Longman Dictionary of Contemporary English
The Collaborative International Dictionary
nuclear fission \nuclear fission\ n. A nuclear reaction in which a heavy atomic nucleus splits into smaller nuclei with the simultaneous release of energy.
n. (context physics English) a nuclear reaction in which a large nucleus splits into smaller ones with the simultaneous release of energy
n. a nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy [syn: fission]
In nuclear physics and nuclear chemistry, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.
Nuclear fission of heavy elements was discovered on December 17, 1938 by German Otto Hahn and his assistant Fritz Strassmann, and explained theoretically in January 1939 by Lise Meitner and her nephew Otto Robert Frisch. Frisch named the process by analogy with biological fission of living cells. It is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments ( heating the bulk material where fission takes place). In order for fission to produce energy, the total binding energy of the resulting elements must be less negative (higher energy) than that of the starting element.
Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes. Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus.
Apart from fission induced by a neutron, harnessed and exploited by humans, a natural form of spontaneous radioactive decay (not requiring a neutron) is also referred to as fission, and occurs especially in very high-mass-number isotopes. Spontaneous fission was discovered in 1940 by Flyorov, Petrzhak and Kurchatov in Moscow, when they decided to confirm that, without bombardment by neutrons, the fission rate of uranium was indeed negligible, as predicted by Niels Bohr; it wasn't.
The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum-tunnelling processes such as proton emission, alpha decay, and cluster decay, which give the same products each time. Nuclear fission produces energy for nuclear power and drives the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes possible a self-sustaining nuclear chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon.
The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very dense source of energy. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over the destructive potential of nuclear weapons may counterbalance the desirable qualities of fission as an energy source, and give rise to ongoing political debate over nuclear power.
Usage examples of "nuclear fission".
No useful release of the energy locked up inside the atomic nucleus would ever be realized if every nuclear fission inside the uranium fuel required a neutron fired in from the outside to trigger it.
Everyone in the world knew about nuclear fission - it was no secret, what real secrets are there in physics, eh?
Perhaps, with their greater mathematical ability and closer comprehension of what actually went on inside the nuclear fission chamber, they had some vivid glimpse of the mind-shattering horror locked up beyond that shield.
Perhaps, with their greater mathematical ability and closer comprehension of what actually went on inside the nuclear fission chamber, they had some vivid glimpse of the mind - shattering horror locked up beyond that shield.
Analyses have shown that a nuclear fission electric propulsion spacecraft of this type could carry thousands of kilograms of payload to the Pluto-Charon system and maybe even the Kuiper Belt with trip times of less than twenty years.
They had just about exhausted their easily worked areas of fossil fuels and relied heavily on nuclear fission power to provide motive transport.
Even the suborbital rockets that they used instead of most aircraft were powered by nuclear fission.
Although such renowned names as America's Robert Millikan and Britain's Sir James Jeans were still ascribing them to some kind of nuclear fission or fusion, Alfvé.
The high whine of both E-motors in the front wheels faded into quiet as beneath the heavy radiation shield, the reactor lowered the rate of nuclear fission to a minimum.
The money runs out on building a nuclear fission plant, but the power has to come from somewhere, so they burn coal.