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
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.