Neutron Absorbers (Poisons)

DOE-HDBK-1113-98

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

238

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,

or "poison".

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.

Enrichment

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