Every atom has a nucleus, a dense cluster of protons (positive charge) and neutrons (no charge), held together by the nuclear force. The process commonly described as “splitting an atom” is nuclear fission, a reaction that targets the nuclei of heavy, unstable elements. This process applies primarily to isotopes like Uranium-235, whose massive size makes them inherently less stable.
Defining Nuclear Fission
Nuclear fission begins when a neutron impacts the nucleus of a heavy, unstable isotope. For a fissile material like Uranium-235, absorbing a low-energy neutron transforms the nucleus into the highly unstable compound nucleus, Uranium-236. The added neutron supplies enough internal energy to overcome the nuclear force. The nucleus then oscillates violently, distorting its shape from a sphere into an elongated form.
The powerful electrostatic repulsion between the numerous positively charged protons forces the elongated structure to divide. The nucleus snaps apart, typically splitting into two smaller, unequal fragments, such as Barium and Krypton nuclei. The resulting fragments are propelled away from each other at extremely high speeds. The initial input of a single neutron triggers the entire division process.
Immediate Products of the Split
A single fission event releases a large amount of energy because the total mass of the products is slightly less than the mass of the starting atom and neutron. This missing mass is converted directly into energy, following mass-energy equivalence. About 80 to 85 percent of the released energy appears as the kinetic energy of the two fission fragments. These fragments rapidly collide with surrounding atoms, transferring kinetic energy and manifesting as intense heat.
The remaining energy is released as radiation and other particles. This includes prompt gamma rays, which are high-energy photons emitted instantaneously from the splitting fragments. Additionally, two or three new neutrons are ejected from the fragments at high velocity. The two new, smaller nuclei, known as fission products, are unstable and highly radioactive, continuing to decay over time and releasing further radiation like beta particles and gamma rays. A single fission event releases approximately 200 million electron volts of energy.
Initiating a Chain Reaction
The neutrons released during the initial split are the mechanism by which the process becomes self-sustaining. These ejected neutrons can collide with other nearby fissile nuclei, causing them to undergo fission. If enough subsequent fissions occur, they release more neutrons, propagating a continuous and rapidly multiplying sequence called a nuclear chain reaction. This propagation is measured by the neutron multiplication factor, known as k.
For a chain reaction to be sustained, the system must achieve criticality, where k equals one. This means that for every fission event, exactly one resultant neutron causes another fission. The minimum amount of fissile material required to reach this steady-state reaction is called the critical mass. If the mass is subcritical, the reaction will quickly die out because too many neutrons escape without causing a new split.
Controlled Versus Uncontrolled Fission
The distinction between the practical applications of nuclear fission lies in managing the chain reaction rate. Controlled fission is the process used in nuclear power reactors to generate a stable, continuous output of energy. In a reactor, the multiplication factor is maintained at k equals one, achieving a steady-state reaction. This stability is accomplished using control rods, typically made of materials like cadmium or boron, which strongly absorb excess neutrons.
By inserting or withdrawing the control rods, operators regulate the number of neutrons available to cause further fission, controlling the reaction rate. Uncontrolled fission is the principle behind nuclear weapons, aiming for an instantaneous, explosive energy release. Fissile material is rapidly forced into a state of supercriticality, where the multiplication factor is significantly greater than one. The resulting exponential increase in the fission rate occurs in a fraction of a second, leading to a massive energy discharge.