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". 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. 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. Hence, the possibility exists for creating a chain reaction. 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.