Factors Affecting Criticality

DOE-HDBK-1113-98

Module 105 Criticality Safety

Lesson Plan

Instructor's Notes

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.