In last night's episode of Eleventh Hour, a doe-eyed lad suffering from kidney cancer started drinking from a natural spring he discovered while fleeing a flash flood. he drank the stuff for three weeks, and during that time his advanced kidney cancer vanished, poof! Local news media told his story and soon the little Montana spring near his home was the destination for desperately sick people from all over the country. Our hero, Jacob Hood, FBI scientist, read about the miracle water and dashed to the scene to debunk the myth, for fear that sick people would skip their treatments in favor of the magic. Along the way he discovered that some domestic terrorists were trying to make a dirty bomb in the basement of the local hospital. To process their radioactive material, they needed heavy water. It was the heavy water that cured the boy. Heavy water is pretty much just like regular water, but instead of hydrogen atoms it has deuterium, which is to say, hydrogen atoms with an extra neutron. The resulting water is, literally, heavier than regular water, as Hood demonstrated when he made heavy water ice cubes and watched them sink to the bottom of a glass of water. The extra neutrons make it a very good tool for absorbing radiation and even reflecting it, which is why it's used in nuclear reaction. But making it in quantity is usually a huge process requiring industrial capabilities. But about one in every 5,000 water molecules is heavy. It has a higher boiling point than regular water, so it's actually feasible to distill heavy water in much the same way one would distill alcohol, as they did in the show, but it would take a quite a while to generate a usable amount with this method. As it happens, it does have some have some application to cancer treatment. Experiments in the 1980s on cancerous mice showed that heavy water increased their lifespans. The research into the uses of heavy water to slow the growth of cancer, kept producingpromising results, and in 2006, an Iranian official even used the medical benefits of drinking heavy water as an excuse for his country to make the stuff (they'd never use it for nuclear weapons, no no, never). But heavy water is actually toxic when drunk in quantity. Because of the deuterium, it slows down some chemical reactions, notably cell division. This makes it useful for slowing the growth of tumors, but dangerous to almost everything else in the body. It also does not yet appear to be a total cure, but more of a very expensive delaying tactic (there are few plants in the world producing the stuff, so it's pricey). By the way, the show tapped the perfectly rational human fear of a domestic terrorist blowing off a dirty bomb. But in the process of learning about heavy water, I discovered that producing, making, and moving a dirty bomb is nigh impossible. A dirty bomb is simply a conventional bomb with a lot of radioactive material strapped to it. Blow up the bomb, and the radioactive material scatters everywhere. Building the bomb isn't especially difficult, nor is, it seems, acquiring radioactive material. But gathering together enough radioactive substance to be dangerous poses it's own special challenges. If the bomb is going to scatter the material all over the place in sufficient quantity to really hurt people, it needs to be really radioactive. And before the bomb is set off, all that stuff is concentrated in one place: on the bomb, and in the perpetrators hands. That much material would probably kill the person before they could really do any damage. Putting enough shielding around the bomb to protect the carrier would make the bomb too heavy to move. This is why so few countries even try to build such a thing, and also probably why no such attack has ever happened.these problems may not be insurmountable, but they're enough to make it a serious challenge.