Reflectors - doe-hdbk-1113-98_reaffirm_2005

DOE-HDBK-1113-98

Module 105 Criticality Safety

Lesson Plan

Instructor's Notes

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 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.