![]() In the case of a nuclear reactor, the number of fissionable nuclei available in each generation is carefully controlled to prevent a “runaway” chain reaction. If the number of fissions in one generation is equal to the number of neutrons in the preceding generation, the system is said to be critical if the number is greater than one, it is supercritical and if it is less than one, it is subcritical. The continuing process whereby neutrons emitted by fissioning nuclei induce fissions in other fissile or fissionable nuclei is called a fission chain reaction. Many heavy atomic nuclei are capable of fissioning, but only a fraction of these are fissile-that is, fissionable not only by fast (highly energetic) neutrons but also by slow neutrons. A portion of these neutrons are captured by nuclei that do not fission others escape the material without being captured and the remainder cause further fissions. In an atomic bomb or nuclear reactor, first a small number of neutrons are given enough energy to collide with some fissionable nuclei, which in turn produce additional free neutrons. As a rule of thumb, the complete fission of 1 kg (2.2 pounds) of uranium or plutonium produces about 17.5 kilotons of TNT-equivalent explosive energy. In addition to this formation of lighter atoms, on average between 2.5 and 3 free neutrons are emitted in the fission process, along with considerable energy. When bombarded by neutrons, certain isotopes of uranium and plutonium (and some other heavier elements) will split into atoms of lighter elements, a process known as nuclear fission. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.This Time in History In these videos, find out what happened this month (or any month!) in history. ![]() #WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find. ![]()
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