Radiological Safety Training for Uranium Facilities
Module 105 - Criticality Safety
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
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
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.