http://www.upi.com/print.cfm?StoryID=20020920-044456-2907r
Analysis: Uranium processing difficult
By Scott R. Burnell
UPI Science News
>From the Science & Technology Desk
Published 9/20/2002 7:45 PM
Transforming uranium ores into fuel for nuclear power plants or the core
of atomic bombs is an extensive process requiring a careful
combination of chemistry and mechanical precision, a factor that should
be considered in the current situation with Iraq.
Uranium is a not-uncommon common element in the Earth's crust. It falls
48th on the list of abundance, placing it a little more than
halfway down the list of the 92 naturally-occurring elements. It is far
more prevalent than silver, for example, according to the U.S.
Department of Energy.
As it sits in the ground, uranium is made up of three isotopes,
differentiated by the number of neutrons in each atom's nucleus.
Nearly all naturally occurring uranium -- 99.3 percent -- is U-238. Most
of the remaining seven-tenths of a percent is U-235, with
U-234 comprising just a trace, the DOE says.
The key factor in uranium's use in nuclear applications is its
instability. Although U-238 is relatively stable, U-235 can be split
apart
into lighter elements. This process, called nuclear fission, can release
huge amounts of energy -- either slowly within a reactor or
nearly instantly in a bomb.
Raw uranium ore contains combinations of uranium and oxygen, as well as
many other elements that basically prevent fission from
occurring in any usable fashion. The chemical mix also prevents uranium
from being separated into its isotopes easily. The first step
requires milling the ore to yield "yellow cake," a substance consisting
of uranium oxides that gets its name from its resemblance to
cooked cake mix.
In order to isolate the U-235, uranium compounds must be stripped of
their oxygen. This requires a series of chemical reactions
involving acids, hydrogen and ammonium hydroxide that replace the oxygen
with fluorine. The result is uranium hexafluoride, an
easily-transported, water-soluble solid with a relatively low boiling
point. UF6 still retains about the normal ratio of U-238 to U-235.
The hexafluoride must undergo enrichment to boost its percentage of
U-235. Standard methods include gaseous diffusion and gas
centrifuge, the latter being quicker, cheaper and the center of
controversy in Iraq's case. Either way, however, increasing amounts of
the heavier U-238 are left behind with each enrichment cycle, raising
the percentage of desired U-235.
The question of whether or not Iraq is at work on a nuclear device is
active because two shipments of high-grade aluminum tubes
bound for Iraq have been intercepted in the past 14 months. The tubes
are components associated with the enrichment process,
although experts remain divided over whether the shipments were
centrifuge parts or meant for somewhat less-threatening uses such
as artillery rocket tubes.
If the parts were destined for a centrifuge, the discovery still means
Iraqi scientists would need an extended period to produce usable
amounts of weapons-grade material. Starting at the "natural" level of
seven-tenths of a percent, uranium is not considered even
nuclear power plant fuel until it reaches about 3-to-5 percent U-235.
Officials do not use the label "highly enriched" until material attains
20 percent U-235. Theoretically, it is possible to create a nuclear
weapon with such material, but even higher enrichment levels are thought
necessary for the simplest devices Iraq is thought to
desire.
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