Radiological Safety Traning for Uranium Facilities
Module 105 - Criticality Safety
MODULE 105 - Criticality Safety
Describe the criticality safety control measures for uranium, including inventory control
Identify c ritica lity moni toring t echniqu es used wi th urani um.
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
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 fissio nable materia l absorbs a ne utron, the al ready unstabl e atom gains addi tional ener gy
and becomes even more unstable. One way the unstable atom can get rid of its excess energy is
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.