Radiological Safety Training for Uranium Facilities
Module 105 - Criticality Safety
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 natural uranium.
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 enrichment of 235U (i.e., the concentration of 235U in
relation to other uranium isotopes), the greater the opportunity for criticality.
The volume of material in which fissile material is in solution can also play an
important role in preventing criticality. For a given concentration or density of
fissile material, the amount of fissile material will increase as the volume
Neutron interaction in an array of containers of fissile material is dependent
upon geometric factors, including: size, shape, and separation of the containers,
as well as the size and shape of the array. Materials that may surround or be
intermingled with the containers are also important. A close-packed array may
be come critical if flooded with water which will thermalize neutrons. Also, a
less closely-packed array may become critical if the water is removed, allowing
less neutron absorption to take place.