Natural uranium contains 0.7205% of the U-235, the fissile isotope of uranium. There are a few U-234 atoms (0.0055%) in the remaining mass of U-238 (99.274%). Uranium-238 does not contribute to slow neutron fission; however, it does react with neutrons to form a fissile isotope of plutonium, Pu-239. 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 3 to 4% U-235. Uranium for nuclear weapons is enriched to greater than 90%.
The Manhattan Project considered four physical processes for uranium enrichment: gaseous diffusion (effusion), electromagnetic separation, liquid thermal diffusion, and centrifugation. The first three were employed at Oak Ridge to produce enriched uranium for the Hiroshima bomb. Centrifugation was abandoned because the technology required to spin a rotator at high speeds was not practical for large-scale separations.
(Courtesy of the Department of Energy)
The gaseous diffusion process is based on molecular effusion, a process that occurs whenever a gas is separated from a vacuum by a porous barrier that contains microscopic holes. A gas passes through the holes because there are more "collisions" with holes on the high-pressure side than on the low-pressure side; the gas flows from the high-pressure side to the low-pressure side. Thomas Graham, a Scottish chemist, observed that the rate of effusion of a gas through a porous barrier was inversely proportional to the square root of its mass. Thus lighter molecules pass through the barrier faster than heavier ones.
The ratio of times it takes the same amounts of two gases, A and B, to effuse through a barrier is
Uranium hexafluoride (UF6), the gaseous compound of uranium, is used in this process. Since there is only one common isotope of fluorine, F-19, the ratio of rates becomes
K-25 Gaseous Diffusion Plant, Oak Ridge, TN
This small difference in rates means that many effusion barriers (stages) are necessary for enrichment. The K-25 gaseous diffusion plant at Oak Ridge required 4000 stages. This plant was one-half mile long and six stories high and covered 43 acres. The production of a suitable barrier was the key to successful separation. The holes must be microscopic (approximately one-millionth of an inch in diameter) and uniform in size. The porosity must always be high to sustain high flow rates and the barrier must not react with the highly corrosive hexafluoride. Nickel and aluminum oxide were 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 can be allowed out.
(Courtesy of the University of California, Berkeley)
The electromagnetic isotope separation (EMIS) process is based on the principle of a simple mass spectrometer, which states that a charged particle will follow a circular path when passing through a uniform magnetic field. Thus 235U+ and 238U+ with the same charge and kinetic energy will have slightly different paths when moving through a magnetic field. This allows for the separation and collection of the isotopes in receivers. 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 from solid uranium tetrafluoride, UF4, that is heated to produce a vapor that is then bombarded with electrons to produce U+ ions. There is a major problem with this process: less than half of the original U-235 is collected in the receivers. The rest is scattered through the calutron and is difficult to recover. At Oak Ridge, two groups of calutrons were used to produce weapons-grade uranium. The first, or alpha calutrons, enriched uranium (8.0% U-235) from the gaseous diffusion plant to between 12 and 20% U-235. The second, or beta calutrons, took the alpha product and enriched it to approximately 90% U-235 (weapons-grade uranium). As difficulties with the gaseous diffusion were overcome, the EMIS process was phased out.
Three of the processes (thermal diffusion, gaseous diffusion, and electromagnetic separation) were used to produce the U-235 for the first atomic bomb. Later, the gaseous diffusion alone became the process for producing both weapons-grade and reactor-grade U-235.
By the spring of 1945, Oak Ridge had shipped approximately 132 lbs. of enriched uranium (approximately 90% U-235) to Los Alamos, New Mexico. This was used in "Little Boy", the bomb dropped on Hiroshima on August 6, 1945. The majority of fission weapons since that time have used plutonium. Uranium enrichment is currently used to produce fuel (3 to 4% U-235) for civilian nuclear reactors.
Complete Bibliography on Uranium
from the ALSOS Digital Library for Nuclear Issues