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What is nuclear fission? It is is a nuclear reaction that splits the nucleus of an atom into smaller, subatomic particles. It often produces free neutrons and photons. Fission of heavy elements can release large amounts of energy as electromagnetic and kinetic energy. For fission to produce energy, the total binding energy of the resulting element has to be lower than that of the original element. Fission is a form of transmutation because the resulting fragments are not the same element used.
Nuclear fission can occur without neutron bombardment(radioactive decay). This type of fission only occurs in a few heavy isotopes. In nuclear devices, all nuclear fission occurs from a neutron bombardment process that results from the collision of two subatomic particles. In nuclear reactions, a subatomic particle collides with an atomic nucleus and causes changes to it, so nuclear reactions are thus driven by the mechanics of bombardment. The isotopes that can sustain a fission chain reaction are called nuclear fuels and are said to be fissle. The most common nuclear fuels are 235uranium and 230plutonium. These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135.
Typical fission events release about two hundred million eV(electron volts) of energy for each fission event. Most chemical fuels only release a few eV. The energy of nuclear fission is released as kinetic energy, fragments, gamma rays, along with a huge amount of heat. Neutron bombs release a larger fraction of their energy as ionizing radiation, but these are all thermonuclear devices which rely on the fusion stage to produce the extra radiation.
Nuclear fission was discovered in 1938 after nearly five decades of work. Work by Ernest Rutherford, Henri Becquerel, Marie Curie, and Pierre Curie elaborated that the nucleus, though tightly bound, could undergo different forms or radioactive decay and could transmute into other elements. All known radioactive processes before fission changed mass of the atomic nucleus by no more than two protons. Einstein’s principle of mass-energy equivalence described the amount of energy released in such processes, but this could not be harnessed on a large scale. After James Chadwick discovered the neutron, Enrico Fermi studied the results of bombarding uranium with neutrons. Leo Szilard realized that fission could be used to create a nuclear chain reaction. If the number of secondary neutrons produced by each fissioning nucleus was greater than one, then each fission reaction could, in theory, trigger two more reactions. Such a system of exponential growth held out the possibility of using uranium fission as a means to generate large amounts of energy, either for civilian or military purposes. The atomic bomb was born.
Nuclear fission has brought us great strides in medicine and energy. It has also brought us the most destructive force that we know of. Hopefully, we have evolved enough to stop killing each other.
We’ve also recorded an entire episode of Astronomy Cast all about Nuclear Forces. Listen here, Episode 105: The Strong and Weak Nuclear Forces.