Radiological Safety Training for Uranium Facilities
Module 105 - Criticality Safety
MODULE 105 - Criticality Safety
Describe the criticality safety control measures for uranium, including
inventory control measures.
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.