Copyright © Karl Dahlke, 2023
What is it like to swim in heavy water, or drink heavy water? First some background.
Water is H2O, two atoms of hydrogen and one atom of oxygen linked together into one molecule. This "linking" is done by electrons; in fact chemistry is done by electrons. The nucleus at the center of an atom influences chemistry hardly at all. Change the nucleus of a hydrogen atom, by adding a neutron for instance, and it still links up with oxygen to make water, because the electron, buzzing around the outer edge of the hydrogen atom, forms the bond.
Most of the hydrogen in the universe has one proton in its nucleus, but approximately one atom in 6,700 has a proton and a neutron. With two particles in the nucleus, this form of hydrogen is called deuterium. Electrons are extremely light, so deuterium is twice as heavy as hydrogen. Water made from deuterium, sometimes designated D2O, has two deuterium atoms and an oxygen atom. It is about 10% heavier than ordinary water, thus "heavy water". In fact ice made from heavy water will sink to the bottom of a glass of ordinary water.
You have just been transported to a cottage next to a lake of heavy water. What is life like?
As mentioned above, the chemistry of water is the same. You can wash your dishes and clothes as usual - even take a shower. Soap up and rinse off, you'll feel nice and clean. Then go swimming in the lake if you wish. It's easy; you can't help but float. Remain motionless, and your head is still above water. But you notice there is no life in the lake, no fish, no plants, not even algae. The water is crystal clear, down to the rocks and sand below. If you had a microscope you might see some bacteria in the water, but that's it.
Go back in the cottage and cook with the heavy water that is piped in from the lake. Make a pot of coffee if you like. The chemistry is the same. Then eat your dinner and drink your coffee. All is well, for a time, but after a few days something goes terribly wrong.
Organic chemistry, the chemistry of life, is subtle and delicate. Everything looks fine in a test tube, but not so in your cells. When 25% of the water in your body is D2O, cell division slows down. Life requires cell division, particularly in the bone marrow (producing blood cells), and the intestinal lining where cells turn over rapidly. Death occurs at 50% D2O concentration, though it is a lingering, unpleasant death, as though you were on high doses of chemotherapy.
If the chemistry is the same, what goes wrong? Well the chemistry is not exactly the same. Almost everything you do in a test tube involves ionic and covalent bonds, but life also uses something called a hydrogen bond, which is much weaker than the other two bonds. It joins large molecules together, gently, and it is central to life. Heavy water slows down these reactions just a bit, just enough to interfere with life processes, especially cell division.
Bacteria grow and multiply using a simpler process, and thus they are able to survive in 100% heavy water. They are a different kingdom of life.
You may have heard of Germany's heavy water experiments, in their efforts to build an atomic bomb. I first heard about this on Star Trek, City on the Edge of Forever. Spock reviews his tricorder readings of earth history, after McCoy changed it. He reports his findings to Kirk as follows.
“This is how history went after McCoy changed it. Here, in the late 1930's. A growing pacifist movement whose influence delayed the United States' entry into the Second World War. While peace negotiations dragged on, Germany had time to complete its heavy-water experiments. … All this lets them develop the A-bomb first. … With the A-bomb, and with their V2 rockets to carry them, Germany captured the world. … And all this because McCoy came back and somehow kept her from dying in a street accident as she was meant to.”
Several years later I studied the Manhattan project, our classified program to develop the atomic bomb. We didn't use heavy water. We didn't need heavy water. Was Germany barking up the wrong tree?
No. There are different ways to build the atomic bomb, and they are all difficult, especially in the context of 1940's technology. (stone knives and bear skins) One path, the path that we chose, requires enriched uranium. It took years of effort, at the secret labs in Oakridge Tennessee, to produce a handful of enriched uranium, just enough for a couple of bombs. On the other side of the world, Germany thought it would be easier to make heavy water, which obviates the need for enriched uranium. Perhaps, but that is also a daunting task. Germany constructed a laboratory in occupied Norway to produce heavy water, and it was making progress. Operation Gunnerside, a secret mission orchestrated by Norway, sabotaged the plant, and set Germany back several months in their efforts to build the bomb. Germany rebuilt the plant, but the allies won the war before they could complete their version of the bomb. These brave Norwegians, just 11 people acting by stealth, may have changed the course of the war. Read more about operation Gunnerside here, it's a fascinating story.