Properties and Relative Hazards

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

2.0 PROPERTIES AND RELATIVE HAZARDS

This chapter presents basic radiological and chemical properties of uranium and discusses the basis

for current control limits. A variety of materials are inherent to uranium handling processes and hazards

characteristic of these materials and processes. The data and discussions are intended to provide a basis

for understanding the changes in hazards as a function of such parameters as enrichment, physical form,

and chemical form.

2.1 NUCLEAR PROPERTIES OF URANIUM

Naturally occurring uranium consists of a mixture of 234U, 235U and 238U isotopes, along with their

decay products. Uranium is relatively abundant in nature. The primary isotopes of uranium are long-

lived alpha-emitters with energies between 4.15 and 4.8 MeV. Their progeny include numerous other

radionuclides, some of which are radiologically significant at uranium facilities, the degree of

significance depending upon the history of the uranium materials and the processing.

Through proper processing, uranium can be used as a fuel in nuclear reactors to generate electricity

on a commercially-viable scale. The 235U isotope is readily fissioned by slow, "thermal" neutrons with

the release of a large amount of energy. The percentage of 235U present (referred to as "enrichment")

determines the fuel reactivity and the criticality hazard of the material. By concentrating the amount of

the 235U isotope in the uranium, the quantity of fuel and the size of the reactor needed for production

decreases. This concentration of natural uranium to enriched uranium is carried out by special processes

such as gaseous diffusion, centrifuging, or laser separation. The uranium by-product of the enrichment

process is reduced in 235U content and is called "depleted" uranium. Uranium is commonly classified by

its 235U enrichment as natural uranium, enriched uranium, or depleted uranium.

Uranium-235 fissions after capturing a thermal (very low energy) neutron. Its fission thermal cross-

section (probability of interaction) is 577 barns (Stehn et al. 1965). Its neutron capture cross section is

101 barns. After capturing a fast neutron, 238U undergoes two successive beta decays to 239Pu which will

also undergo thermal fission (thermal cross-section = 741 barns). Pressurized heavy-water reactors

function with natural uranium isotopic composition. Other types of reactors require some 235U

enrichment.

2.1.1 Isotopic Characterization

Natural uranium consists of three isotopes: 238U, 235U, and 234U. All three radionuclides undergo

radioactive decay by alpha particle emission. The 235U isotope (and 234U to a much lesser degree and at

lower energy) emits gamma radiation as well. The natural abundances of these uranium isotopes, as

well as the weight percentages of the isotopes in enriched (typic al commercial nuclear power reactor

enrichment) and depleted uranium, are listed in Table 2-1.

2-1