Nuclear - Energy Policy Analysis: A Conceptual Framework - page 45

or felted fabric bags, and the pressure increases until the dust is removed by manual or automatic
means, such as rapping, shaking gear, or automatic flow reversal mechanisms. Wet solvent extrac-
tion systems also require absorption towers for removing oxides of nitrogen. Gaseous effluents
escaping from the scrubber and bag filter systems are released to the environment through stacks
without further treatment (USEPA 1973a, 15-16).
Processes that produce liquid wastes include high-efficiency centrifuges; chemical treatments
for flocculation, precipitation, and neutralization; filtration; and settling basins or ponds. Dissolved
waste matter is usually in the form of ions, which may be separated from the waste stream by
passing it though beds of small spheres of chemically treated resins on which ions are absorbed.
Flocculation and neutralization chemicals may be added to the waste stream, causing dissolved
wastes to form particles that may settle out or be separated from water by filters or centrifuges.
Settling basins or ponds of about four feet depth may be several acres in size and are preferred
because their operation entails little power or maintenance costs. Such structures must be lined
with clay soils or artificial liners of plastic or chemically treated fabric to prevent seepage into
the ground and contamination of groundwater (USEPA 1973a, 18).
Enrichment
Conversion is followed by isotopic enrichment, in which the uranium-235 concentration of the
uranium feed is increased to the design specification (usually 2 to 4 percent uranium-235) of
the power reactor. In the United States a gaseous diffusion process is used, where volatile UF 6
is pumped under pressure through a system of numerous (e.g., ~1,700) porous membranes that
discriminate against the passage of heavier isotopes of uranium-238, allowing more uranium-235
to pass, thereby producing a fuel with a higher concentration of uranium-235 (USEPA 1973a,
96). This requires compressors driven by electric motors utilizing large quantities of electricity
and generating substantial process heat that requires cooling. During enrichment, liquid, gaseous,
and airborne material impurities are removed from effluents with cold traps and aluminum traps,
descriptions of which are not publicly available (USEPA 1973a, 16) for security reasons. There
is only one operating gaseous diffusion plant in the United States, at the Department of Energy's
Paducah, Kentucky, site, operated by the U.S. Enrichment Corporation (USEC), which was cre-
ated as a government corporation by the Energy Act of 1992 and privatized in 1996 (USNRC
2011c).
An alternative enrichment technique requires use of numerous gas centrifuges, a large number of
rotating cylinders interconnected to form cascades. UF 6 is placed in a cylinder, which is then rotated
at a high speed, creating a strong centrifugal force that draws more of the heavier gas molecules
containing the U-238 toward the wall of the cylinder, while lighter gas molecules containing the
U-235 tend to collect closer to the center. Slightly enriched U-235 is then extracted and fed into
the next higher stage of centrifuges. Two gas centrifuge enrichment plants are currently under
construction in the United States, one by USEC at Piketon, Ohio, and another in Lea County, New
Mexico (USNRC 2011c). These facilities will require a great deal of electricity to run.
A third alternative separates the U-235 isotope from uranium through photoexcitation by using
specially tuned lasers that are able to ionize a specific isotope, changing its properties and allowing
it to be separated from the rest of the uranium. A proposal to construct a laser enrichment plant
in Wilmington, North Carolina, was pending in the U.S. Nuclear Regulatory Commission (NRC)
licensing process as this topic went to press (USNRC 2012). Electric utilities have expressed
concern about the limited uranium enrichment capacity in the United States and their resulting
dependence on foreign sources of enriched uranium fuel (NERC 2009, 369). In each alternative,