Module 105 Criticality Safety
When neutrons from one fission cause fission in another
atom, it is called a chain reaction. If the chain reaction is
self-sustaining, we call this criticality. Criticality is a self-
sustaining nuclear chain reaction. This is an obvious
radiation hazard because of the amount of energy given off
as gamma radiation and other forms.
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:
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."
For example: a solid sphere,
To avoid a criticality event, the fissile material
such as a billiard ball, has a
must not be placed in a shape, or geometry, that is
much lower surface-to-volume
favorable to criticality. In general, the lower the
ratio than a thin rectangular
surface-to-volume ratio is, the greater the
shape, such as a piece of paper.
opportunit y for criticality.