Module 105 - Criticality Safety

DOE-HDBK-1113-98

Radiological Safety Traning 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 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

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 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

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.