Module 105 Criticality Safety
Sometimes, neutrons that are emitted from the fissile
material may run into or otherwise interact with an
atom outside the fissile material and be "bounced
back" or "reflected" into the fissile material. Materials
such as water, graphite (a form of carbon), and
beryllium are good at reflecting neutrons. If the
uranium material is surrounded by these reflector
materials, criticality is easier to obtain. Accordingly, it
is undesirable to store fissile material where there is
potential for these materials to be present.
Another factor that affects criticality is the speed of
the neutrons from fission. Neutrons that are traveling
at about the same speed as the atoms in surrounding
materials are more easily absorbed by fissile
materials. Materials that slow the neutrons are known
as moderators. Examples of good moderators include
water and graphite.
For an example of moderation, consider 235U. This
uranium isotope absorbs slow neutrons (also called
"thermal" neutrons; these neutrons travel at the same
speed as their surroundings) with a rather high
probability for absorption. However, 238U only
absorbs fast neutrons (those neutrons with high
energies that travel faster than their surroundings).
Normally fast neutrons are quickly moderated to
lower energies, so that 238U will not go critical under
normal conditions. The fast neutrons must be
moderated, or slowed, to allow 235U to go critical.