Enrichment

U-238 More than 99% of the Uranium in the ground is U-238. U-238 doesn’t fission.

U-235 Less than 1% of the Uranium in the ground is U-235. U-235 fissions!

Need more U-235 in the mix Enrichment is the name for the process in which the concentration of U-235 is increased in a mixture of uranium isotopes. The concentration of U-235 is increased by removing U-238. U – DU = LEU Depleted uranium (DU) is the name for the U-238 that must be discarded in order to increase the concentration of the U-235 isotope.

LEU Low Enriched Uranium (LEU) is about 5% U-235. LEU will sustain a slow fission reaction. LEU cannot explode.

HEU High Enriched Uranium (HEU) is enriched to more than 20% U-235. HEU explodes. Atomic bombs can be made of HEU.

HEU HEU is used as fuel for small research reactors, small reactors producing radioactive elements for medical uses, and some types of naval propulsion (such as US submarines).

HEU on Hiroshima The atomic bomb that the US exploded over Hiroshima, Japan on August 6, 1945, had HEU as its explosive material.

HEU over Hiroshima

HEU weapon, easy design, never had been tested before Hiroshima

Enrichment is exponential 0.7% 1.4% 2.8% 5%10% 20% 40% 80% ^ ^ (nice for reactors) (nice for bombs)

How is uranium enriched? Gaseous diffusion (World War II to present)

Gaseous diffusion HEU for Hiroshima bomb Massive industrial facility Requires a lot of land and electricity Hard to hide

The K-25 plant, located on the southwestern end of the Oak Ridge reservation, used the gaseous diffusion method to separate uranium-235 from uranium-238. Based on the well-known principle that molecules of a lighter isotope would pass through a porous barrier more readily than molecules of a heavier one, gaseous diffusion produced through myriads of repetitions a gas increasingly rich in uranium-235 as the heavier uranium-238 was separated out in a system of cascades. Although producing minute amounts of final product measured in grams, gaseous diffusion required a massive facility to house the hundreds of cascades and consumed enormous amounts of electric power.

Begun in June 1943 and completed in early 1945 at a cost of $500 million, the K-25 plant employed 12,000 workers. The U-shaped K-25 building measures half a mile by 1,000 feet and is larger than the Pentagon. Construction began before completion of the design for the process. Due to construction needs at K-25 and elsewhere on the reservation, the town of Oak Ridge, originally designed for 13,000 people, grew to 50,000 by summer 1944.

Gaseous diffusion was one of three isotope separation processes that provided uranium-235 for the Hiroshima weapon (Little Boy) - the other two being electromagnetic separation and liquid thermal diffusion. The S-50 liquid thermal diffusion plant, using convection to separate the isotopes in thousands of tall columns, was built next to the K-25 power plant, which provided the necessary steam. Much less efficient than K-25, the S-50 plant was torn down after the war. Gaseous diffusion was the only uranium enrichment process used during the Cold War. K-25 was the prototype for later Oak Ridge plants and those at Paducah and Portsmouth.

How is uranium enriched? Centrifuges (now)

Centrifuges in the P5 + 1 Iran Agreement Limited enrichment would mean limits on the numbers and types of centrifuges. Shortly before the comprehensive negotiations began, Iran was estimated to have 19,000 centrifuges installed, mostly first generation IR-1 machines, with about 10,000 of them operating to increase the concentration of U-235. The Iranians strive to expand their enrichment capacity by a factor of 10 or more while the six powers (P5+1) aim to cut the number of centrifuges to no more than a few thousand.

Centrifuge drawing

Centrifuges – need a lot!

Laser Enrichment Separation Anxiety (ironic name) Smaller Use less energy Easier to conceal May end up cheaper

How is uranium enriched? Lasers (controversial) GE is doing laser enrichment in Wilmington, North Carolina slows-pace-of-uranium-enrichment