between the many positively charged protons overcome the strong nuclear force that usually binds the nucleus together.
Within an incredibly short span of time, the unstable U-236 nucleus splits apart, or fissions, into two smaller, roughly equal-sized atomic nuclei, which are called fission fragments. These fragments are typically atoms found around the middle of the periodic table, such as Barium and Krypton. The fission fragments fly apart at high speed, possessing a large amount of kinetic energy, which is the immediate source of the heat generated by the process.
Along with the fission fragments, two or three additional, high-energy neutrons are instantly ejected from the nucleus. The process also emits gamma rays, a form of high-energy electromagnetic radiation. On average, a single fission event releases approximately 200 million electron-volts of energy, primarily in the form of the kinetic energy of the fragments.
Achieving a Self-Sustaining Reaction
The neutrons released during fission are the crucial element that allows the process to become a continuous, self-perpetuating phenomenon known as a chain reaction. For the reaction to sustain itself, at least one of the newly released neutrons must strike and induce fission in another adjacent fissile nucleus. If the number of neutrons produced is greater than the number lost, the reaction will accelerate exponentially.
The key to achieving a stable, continuous reaction is maintaining “criticality,” where the neutron multiplication factor, ‘k’, is exactly equal to one. If ‘k’ is less than one, the reaction is “subcritical” and will quickly die out. If ‘k’ is greater than one, the reaction becomes “supercritical,” leading to an uncontrolled release of energy.
The physical parameter that determines whether a sustained chain reaction is possible is the “critical mass.” This is the minimum amount of fissile material required to ensure that enough neutrons are captured to keep the reaction going rather than escaping into the surroundings.
The necessary critical mass depends on factors like the density, shape, and purity of the material, and the presence of any surrounding neutron-reflecting material. A larger surface area allows more neutrons to escape, meaning a spherical shape, which has the lowest surface area-to-volume ratio, requires the smallest critical mass. In a controlled nuclear environment, the material is kept at or near the critical mass threshold, allowing for a steady, manageable energy output.
Harnessing the Splitting Process
To utilize the enormous energy from the controlled chain reaction, specialized technology is required, primarily a nuclear reactor. The reactor core is where sustained fission takes place, managing the reaction rate and transferring the resulting heat. The physical fuel is typically uranium, processed into small ceramic pellets and sealed inside metal tubes called fuel rods.
Inside the reactor, two types of components control the neutrons released during fission: the moderator and the control rods.
The Moderator
The moderator is often light water, heavy water, or graphite. Neutrons are released at high speeds, but the U-235 nucleus is much more likely to absorb a slow-moving neutron to induce fission. The moderator slows these fast neutrons down, increasing the probability of continued fission and sustaining the chain reaction.
Control Rods
Control rods are made from materials like boron or cadmium, which absorb neutrons without undergoing fission. The operator can insert or withdraw these rods into the core to adjust the reaction rate precisely. Inserting the rods absorbs more neutrons, slowing the fission rate and reducing the power output, while withdrawing them has the opposite effect.
The heat generated by the kinetic energy of the fission fragments is continuously removed from the reactor core by a circulating coolant, typically water. This heated fluid is then used to create steam, which drives a turbine connected to an electrical generator.
Fission Byproducts and Energy Output
The two smaller nuclei created from the split are the fission fragments, which are inherently unstable and radioactive, constituting the primary nuclear byproduct. These fragments undergo subsequent radioactive decay, which continues to release heat even after the chain reaction has been intentionally stopped.
The vast energy release from fission is explained by Albert Einstein’s mass-energy equivalence principle, E=mc². When the heavy nucleus splits, the combined mass of the resulting fission fragments and released neutrons is slightly less than the mass of the original target nucleus and the incident neutron. This small difference in mass, known as the mass defect, is converted directly into the immense energy released during the reaction.
This nuclear process yields an energy density that is millions of times greater than that of chemical combustion. For instance, the fission of one kilogram of Uranium-235 produces approximately 2.5 million times the energy produced by burning one kilogram of coal.