A nuclear reaction is a process that changes the identity or characteristics of an atomic nucleus, releasing an immense amount of energy. Unlike chemical reactions, which rearrange electrons, nuclear reactions involve changes to the dense core of the atom where protons and neutrons reside. The reaction is a microscopic event, often visualized using physics models. This process harnesses the powerful forces that bind the nucleus together, resulting in energy yields millions of times greater than conventional energy sources.
Defining Fission and Fusion
The two primary types of nuclear reactions that release energy are fission and fusion, which are opposite processes. Nuclear fission involves the splitting of a heavy, unstable nucleus, such as Uranium-235, into two smaller, lighter nuclei. This process releases energy and additional particles. Fission is the reaction currently used in all commercial nuclear power plants to generate electricity.
Nuclear fusion is the process of combining two light atomic nuclei to form a single, heavier one. The fusion of hydrogen isotopes into helium is the energy source of the sun and stars, releasing a massive amount of energy. To achieve fusion, light nuclei must be forced together under extreme temperature and pressure to overcome their natural electrostatic repulsion. Although fusion yields more energy than fission, these necessary conditions are extremely difficult to replicate and sustain on Earth.
Visualizing the Atomic Mechanism
Fission, the most common nuclear reaction in current use, is visualized as a subatomic collision and breakup. The process begins when a free neutron strikes a heavy nucleus, such as Uranium-235. Upon absorbing the neutron, the nucleus becomes highly unstable and begins to vibrate, transforming into an energized isotope. This unstable nucleus then splits into two smaller, fast-moving fragments, which are the nuclei of new elements.
This splitting releases a tremendous amount of kinetic energy and two or three new high-speed neutrons. These freed neutrons can strike other nearby Uranium-235 nuclei, causing them to split and creating a self-sustaining nuclear chain reaction. In a nuclear reactor, this chain reaction is carefully controlled to maintain a steady heat output. If uncontrolled, the reaction multiplies exponentially in a fraction of a second.
Modeling fusion reactions involves visualizing plasma, a superheated gas. In plasma, nuclei move so fast that they overcome their mutual repulsion and are forced to merge.
Observable Macroscopic Effects
Since the atomic mechanisms are invisible, the observable effects of a nuclear reaction are seen in the large-scale energy output and the physical environment surrounding the process. The most immediate result is the immense release of thermal energy, which manifests as intense heat. In power generation facilities, this heat is used to boil water, creating steam that spins turbines to produce electricity. Consequently, what is visible at a nuclear power plant are primarily the large cooling towers releasing plumes of steam or water vapor.
The massive energy release in fusion is most visibly seen in stars, where the reaction produces intense light and heat. Another tangible effect of both fission and fusion is the creation of new elements, or the transmutation of matter, as the original nuclei are fundamentally changed. A measurable byproduct of nuclear reactions is the release of radiation, such as gamma rays and other particles. This requires the reactions to be housed in extremely robust, shielded containment structures.