Radiological Safety Training for Uranium Facilities
Module 101 Properties of Uranium
Uranium is radioactive. Partially because of its size, the nucleus of a uranium atom
is unstable. It reduces its size either by alpha particle emission or by nuclear fission,
in which the uranium nucleus splits, primarily, into two smaller fission products.
Both processes release energy, which can be helpful or harmful depending on how
they are controlled.
All isotopes of uranium are fissionable, which means they can be fissioned by fast
neutrons. Two isotopes, 233U and 235U, are fissile, which means they can also be
fissioned by slow (thermal) neutrons. A fissile material can be involved in a
criticality accident, resulting in the release of a lethal amount of radiation.
Criticality is discussed in more detail in Module 105 Criticality Safety.
The primary isotopes of uranium are all long-lived alpha emitters. However, several
other radionuclides can be radiologically significant at uranium facilities, depending
on the history of the uranium materials and the processing. These other
radionuclides include the following beta emitters: 234Th, 234mPa, 231Th, and
99Tc. The degree of enrichment also affects the controls that are required for
external radiation exposure because of the increase in the amount of gamma-
emitting 235U that is present. The uranium daughter products may also include
some low-energy gamma and x-ray radiation. For example, the daughter products of
232U represent a potential gamma-emission hazard.
Although there are several isotopes of uranium, only three exist naturally, and all
three are radioactive. See the table below for half-lives and natural percent
abundance for important uranium isotopes in the nuclear fuel cycle.