Natural uranium contains 0.7205% U-235, the fissile isotope of uranium. The remaining mass includes 99.274% U-238 and a small amount of U-234 (0.0055%). Uranium-238 does not contribute to slow neutron fission; however, it can react with neutrons to form a fissile isotope of plutonium, Pu-239. Thus U-238 is known as a fertile material, i.e., one that can produce fissile materials. Although U-235 and U-238 are chemically identical, they differ slightly in their physical properties, most importantly mass. This small mass difference allows the isotopes to be separated and makes it possible to increase (enrich) the percentage of U-235 in uranium. Most civilian power reactors use enriched uranium fuel containing 0.8 to 8.0% U-235, known as low enriched uranium (LEU). Weapons grade uranium must contain highly enriched uranium (HEU) with an isotopic concentration greater than 90% U-235.
Figure 1 – Isotopic Separation by Gaseous Diffusion
With permission from Informationkreis KernEnergie, Berlin
The ratio of times it takes the same amounts of two gases, A and B, to effuse through a barrier is
Figure 2 – The K25 Gaseous Diffusion Plant at Oak Ridge, Tennessee
Courtesy of Atomic Archive
The production of a suitable barrier material was the key to successful separation. The holes in the barrier must be microscopic (approximately one-millionth of an inch in diameter) and uniform in size. Its porosity must sustain high flow rates, and it cannot react with the highly corrosive hexafluoride gas. Nickel and aluminum oxide were found to be best suited for barrier materials. Diffusion equipment is large and consumes significant amounts of energy. The entire system must be leak free; no air can be allowed in and no uranium hexafluoride allowed out.
Figure 3 – Electromagnetic Isotope Separation
Courtesy of the University of California, Berkeley
EMIS units, known as "calutrons", were operated in the Y-12 area of Oak Ridge. Because EMIS is a batch process, with each unit requiring a long time to produce small amounts of U-235, many units were used to produce the first fissionable uranium. Uranium ions for the EMIS are generated by heating solid uranium tetrachloride, UCl4, to produce a vapor that is then bombarded with electrons to produce positively charged ions. There was a major problem with this process: less than half of the original U-235 was collected in the receivers. The rest was scattered throughout the calutron and was difficult to recover. At Oak Ridge, two groups of calutrons were used to produce weapons-grade uranium. The first group, or alpha calutrons, enriched uranium (7.0% U-235) from the gaseous diffusion plant to between 12 and 20% U-235. The second group, or beta calutrons, took the alpha product and enriched it to approximately 90% U-235 (weapons-grade uranium). As difficulties with the gaseous diffusion process were overcome, the EMIS process was phased out.
Figure 4 – Isotopic Separation by Thermal Diffusion
Courtesy of the U.S. Department of Energy
Production of Enriched Uranium for the First Atomic Bomb
Figure 5 – Methods Used to Produce Enriched Uranium for the First Atomic Bomb
In centrifugation, gaseous UF6 is fed into a centrifuge unit consisting of a cylindrical rotor spinning at a high speed inside an evacuated casing (Figure 6). The centrifugal force in the rapidly spinning rotor causes a partial separation of the UF6, with the heavier U-238 molecules becoming slightly more concentrated around the outside walls, while the concentration of the lighter U-235 molecules increases around the middle of the tube. The separation is facilitated by a relatively slow axial countercurrent flow in the rotor that moves the molecules enriched in U-235 to one end and the depleted molecules containing increased concentration of U-238 to the other. The separation can be enhanced further by heating the lower end of the casing, creating convection currents that move the U-238 down and the U-235 up.
Figure 6 – Isotopic Separation by Centrifugation
Courtesy of Informationkreis KernEnergie, Berlin
Although it is possible to obtain significantly more enrichment from a single centrifuge than from a single gaseous diffusion stage, this process must be repeated in a series of connected centrifuges known as a cascade (Figure 7) in order to obtain the desired concentration of enriched uranium. The slightly enriched stream is fed to the next higher stage, while the depleted stream is recycled back to the preceding stage. Cascades containing several hundred or even thousands of units are the basic components of a centrifuge enrichment facility.
Figure 7- A Centrifuge Cascade
Courtesy of Wikimedia Commons
Cascades of individual centrifuges constructed to produce LEU for reactors can be easily reconfigured to produce HEU for weapons. It is also possible to recycle LEU through a cascade for additional enrichment without changing its configuration. Thus, any centrifuge enrichment facility has the potential to produce HEU for weapons, a major concern with respect to nuclear weapons proliferation. Pakistan produced enriched uranium for its nuclear weapons using centrifuge technology acquired by the infamous metallurgist and physicist, A. Q. Khan. Concerns surrounding Iran’s uranium enrichment program center on its cascades of centrifuges at Natanz. The Iranian government claims that it will produce only LEU for nuclear power but there is evidence that Iran is attempting to manufacture HEU for nuclear weapons.
|2005-2009 Kennesaw State University
Principal Investigator Laurence Peterson
Project Director Matthew Hermes