Reflectors - hdbk1113cn10099

DOE-HDBK-1113-98

Module 105 Criticality Safety

Lesson Plan

Instructor's Notes

Reflectors

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.

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