Module 105 - Criticality Safety

DOE-HDBK-1113-98

Radiological Safety Traning 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 occurre nce and magnitud e of a critic ality:

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 f avorabl e to crit icalit y. In general , the lowe r the sur face-to-vo lume

ratio is, th e greater the o pportunity f or critical ity.

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.

Moderators

Another factor that affects criticality is the speed of the neutrons from fission. Neutrons

that are traveling at about the same speed as the atoms in surrounding materials are more