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Factors Affecting Criticality - hdbk1113cn10098
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Radiological Safety Training for Uranium Facilities - index
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DOE-HDBK-1113-98
Module 105 Criticality Safety
Lesson Plan
Instructor's Notes
3.
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.
4.
Moderators
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
62


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