Uranium - Toxicology of Metals - page 863

for digestion can be eliminated and sample preparation
time reduced to <1 hour by use of an appropriate che-
lation or ion exchange column with suffi cient prewash
and postwash volume, and adjusting the mass offset to
the high mass end of the spectral peak in magnetic sec-
tor instruments (Pappas et al ., 2006). Quadrupole ICP-
MS units can give accurate total uranium and isotopic
ratios for urine concentrations above approximately
50 ng/L, whereas high-resolution, double-focusing mag-
net units resolve individual isotopes more accurately.
The latter can achieve a limit of detection <3 ng/L in
a 100-

pitchblende, torbernite, tyuyamunite, and uraninite).
Once mined, uranium ore is milled by grinding the
ore into fi ne sand, the U is removed by acid leach or
alkaline roast, and its crude oxide is formed through
chemical separation and precipitation as U 3 0 8 . Low
extraction effi ciencies (5-15%) in the early 1940s quickly
increased during World War II as the understanding of
uranium chemistry improved and can now approach
90%. Uranium can also be recovered from unconsoli-
dated deposits or suffi ciently rich mill tailings through
the process of in situ leach mining that involves creat-
ing underground barrier curtains, injecting chemicals
to dissolve the uranium, vacuum recovering the liquor,
and treating it to remove uranium.
Most uranium oxide is directed toward the nuclear
energy industry for which it is separated into enriched
and depleted U fractions. The most effective methods
are ultracentrifugation followed by gaseous diffusion,
with laser separation being studied. The United States
uses gaseous diffusion that involves converting ura-
nium oxide to uranium hexafl uoride (that sublimes
near room temperature) and passing the gas through
a long series of membrane fi lters. Diffusion rates are
higher for 234 UF and 235 UF 6 than 238 UF 6 , causing incre-
mental separation of the mixture into enriched and
depleted fractions that have different ratios of the three
natural isotopes. Enriched U contains higher than
natural proportions of 235 U and 234 U (i.e., greater than
0.72 and 0.0055%, respectively), making it fi ssionable
and more radioactive, whereas the depleted U (DU)
contains less of these isotopes and is suitable for more
conventional industrial uses. The degree of enrichment
is determined by the desired use, with 3% being suit-
able for commercial power reactors, and >95-97%
used for warship propulsion systems and weapons.
European producers achieve this separation to pro-
duce enriched uranium by ultracentrifugation. For this
method, each fraction (enriched uranium or depleted
uranium) can be stored as the hexafl uoride or con-
verted to oxide or metal. The oxide is often prepared
for use as power reactor fuel by shaping and sintering
the material to form cylindrical fuel pellets, which are
stacked in rods and arrayed in bundles that are placed
inside the reactor vessel. Uranium metal has industrial
applications (e.g., counterweights), as well as military
uses. Natural, enriched, and depleted U are chemically
and biologically the same, but the shift in 234 U (the
most radioactive isotope) makes enriched U the most
and DU the least radioactive.
Uranium production (as U 3 O 8 ) peaked in the United
States at 2 × 10 10 g in 1980 and then decreased to 9 × 10 9 g
(2 million pounds) in 2003. Imports have remained rel-
atively fl at at 9-13 × 10 9 g/year. There has been a glo-
bal shift by production location and a price increase as

L sample (Pappas et al ., 2002). Both have some
diffi culty resolving isotopes whose masses are mul-
tiples of that for the carrier gas. ICP-QMS was used
to determine that black rain fallout stains on concrete
from the atomic bomb detonation over Hiroshima in
1945 contained a higher proportion of 235 U than natural
uranium, confi rming that the fi ssion was not complete
(Fujikawa et al ., 2003).
Measuring the relative ratios of the natural uranium
isotopes and anthropogenic uranium isotopes (e.g.,
236 U) along with trace radionuclides is a method that
can be used as part of a forensic evaluation to identify
potential sources of uranium in a sample. Because of
the wide natural variability in uranium isotopic ratios,
the reliability of this method will depend in part on the
ability to fi nd and accurately measure the isotopic
ratios in uncontaminated reference samples.
Examples for demonstrating the consequence of
converting between mass and radioactivity without
considering the isotopic ratios can be found in U.S.
EPA Environmental Radiation Data Reports. Report
78 for April-June 1994, gives Albany, NY, precipitation
concentrations for 234 U, 235 U, and 238 U of 0.016 pCi/L
(0.00059 Bq/L), 0.001 pCi/L (0.000037 Bq/L), and
0.002 pCi/L (0.000074 Bq/L), which totals 0.019 pCi/L
(0.00068 Bq/L) (EPA, 1996). The use of 0.67 pCi/

µ

µ g

(0.025 Bq/ g) as a generic conversion factor results

in a mass concentration of 0.028

µ

g/L. Alternately,
the use of respective isotopic conversion factors of
6240 pCi/

µ

µ g (231 Bq/ µ g), 2.16 pCi/ µ g (0.080 Bq/ µ g),

and 0.336 pCi/ µ g (0.012 Bq/ µ g) yields mass concentra-

tions of 0.0000026 µ g/L, 0.00046 µ g/L, and 0.0060 µ g/L,

which total 0.0065 µ g/L, or a factor of 4 less than by the

other method.

3 PRODUCTION AND USES

3.1 Production
There are more than 100 minerals known to con-
tain uranium, but the bulk of uranium production
comes from seven (autunite, carnotite, comminite,