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Regarding Uranium Enrichment:

Chemical elements are defined by their Atomic Number, which is the number of protons in the nucleus of each atom. Two atoms with different numbers of protons are by definition different elements. Atoms also include electrons (whose negative charge offsets the positive charge of the protons) and neutrons (which have no charge). Electrons are nearly weightless and live in concentric "shells" around the nucleus. Neutrons weigh the same as protons and, like them, live in the nucleus. The Atomic Weight of an atom is the number of protons and neutrons in the nucleus. Isotopes of an element contain the same number of protons but different numbers of neutrons. For example U-237 has an Atomic Weight of 237.

The reason uranium is radioactive is that it is unstable. It emits particles as part of a long slow process called radioactive decay whereby each atom loses protons and thus turns into a different element. In the case of uranium the element it turns into isn't stable either and it keeps decaying until it turns into lead. We consider lead to be heavy because it is one of the heaviest elements that is stable and relatively common.

The process of decay is slightly different for the different isotopes of uranium. Some isotopes emit considerably more energy over a given time span than others, and this difference is important when creating nuclear fission, which is the sudden splitting of atomic nuclei into two or more smaller nuclei of lighter elements. Fission can cause an explosion or, if contained, a sustainable reaction useful for generating power (as well as various scientific purposes).

The concept of enrichment is kind of a metaphor based on milk. We call cream richer than skim milk because it contains more fat. Because it contains more fat it is lighter and naturally rises to the top, where it can be skimmed off and separated from the "whole" milk. We call certain isotopes richer because they produce more radiation. Because they have different numbers of neutrons per atom, different isotopes have different Atomic Weights and can be separated by weight just like the components of milk - under the right conditions.

I'm not sure how they make uranium, a solid metal, into something that acts like a liquid, but I do know that unlike milk it does not spontaneously separate by weight under normal gravity. In order to promote this separation they use a centrifuge, which is simply a wheel that holds the uranium and spins it very quickly. The material in the centrifuge wants to move in a straight line, and would do so if the wheel fell apart, but instead is continually redirected into a curved path. The force that the uranium exerts against the wheel in resisting this change of direction is called centrifugal force, and in many ways it acts like gravity, causing the uranium to separate into isotopes like milk separating into cream and skim milk. In this way, after many rounds of spinning and skimming, a more "rich" blend of isotopes can be extracted from the naturally occurring mix.

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