Radioactive Properties - hdbk1113cn10117

DOE-HDBK-1113-98

Radiological Safety Training for Uranium Facilities

Module 101 - Properties of Uranium

Hydrogen gas is generated from uranium in water, and this may also produce a pressure buildup

situation. Because the hydrogen buildup may also be a fire hazard, it is discussed later in this

module in the Chemical Properties section.

C. Radioactive Properties

Uranium in its pure metal form is a silvery, gray metal and is the heaviest naturally occurring

element. There ar e 18 sepa rate is otopes o f uraniu m. Isotopes are eleme nts that have the s ame

number of protons, but different numbers of neutrons. For example, 235U has 92 protons with 143

neutrons and 238U has 92 protons with 146 neutrons.

Uranium i s radioa ctive. Pa rtiall y because of its si ze, the nu cleus of a uraniu m atom is uns table. 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 fissiona ble, which means they can be fissioned by fast neutr ons. 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 radiatio n. Criticali ty is discusse d in more detail in Module 105 - Cri ticality Saf ety.

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:

234

Th, 234mPa, 231Th, and 99Tc. The degree of enrichment also affects the controls that are required for

235

external ra diation expo sure because of the increa se in the amount of gamma-emitting

U 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.