Uranium/ special nuclear material

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.