Module 105 Criticality Safety
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
U 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 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 found in
natura l uranium.