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.
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 shim 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.
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