Module 105 Criticality Safety
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 increases.
Neutron interaction in an array of containers of fissile
material depends on geometric factors, including size,
shape, and separation of the containers, as well as the
size and shape of the array itself. 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.