Tuesday, January 29, 2013

Chain reactions; Enriching Uranium

When we started talking about fission, we needed to understand chain reactions, and of course the obvious example is with dominoes. We tried several different configurations and experimented with putting rulers in to simulate control rods.

Here's how I demonstrated the enrichment process for Uranium. This was all new to me, although naturally I'd heard of "enriched Uranium." Here's a good explanation of how the Uranium centrifuge works. By the way, there were some really terrible books about nuclear power at the library. I feel that Atoms and Molecules by Molly Aloian deserves special dishonorable mention for its reversal of U-235 and U-238 (it repeatedly referred to U-235 as the most abundant isotope, and U-238 as the one you need for fission----which is exactly WRONG), spelling errors (e.g. "canon" for "cannon"), and general hysterical tone (YOU DECIDE: "Is radioactive waste going to KILL US ALL?"). I really hate pseudo-scientific books like that. Nuclear energy seems to be a hot subject for it, unfortunately. But generally, I could get what information we needed from the books and leave out the other stuff, so it was okay.

Anyway, here's how it works with getting Uranium ready for fission.
First, the Uranium ore is in deposits all over the world. The yellow beads represent U-235, the rarer isotope, and the wood beads are U-238, the most abundant natural source. (U-235 is disproportionately represented here, actually. I think it's only 1-2% of the world's uranium.)

They mine the ore and gather it. (Um, this is a simplified explanation of the process. :))

Then they add chemicals to make it into a gas. The U-235 is sliiiiightly lighter, but gravity alone won't separate the two isotopes. They have to use a strong centrifugal force, which they do by spinning the gas in a centrifuge. The lighter U-235 tends to stay in the middle while the heavier 238 spins to the outer edges. You actually have to do this thousands of times before the separation becomes pronounced enough, so the gas goes from centrifuge to centrifuge, becoming more concentrated in U-235 each time.

Finally you have a solution that is mostly (or, MORE than before) composed of U-235. This is the enriched uranium----the pile of yellow beads. And you also have depleted Uranium, the U-238 left over. Both have their uses, but the enriched Uranium is what's used for nuclear power plants and for nuclear weapons.

And this video from The Onion is just for your enjoyment. :)

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