Radioactive Properties cont'd - doe-hdbk-1113-98_reaffirm_2005

DOE-HDBK-1113-98

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.