Radiological Safety Traning for Uranium Facilities
Module 105 - Criticality Safety
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
U will not go critical under normal conditions. The fast neutrons must be moderated,
or slowed, to allow 235U to go critical.
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,
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
Enrichment is the separation of isotopes. With uranium, enrichment is typically referred
to as increasing the percentage (by weight) of the 235U isotope in material to greater than
that fou nd in nat ural ura nium.
Obviously, the enrichment of uranium plays an important role in criticality because the
amount of fissile material available for criticality is greater. For example, the higher the