nuclear fission

nuclear fission is a fundamental nuclear reaction in which the nucleus of a heavy atom splits into two or more lighter nuclei, releasing a significant amount of energy. The process, often initiated by neutron absorption, is the principle behind nuclear power generation and nuclear weapons. It was first achieved artificially in 1938 by Otto Hahn and Fritz Strassmann, with the theoretical explanation provided by Lise Meitner and Otto Robert Frisch. The discovery directly led to major historical developments including the Manhattan Project and the subsequent Atomic bombings of Hiroshima and Nagasaki.

History

The scientific journey began with Enrico Fermi's experiments bombarding uranium with neutrons in 1934, which produced confusing results later understood to be fission products. The definitive discovery occurred at the Kaiser Wilhelm Institute for Chemistry in Berlin, where Hahn and Strassmann identified barium after neutron irradiation of uranium. Meitner, then in exile in Sweden, and Frisch correctly interpreted this as nuclear splitting, coining the term. This breakthrough was rapidly communicated within the global physics community, notably to Niels Bohr who announced it at a conference in Washington, D.C.. Wartime efforts culminated in the establishment of the Manhattan Project, which constructed production facilities at Oak Ridge National Laboratory and Hanford Site and developed the first weapons at Los Alamos National Laboratory. Post-war developments were shaped by figures like Hyman G. Rickover in naval propulsion and Dmitriy Ustinov in the Soviet atomic bomb project, leading to the expansion of civilian power programs.

Physical overview

The process typically occurs in heavy isotopes like uranium-235 or plutonium-239 when they absorb a neutron, becoming unstable and deforming into an elongated shape before splitting. This split releases kinetic energy in the fission fragments, additional free neutrons, and gamma rays. The emitted neutrons can induce further fission in a chain reaction, a principle critical for sustained energy release. The total energy released vastly exceeds that from chemical reactions because it derives from the conversion of mass according to Albert Einstein's mass–energy equivalence formula. Key measurements of the process include the fission cross-section and the number of neutrons emitted per fission, which vary between isotopes. The fission products themselves are often radioactive, undergoing subsequent beta decay.

Fission reactors

These devices are designed to maintain a controlled, self-sustaining chain reaction. Most commercial power reactors, such as pressurized water reactors and boiling water reactors, use low-enriched uranium dioxide fuel moderated by light water. Other designs include the CANDU reactor, which uses heavy water as a moderator, and advanced concepts like the sodium-cooled fast reactor. Critical components include fuel assemblies, control rods made of materials like boron or cadmium to absorb neutrons, and a coolant to transfer heat. The generated heat produces steam to drive turbines connected to electrical generators. Major reactor vendors and designers include Westinghouse Electric Company, Framatome, and Rosatom. Research reactors, such as those at the Institut Laue–Langevin, are used for scientific experiments rather than power production.

Applications

The primary non-military application is the generation of electricity in nuclear power plants, which supply a major portion of the baseload electricity in countries like France and the United States. Fission also provides propulsion for naval vessels, most notably in United States Navy aircraft carriers and submarines like the USS Nautilus (SSN-571). Radioisotopes produced as fission products or through neutron activation have widespread uses in medicine for diagnostics and treatments, in industry for radiography, and in scientific research as tracers. The process is also the energy source for proposed nuclear thermal rockets for space exploration. Historically, the most consequential application was in nuclear weapons, such as the Little Boy device dropped on Hiroshima.

Safety and environmental impact

Reactor safety is governed by principles including multiple physical barriers, defense in depth, and stringent regulations from bodies like the Nuclear Regulatory Commission and the International Atomic Energy Agency. Major accidents at Chernobyl, Three Mile Island, and Fukushima have profoundly influenced safety standards and public perception. A primary long-term challenge is the management of high-level radioactive waste, such as spent nuclear fuel, with proposed solutions including deep geological repositories like Yucca Mountain or Onkalo. The fission process itself does not emit greenhouse gases, offering a low-carbon energy alternative, but the full fuel cycle involves environmental considerations from uranium mining, exemplified by operations at Ranger Uranium Mine, to eventual decommissioning of facilities like Sellafield.

Category:Nuclear physics Category:Nuclear technology Category:Energy