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

DOE-HDBK-1113-98

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.