Toxicological Profile for Uranium | Slides Law

URANIUM 263

4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION

4.1 CHEMICAL IDENTITY

Uranium is a naturally occurring element that makes up approximately 2–4 ppm of the earth=s crust. It is

more plentiful than silver and about as abundant as molybdenum or arsenic. Uranium is an actinide

element, and has the highest atomic mass of any naturally occurring element. In its refined state, it is a

heavy, silvery-white metal that is malleable, ductile, slightly paramagnetic, and very dense, second only

to tungsten. In nature, it is found in rocks and ores throughout the earth, with the greatest concentrations

in the United States in the western states of Colorado, Arizona, Wyoming, Texas, Utah, and New Mexico

(Lide 2008). In its natural state, crustal uranium occurs as a component of several minerals, such as

carnotite, uraninite, and pitchblend, but is not found in the metallic state. The chemical information for

uranium metal is listed in Table 4-1.

4.2 PHYSICAL, CHEMICAL, AND RADIOLOGICAL PROPERTIES

The physical properties of uranium and uranium compounds important in the nuclear fuel cycle and

defense programs are listed in Table 4-2. The percent occurrence and radioactive properties of naturally

occurring isotopes of uranium are listed in Table 4-3. The two decay series for the naturally occurring

isotopes of uranium are shown in Table 4-4.

Metallurgically, uranium metal may exist in three allotropic forms: orthorhombic, tetragonal, or body-

centered cubic (Lide 2008), and may be alloyed with other metals to alter its structural and physical

properties to suit the application. Like aluminum metal powder, uranium metal powder is autopyrophoric

and can burn spontaneously at room temperature in the presence of air, oxygen, and water. In the same

manner, the surface of bulk metal, when first exposed to the atmosphere, rapidly oxidizes and produces a

thin surface layer of UO2, which resists oxygen penetration and protects the inner metal from oxidation.

At temperatures of 200–400°C, uranium powder may self-ignite in atmospheres of CO2 and N2. In order

to prevent autoignition, uranium machining chips can be stored in open containers and under machine oil

or water to prevent hydrogen gas buildup. Burning uranium may be placed under water until

extinguished, which may be delayed by hydrolysis of the water, which provides some oxygen and

hydrogen for continued burning. Water spray, CO2, and halon are ineffective, and halon discharge can be

explosive and produce toxic gases (DOE 2001).

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4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION

4.1 CHEMICAL IDENTITY

Uranium is a naturally occurring element that makes up approximately 2–4 ppm of the earth=s crust. It is

more plentiful than silver and about as abundant as molybdenum or arsenic. Uranium is an actinide

element, and has the highest atomic mass of any naturally occurring element. In its refined state, it is a

heavy, silvery-white metal that is malleable, ductile, slightly paramagnetic, and very dense, second only

to tungsten. In nature, it is found in rocks and ores throughout the earth, with the greatest concentrations

in the United States in the western states of Colorado, Arizona, Wyoming, Texas, Utah, and New Mexico

(Lide 2008). In its natural state, crustal uranium occurs as a component of several minerals, such as

carnotite, uraninite, and pitchblend, but is not found in the metallic state. The chemical information for

uranium metal is listed in Table 4-1.

4.2 PHYSICAL, CHEMICAL, AND RADIOLOGICAL PROPERTIES

The physical properties of uranium and uranium compounds important in the nuclear fuel cycle and

defense programs are listed in Table 4-2. The percent occurrence and radioactive properties of naturally

occurring isotopes of uranium are listed in Table 4-3. The two decay series for the naturally occurring

isotopes of uranium are shown in Table 4-4.

Metallurgically, uranium metal may exist in three allotropic forms: orthorhombic, tetragonal, or body-

centered cubic (Lide 2008), and may be alloyed with other metals to alter its structural and physical

properties to suit the application. Like aluminum metal powder, uranium metal powder is autopyrophoric

and can burn spontaneously at room temperature in the presence of air, oxygen, and water. In the same

manner, the surface of bulk metal, when first exposed to the atmosphere, rapidly oxidizes and produces a

thin surface layer of UO 2 , which resists oxygen penetration and protects the inner metal from oxidation.

At temperatures of 200–400°C, uranium powder may self-ignite in atmospheres of CO 2 and N 2. In order

to prevent autoignition, uranium machining chips can be stored in open containers and under machine oil

or water to prevent hydrogen gas buildup. Burning uranium may be placed under water until

extinguished, which may be delayed by hydrolysis of the water, which provides some oxygen and

hydrogen for continued burning. Water spray, CO 2 , and halon are ineffective, and halon discharge can be

explosive and produce toxic gases (DOE 2001).

Table 4-1. Chemical Identity of Uranium Metal

Value Reference

Table 4-2. Physical and Chemical Properties of Selected Uranium Compounds

Property

Uranium

hexafluoride

Uranium

tetrachloride

Uranyl

fluoridec

Uranyl acetate,

dihydrate

Uranyl nitrate

hexahydrate

UO 2 (NO 3 ) 2 6H 2 O

Table 4-2. Physical and Chemical Properties of Selected Uranium Compounds

Property

Uranyl nitrate

(not hexahydrate)

Ammonium

diuranate

Uranium

peroxide Uranyl acetate

Table 4-4.

U and

U Decay Series Showing Sources and Decay Products

238 Uranium-238 series, includes uranium-234 series Uranium-235 series

Uranium can exist in five oxidation states: +2, +3, +4, +5, and +6 (Lide 2008); however, only the +4 and

+6 states are stable enough to be of practical importance. Tetravalent uranium is reasonably stable and

forms hydroxides, hydrated fluorides, and phosphates of low solubility. Hexavalent uranium is the most

stable state, and the most commonly occurring state is U3O 8 , although there are a few localized storage

locations for anthropogenic uranium hexafluoride (UF 6 ) in the United States (DOE 2011a). Major

compounds of uranium include oxides, fluorides, carbides, nitrates, chlorides, acetates, and others. One

of the characteristics of UO 2 +2^ ions is their ability to fluoresce under ultraviolet light.

Although the element uranium was discovered in 1789 by Klaproth, who named it “uranium” after the

newly discovered planet Uranus, it was not until 1896 that Becquerel discovered that uranium is

radioactive. There are 22 known isotopes of uranium, only 3 of which occur naturally (NNDC 2011).

These three isotopes, 234 U, 235 U, and 238 U, have relative mass abundances within the earth=s undisturbed

crustal rock of 0.005, 0.72, and 99.275%, respectively. One gram of natural uranium having this relative

isotopic abundance has an activity of 0.69 μCi. Of this 0.69 μCi, 49.0% of the activity is attributable to

234 U, 2.27% of the activity is attributable to 235 U, and 48.7% of the activity is attributable to 238 U (Agency

for Toxic Substances and Disease Registry 2011). This ratio is for undisturbed crustal rock only.

Although the relative mass abundance of 234 U is only 0.005%, it accounts for approximately one-half of

the total activity. The relative isotopic abundances given above can be altered to some extent by natural

processes that are not fully understood, but which can cause different ratios in air, water, and soil as

demonstrated in EPA reports (EPA 1994a, 2007).

235 U is an isotope of particular interest because it is fissile (capable of being fissioned) and, consequently,

can sustain a nuclear chain reaction in the presence of appropriate energy neutrons. The predominant