Environmental Engineering Reference
In-Depth Information
Membranes with small pores allow gaseous diffusion to occur. The rate of diffusion is a function
of pressure and temperature and is also a function of the molecular diameter of the diffusing gas.
235 UF 6 has a slightly smaller diameter than 238 UF 6 . Thus, the lighter isotope diffuses somewhat
faster through the pores than the heavier isotope. In one passage, the enrichment factor is very small,
but by forcing the gases to pass many membranes (“stages”), almost any degree of enrichment can
be achieved. For light-water-moderated nuclear reactors, the desired enrichment is 3-4%. If the
enrichment factor is a fraction of a percent per stage, hundreds to thousands of stages are required
for the desired enrichment level. This requires large and expensive facilities with huge electricity
consumption for the vacuum pumps and the blowers that force the gases through the membranes.
Therefore, gaseous diffusion facilities are usually located near a supply of abundant and cheap
electricity. In the United States, gaseous diffusion plants are located in Oak Ridge, Tennessee, and
Hanford, Washington, where large hydroelectric dams are located nearby.
In the gaseous diffusion process about 85% of the uranium feed is rejected as depleted uranium.
Currently, these “tails” are stockpiled for eventual future use in fast breeder reactors.
The gaseous diffusion facilities were established during the second world war for the pro-
duction of weapons-grade highly enriched uranium. The relatively low enriched uranium used for
power plant reactors is a byproduct of the wartime effort. Critics of nuclear power plants often
charge that were enriched uranium not available from diffusion plants constructed and operated by
governments, nuclear electricity would be far more expensive, or not available at all.
Recently, in the United States and Europe, enrichment plants have been built that work on the
principle of a gaseous centrifuge . When the uranium gases are spun very fast in a centrifuge, the
heavier 238 UF 6 spreads toward the edges of the centrifuge while the lighter 235 UF 6 concentrates
toward the center. Here, the enrichment is dependent on the rate of revolutions and the time spent
in the centrifuge. Gaseous centrifuges are less energy-consuming than diffusion plants and require
lower capital investment.
Development is in progress in various countries on laser enrichment of uranium. In this process,
metallic uranium is vaporized in an oven. A stream of vaporized uranium atoms exits the oven port.
A laser beam with a very narrow wavelength band is shone unto the atomic stream to differentially
excite to a higher electronic state only 235 U, but not the heavier isotope. The excited atom is then
ionized with ultraviolet light. The ionized 235 U is collected on a negatively charged plate.
After enrichment, the UF 6 gas or metallic uranium is converted to uranium dioxide UO 2 . This
is a ceramic-like material that is fabricated into pellets. The pellets are loaded into fuel rods. A
1000-MW power plant needs about 75 metric tons of uranium dioxide pellets per load.
6.5.3
Spent Fuel Reprocessing and Temporary Waste Storage
In boiling and pressurized water reactors the fuel stays in the reactor for 2-3 years, generating
electricity. After that period, the level of fission products and other neutron absorbers has built
up, and the fission reaction has slowed down, with a concomitant decline in steam and electricity
production. At that time, the fuel rods have to be replaced with fresh ones. In CANDU-type
reactors, fuel rods have to be replaced every 18 months. The retrieved fuel rods emit a high level of
radiation because of the radioactive fission products and other neutron-activated isotopes that have
accumulated in the spent fuel rods. Generally, the extracted fuel rods are stored in the containment
vessel of the power plant, in steel and concrete-walled water pools or dry casks.
Once the radioactive level of the spent fuel has declined sufficiently to be handled by remote
control and proper shielding, it should be taken away to a permanent disposal site. Unfortunately, in
 
 
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