Uranium/ Special Nuclear Material
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SPECIAL NUCLEAR MATERIA L
“Special nuclear material” (SNM) is defined by Title I of the Atomic Energy Act of 1954 as plutonium, uranium-233, or uranium enriched in the isotopes uranium-233 or uranium-235. In 1789, Uranium was discovered in the mineral called pitchblende, by a German chemist named Martin Klaproth. It was named after the planet Uranus, which had been discovered eight years earlier. Uranium-233 and plutonium are formed in nuclear reactors because they do not occur naturally. It has to be taken from highly radioactive spent fuel by chemical separation. Uranium-233 can be produced in special reactors that use thorium as fuel. Only small quantities of uranium-233 have ever been made in the United States. No U.S. commercial plutonium reprocessing plant is currently licensed by the U.S. Nuclear Regulatory Commission for operation. Uranium enriched in uranium-235 is created by an enrichment facility. The NRC regulates two gaseous diffusion enrichment plants operated by the U.S. Enrichment Corporation.
The gaseous diffusion process is the current method used by the United States to enrich uranium. There are two gaseous diffusion plants in the United States. One is located in Portsmouth, Ohio but was shut down in March 2001, and the other is in Paducah, Kentucky. This plant has produced enriched uranium continuously since November 1952. It is operated by the United States Enrichment Corporation (USEC) which was created as a government corporation under the Energy Act of 1992 and privatized by legislation in 1996
Natural uranium contains 99% U238 and only about 0.7% U235 by weight.
Gaseous Diffusion
The uranium enriched in uranium-235 is required in commercial light water reactors to produce a controlled nuclear reaction. Gaseous diffusion is one way to enrich uranium.
The gas separates by slowly flowing through small holes. (molecular effusion) In a vessel containing a mixture of two gases, molecules of the gas with lower molecular weight travel faster and strike the walls of the vessel more frequently. The walls of the vessel can be penetrated, so more of the lighter molecules flow through the barrier than the heavier molecules. The gas that escapes the vessel is enriched in the lighter isotope. One barrier isn’t enough to do the job, though. It takes many hundreds of barriers, one after the other, before the UF6 gas contains enough uranium-235 to be used in reactors. At the end of the process, the enriched UF6 gas is withdrawn from the pipelines and condensed back into a liquid that is poured into containers. The UF6 cools down and solidifies before it is transported to fuel fabrication facilities where it is turned into fuel assemblies for nuclear power reactors.
The enriched uranium fuel begins in the form of a hard ceramic pellet about Đ… in. long with the circumference of a pencil. The pellets are put into zirconium tubes. 250 of these tubes are fastened together into fuel assemblies which are used in the nuclear reactor for about 18 months. Then the fuel elements inside the tubes lose their ability to produce enough energy to sustain a nuclear reactor. These uranium-bearing fuel elements inside are referred to as spent nuclear fuel. Once the spent fuel is removed from the reactor the fission process has stopped, but the spent fuel assemblies still generate significant amounts of radiation and heat. Spent fuel is so hazardous, it must be shipped in containers or casks that shield and contain the radioactivity and drive away heat.
Over the last 30 years, thousands of shipments of commercially generated spent nuclear fuel have been made throughout the United States without causing any radiological releases to the environment or harm to the public. Most of these shipments occur between different reactors owned by the same utility to share storage space for spent fuel, or they may be shipped to a research facility to perform tests on the spent fuel. In the near future, because of a potential high-level waste repository being built, the number of these shipments by road and rail is expected to increase.
TYPICAL SPENT FUEL TRANSPORTATION CASKS
Generic Truck Cask for Spent Fuel
Typical Specifications