Factors Affecting Criticality - doe-hdbk-1113-98_reaffirm_2005

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:

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."

For example: a solid

sphere,

such as a billiard ball, has a

To avoid a criticality event, the fissile material must

not be placed in a shape, or geometry, that is favorable much lower surface-to-

volume ratio than a thin

to criticality. In general, the lower the surface-to-

rectangular shape, such as

volume ratio is, the greater the opportunity for

a piece of paper.

criticality.