In the nucleus of each atom of uranium-235 (U-235) are 92 protons and 143 neutrons, for a total of 235. The arrangement of particles within uranium-235 is somewhat unstable and the nucleus can disintegrate if it is excited by an outside source. When a U-235 nucleus absorbs an extra neutron, it quickly breaks into two parts. This process is known as fission (see diagram below). Each time a U-235 nucleus splits, it releases two or three neutrons. Hence, the possibility exists for creating a chain reaction.
The MIT Research Reactor is used primarily for the production of neutrons. When it is in operation, the central active core contains a huge number of neutrons traveling in every direction at very high speeds.
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. Boron has the property of absorbing neutrons without re-emitting any. 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. To put the reactor into operation, the control blades are raised very slowly. 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.
In the MIT reactor, one other group of components is essential to the maintaining and controlling a chain reaction. 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". 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 (D20), and (3) high-purity graphite, both of which are excellent at slowing neutrons without absorbing them.