Moderators

DOE-HDBK-1113-98

Radiological Safety Training for Uranium Facilities

Module 105 - Criticality Safety

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, 235U 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.

5. Neutron Absorbers (Poisons)

If neutron absorbers are present (i.e., atoms and molecules with relatively high

neutron absorption coefficients), these materials will remove neutrons from

being available to begin or sustain criticality. Boron is an example of a

frequently used neutron absorber, or "poison".

6. Concentration or Density of Fissile Material

As the concentration or density of fissile material increases, the opportunity for

criticality increases because of an increased likelihood of neutron interaction

with the fissile material.