Radiological Safety Training for Uranium Facilities
Module 105 - Criticality Safety
Factors Affecting Criticality
Criticality depends on several factors, including the enrichment of the material, the
shape of the material, and surrounding materials, that may help or hinder fission.
Several factors which affect the occurrence and magnitude of a criticality:
1. Quantity of Fissile Material
When dealing with criticality, a common question is "How much material can I
work with and still be safe?" There is some amount of the fissile material
needed to have a criticality. This amount is called the "critical mass."
To avoid a criticality event, the fissile material must not be placed in a shape, or
geometry, that is favorable to criticality. In general, the lower the surface-to-
volume ratio is, the greater the opportunity for criticality.
Sometimes, neutrons that are emitted from the fissile material may run into or
otherwise interact with an atom outside the fissile material and be "bounced
back" or "reflected" into the fissile material. Materials such as water, graphite (a
form of carbon), and beryllium are good at reflecting neutrons. If the uranium
material is surrounded by these reflector materials, criticality is easier to obtain.
Accordingly, it is undesirable to store fissile material where there is potential for
these materials to be present.