Enrichment - doe-hdbk-1113-98_reaffirm_2005

DOE-HDBK-1113-98

Radiological Safety Training for Uranium Facilities

Module 105 - Criticality Safety

7. 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 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.

8. Volume

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.

9. Interaction

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.