![]() Three types of moderators are used at the MIT reactor: (1) ordinary or "light" water that is also used to cool the reactor core, (2) deuterated or heavy water (D 20), and (3) high-purity graphite, both of which are excellent at slowing neutrons without absorbing them. When carefully controlled, a self-sustaining critical reaction of nuclear fission can generate power for a long timeuntil the nuclear fuel becomes depleted of fissionable atoms. In nuclear fission the nucleus of an atom breaks up into two lighter nuclei. The process is accompanied by the release of a large amount of energy. Since U-235 nuclei do not readily absorb the high energy neutrons that are emitted during fission, it is necessary to slow the neutrons down with a "moderator". nuclear fission, subdivision of a heavy atomic nucleus, such as that of uranium or plutonium, into two fragments of roughly equal mass. In the MIT reactor, one other group of components is essential to the maintaining and controlling a chain reaction. As fewer and fewer neutrons are absorbed, more and more neutrons are available to cause the splitting of uranium nuclei, until finally enough neutrons are available to sustain a chain reaction. To put the reactor into operation, the control blades are raised very slowly. When the control blades are fully inserted, they absorb so many neutrons from the uranium that there are not enough to allow a chain reaction to continue. Nuclear fission is a leading technology used for energy production in several countries, including, the UK, Japan, Russia, China, the USA, and Canada. Boron has the property of absorbing neutrons without re-emitting any. The rate of fissions in the uranium nuclei in the MIT reactor is controlled chiefly by six control blades of boron-stainless steel which are inserted vertically alongside the fuel elements. When it is in operation, the central active core contains a huge number of neutrons traveling in every direction at very high speeds. The MIT Research Reactor is used primarily for the production of neutrons. It has great potential as a source of power. Hence, the possibility exists for creating a chain reaction. When the nucleus of an atom splits into lighter nuclei through a nuclear reaction the process is termed as nuclear fission. When an atom splits into two parts, either through natural decay or when instigated within a lab, it releases energy. Each time a U-235 nucleus splits, it releases two or three neutrons. This process is known as fission (see diagram below). ![]() When a U-235 nucleus absorbs an extra neutron, it quickly breaks into two parts. The arrangement of particles within uranium-235 is somewhat unstable and the nucleus can disintegrate if it is excited by an outside source. In the nucleus of each atom of uranium-235 (U-235) are 92 protons and 143 neutrons, for a total of 235.
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