Preparation of nuclear fuel By far the most important application of Grahams

Preparation of nuclear fuel by far the most important

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Preparation of nuclear fuel. By far the most important application of Graham's law is in the preparation of fuel for nuclear energy reactors. The process of isotope enrichment increases the proportion of fissionable, but rarer, 235 U (only 0.7% by mass of naturally occurring uranium) to the nonfissionable, more abundant 238 U (99.3% by mass). Because the two isotopes have identical chemical properties, they are extremely difficult to separate chemically. But, one way to separate them takes advantage of a difference in a physical property—the effusion rate of gaseous compounds. Uranium ore is treated with fluorine to yield a gaseous mixture of 238 UF 6 and 235 UF 6 that is pumped through a series of chambers separated by porous barriers. Molecules of 235 UF 6 are slightly lighter ( = 349.03) than molecules of 238 UF 6 ( = 352.04), so they move slightly faster and effuse through each barrier 1.0043 times faster. Many passes must be made, each one increasing the fraction of 235 UF 6 , until the mixture obtained is 3–5% by mass 235 UF 6 . This process was developed during the latter years of World War II and 2. IEB Wireframe ... 9 of 16 11/3/14, 10:37 AM
produced enough 235 U for two of the world's first atomic bombs. Today, a less expensive centrifuge process is used more often. The ability to enrich uranium has become a key international concern, as more countries aspire to develop nuclear energy and nuclear arms. The Process of Diffusion Closely related to effusion is the process of gaseous diffusion, the movement of one gas through another. Diffusion rates are also described generally by Graham's law: For two gases at equal pressures, such as NH 3 and HCl, moving through another gas or a mixture of gases, such as air, we find The reason for this dependence on molar mass is the same as for effusion rates: lighter molecules have higher average speeds than heavier molecules, so they move farther in a given time. If gas molecules move at hundreds of meters per second (see Figure 5.14), why does it take a second or two after you open a bottle of perfume to smell it? Although convection plays an important role in this process, another reason for the time lag is that a gas particle does not travel very far before it collides with another particle (Figure 5.22). Thus, a perfume molecule travels slowly because it collides with countless molecules in the air. The presence of so many other particles means that diffusion rates are much lower than effusion rates. Imagine how much quicker you can walk through an empty room than through a room crowded with other moving people. Figure 5.22 Diffusion of gases. When different gases ( black , from the left, and green , from the right) move through each other, they mix. For simplicity, the complex path of only one black particle is shown (in red ). In reality, all the particles have similar paths.

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