Module 105 - Criticality Safety - doe-hdbk-1113-98_reaffirm_2005

DOE-HDBK-1113-98

Radiological Safety Training for Uranium Facilities

Module 105 - Criticality Safety

MODULE 105 - Criticality Safety

Objectives

EO8

Describe the criticality safety control measures for uranium, including

inventory control measures.

EO9

Identify criticality monitoring techniques used with uranium.

Explanation of Criticality

Uranium is a fissionable material, which means that it can undergo nuclear fission.

Nuclear fission is a process in which a very heavy unstable atom primarily splits in

two, or "fissions". When an atom fissions, one large atom primarily becomes two

smaller atoms, between one and seven neutrons are given off, and a great deal of

energy in radiation and other forms, such as the kinetic energy of the fission

fragments, is released.

Some unstable atoms, such as 235U, undergo a small amount of fission without any

outside influences. This small amount of spontaneous fission does not present a

significant hazard on its own, but the neutrons from this fission may be absorbed by

other fissionable atoms. When an atom of fissionable material absorbs a neutron,

the already unstable atom gains additional energy and becomes even more unstable.

One way the unstable atom can get rid of its excess energy is through fission.

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.