Dear Science,

Despite going into the social sciences, I've always been fascinated by nuclear physics. You mentioned heavy water in a recent post on Slog, and it made me want to ask some questions about the stuff: How is it made? Is it toxic at all—that is, could I drink it and notice any difference? Is it actually heavier than "normal" water? If it wouldn't kill me to drink the stuff, is it expensive and something I could buy a gallon of in order to impress my geek friends?

If you want to build a backyard nuclear reactor, heavy water is your best bet. It's all about the hydrogen. Water—heavy or light—comprises an oxygen atom and two hydrogen atoms. Most hydrogen nuclei are just lonely little protons. (If you know who Ronald McDonald is, but don't know this about hydrogen, please go back in time and slap yourself.) Rarely, a neutron or two accompanies the proton in hydrogen—doubling or tripling its weight. Heavy water is just regular old water, in which at least one of the hydrogen atoms has a neutron.

So, is heavy water actually heavier than regular water? Yep, but not by much. You drink heavy water all the time. It's present in all water in teeny, tiny amounts; 1 in about 3,000 water molecules is actually heavy water. Since the chemical properties of the heavier hydrogen-nucleus-with-a-neutron are slightly different, heavy water starts to gum up all manner of body parts. Eventually, if you drank enough purified heavy water—more than 20 gallons, at least a quarter heavy—you'd die. A disgruntled Canadian tried to kill his coworkers this way. He wasn't all that successful.

Making heavy water is all about separating out the heavy water from the light, through distillation or some other trickery. Given how close chemically heavy water is to light, this is hideously expensive. Only a few plants around the world can even do it. Buying five liters of heavy water where both hydrogen atoms have the extra neutron would cost you about $2,000. On the plus side, you could impress your geek friends with sinking heavy-water ice cubes.

Okay, but I promised you a backyard nuclear reactor. If you want to start up a nuclear chain reaction, you need to purchase plutonium (difficult, as even the Libyans aren't selling any more), enrich uranium for its more fissionable isotope 235, or use unenriched uranium with heavy water. Given the choice between enriching uranium—which has a tendency to explode violently when piled too high—or enriching water (for heavy water), the sensible among us choose the latter.

Incendiarily Yours,

Science

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