The Fission Process
In the nucleus of each atom of Uranium-235 fuel are 92 protons and 143 neutrons, a total of 235 particles so fantastically small that their size is difficult to imagine. Around this nucleus whirl 92 electrons, which are even smaller particles. If the nucleus were as big as a baseball, an electron on its outer rim would be a mere speck nearly a mile away.
The arrangement of particles within uranium is unstable and the nucleus disintegrates easily. When the nucleus absorbs an extra neutron, it breaks into two parts or splits. This process is known as fission (see diagram below). Each time a 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 veritable horde of neutrons traveling in every direction at very high speeds.
The rate of fissions in the uranium nuclei 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 reemitting any. When the control blades are fully inserted, they absorb so many neutrons from the uranuim that there are not enough to cause a chain reaction. To put the reactor into operation, the control blades are raised very slowly. As they absorb fewer and fewer neutrons, more and more neutrons are available to cause the splitting of uranium nuclei, until finally enough neutrons are being released to sustain a chain reaction.
In addition to the fuel and control blades, one other factor is essential to the operation of the reactor. This is a moderator-coolant, which is ordinary or "light" water in the case of the MITR-II. Since uranium nuclei do not readily absorb neutrons moving at the high speeds with which they leave fissioning nuclei, it is necessary to slow them down with a "moderator". For this purpose about one-half the volume of the reactor core consists of water.
The Nuclear History Website, Available online at: http://nuclearhistory.tripod.com/secondary_pages/fission.html