nuclear chain reaction
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| nuclear chain reaction | |
|---|---|
| Name | Nuclear chain reaction |
| Caption | The first artificial Chicago Pile-1 achieved criticality in 1942. |
| Field | Nuclear physics |
nuclear chain reaction. A nuclear chain reaction is a process where one nuclear fission event triggers subsequent fission events in a self-sustaining sequence. This phenomenon, central to both nuclear reactor operation and nuclear weapon design, relies on the release of neutrons from splitting heavy atomic nuclei like uranium-235 or plutonium-239. The ability to initiate and control such reactions was a pivotal achievement of the Manhattan Project, leading to profound technological and geopolitical consequences throughout the Cold War.
Overview
The process fundamentally involves fissile material, moderator materials like graphite or heavy water, and precise conditions to manage neutron population. Key historical demonstrations include the operation of Chicago Pile-1 under Enrico Fermi and the detonation of the Trinity (nuclear test) device. Subsequent developments were spearheaded by institutions like the Atomic Energy Commission and organizations such as Los Alamos National Laboratory. The science underpinning these reactions draws heavily from earlier work by physicists including Otto Hahn, Lise Meitner, and Niels Bohr.
Physics and mechanism
Fission occurs when a nucleus, such as uranium-235, absorbs a neutron, becomes unstable, and splits into lighter fission products while releasing additional neutrons and a significant amount of energy described by Einstein's mass–energy equivalence. These newly released neutrons, if not lost, can induce further fission events in nearby fissile atoms. The cross section (physics) for neutron absorption is a critical parameter, influenced by neutron energy and the surrounding medium. Materials like cadmium or boron have high absorption cross sections and are used as control rods. The neutron moderator slows down fast neutrons to thermal neutron energies, increasing the probability of fission in fuels like uranium-235.
Criticality and control
The state of a system is defined by its neutron multiplication factor. A critical mass is the minimum amount of fissile material needed to maintain a self-sustaining chain reaction, a value dependent on geometry, enrichment, and presence of reflectors like beryllium. Supercriticality leads to an exponentially increasing reaction rate, essential for nuclear weapon yields. In reactors, precise control is achieved by inserting or withdrawing control rods and using neutron poisons to manage reactivity. Incidents involving unintended criticality, such as the SL-1 accident or the Tokaimura nuclear accident, highlight the hazards of miscalculation. Computational models from Monte Carlo method simulations to deterministic codes are used for criticality safety analyses.
Applications
The primary peaceful application is in nuclear power generation, with reactor designs ranging from Pressurized water reactor systems like those at Three Mile Island to CANDU reactors using heavy water. Naval propulsion for vessels like the USS Nautilus (SSN-571) also utilizes controlled chain reactions. In contrast, uncontrolled reactions form the basis for nuclear weapons, such as the Little Boy device dropped on Hiroshima and the Fat Man implosion device used on Nagasaki. Research reactors, including the B Reactor at the Hanford Site, have been used for isotope production and scientific experiments.
Safety and hazards
Uncontrolled chain reactions pose severe risks of acute radiation syndrome and widespread contamination. Major accidents like the Chernobyl disaster and the Fukushima Daiichi nuclear disaster involved catastrophic failures of control systems. The International Atomic Energy Agency establishes safety standards, while national bodies like the Nuclear Regulatory Commission oversee reactor operations. Criticality accidents in facilities such as the Mayak plant have resulted in fatalities. Long-term hazards include the management of spent nuclear fuel and plutonium from dismantled weapons under treaties like START I.
History and development
The theoretical possibility was identified following the discovery of nuclear fission by Otto Hahn and Fritz Strassmann in 1938, with interpretation provided by Lise Meitner and Otto Robert Frisch. The first artificial, controlled chain reaction was achieved by a team led by Enrico Fermi at the University of Chicago in 1942. This success directly fueled the Manhattan Project, culminating in the Trinity (nuclear test) and the bombings of Hiroshima and Nagasaki. Post-war development was marked by the Atoms for Peace initiative, the construction of the Shippingport Atomic Power Station, and the proliferation of nuclear technology during the Cold War, involving figures like J. Robert Oppenheimer and Andrei Sakharov.