Heavy Water Technology
█ LARRY GILMAN
Heavy water is water (H 2 O) in which oxygen is bound to atoms of the hydrogen isotope deuterium ( 2 H). Heavy water is so named because it is significantly more dense (>1.1 g/cm 3 ) than ordinary ("light") water, 1 H 2 O (1 gm/cm 3 ). Heavy water is not radioactive and has the same chemical properties as light water; a person could drink a glass of heavy water without harm. However, heavy water is better than light water at moderating (slowing) neutrons, which makes it useful in some nuclear reactor cores. Its scarcity during World War II, partly assured by bombing raids and daring Allied commando missions to destroy heavy-water production facilities, interfered critically with the German and Japanese nuclear programs.
Deuterium and tritium. All hydrogen atoms have atomic number 1, that is, one proton in the nucleus; common or light hydrogen also has mass number 1, that is, its nucleus consists solely of a lone proton. Deuterium ( 2 H) has atomic number 1 and mass number 2, because its nucleus contains one proton plus one neutron. The presence of the neutrons in the deuterium atoms of heavy water is what makes it "heavy" (i.e., more dense than common water). Tritium ( 3 H) is an isotope of hydrogen whose nuclei contain one proton plus two neutrons. Tritium can also combine with oxygen to form heavy water, but tritium is much rarer than deuterium, so virtually all heavy water consists of 2 H 2 O (deuterium oxide). Tritium heavy water is radioactive and has been used as a tracer in certain biological experiments.
About .015% of the hydrogen atoms in natural water are deuterium atoms. Heavy water is produced by using electricity to break up water molecules, releasing its hydrogen as gas. (This process is known as electrolysis.) Deuterium oxide molecules are more resistant to electrolysis than light-water molecules, so electrolysis of a volume of water tends to increase its concentration of heavy water. By repeated concentration steps, almost pure heavy water can be obtained. Heavy water can also be extracted from natural water by repeated evaporation steps, as its heavier molecules are less volatile than those of light water (i.e., less likely to gain enough kinetic energy in random molecular collisions to leave the surface of a liquid mass). The electrolysis method was important during World War II, but evaporation methods are used today because they are less expensive.
Neutron moderation. The utility of heavy water in nuclear reactors arises from its ability to slow down or moderate neutrons. Slow or thermal neutrons are more likely to cause unstable nuclei (e.g., of uranium) to fission upon impact; however, neutrons emitted by fissioning nuclei generally have high velocities. To make a nuclear chain reaction sustainable, therefore, it is often desirable to slow down or moderate neutrons released by fissioning nuclei. Slowed-down neutrons are termed thermal neutrons, and reactors that employ a moderator to produce thermal neutrons are termed thermal reactors. (Other reactor designs are also possible.) Interposing a neutron-slowing substance or moderator between thin rods filled with nuclear fuel is a common feature of thermal reactor cores. Most of the neutrons released by fissioning nuclei in the fuel rods escape quickly from the thin rods and collide with atoms in the moderator before passing into other fuel rods; these collisions impart some of the neutrons' kinetic energy to atoms in the moderator. This heats the moderator, and some of the slowed neutrons go on to enter fuel rods and to cause nuclei to fission in them.
Several substances have been used as moderators in nuclear reactors, especially carbon (in the form of graphite), light water, heavy water, and beryllium. Heavy water is a desirable moderator for several reasons. It has excellent moderation properties and, being a liquid, can act simultaneously as a coolant to transfer heat out of the core to a power-generation loop.
Today, most power-generating reactors in the world utilize light water as a moderator. Light water has less desirable moderation properties than heavy water, but the fact that it is essentially free, while heavy water is expensive, gives it an advantage. However, one class of modern reactor—the Canadian CANDU (CANada Deuterium Uranium) reactor type—uses heavy water as a moderator. A CANDU reactor core consists of a stack of horizontal fuelrod assemblies immersed in a large holding tank full of heavy water that serves to reduce stray radiation in the vicinity of the unit. Hot heavy water circulates through tubes stacked between the fuel-rod assemblies, acting both to moderate neutrons in the core and to carry away heat energy. The circulating heavy water is under high pressure to keep it from flashing to steam. After being heated in the reactor core, it is passed through a heat exchanger, a device which allows hot water to circulate on one side of a thin metal barrier and relatively cool water to circulate on the other; heat is conducted through the metal from the hotter to the cooler water, which is then pumped away and allowed to expand into steam to drive turbines. The turbines, in turn, drive generators that make electricity.
Heavy water during World War II. During the early days of nuclear fission, in the 1930s and early 1940s, scientists struggled with what is today a routine task: the production of a sustained, controlled nuclear chain reaction in a reactor core. It took intense research to discover that a moderator was required at all. Graphite was known to be a good moderator, and some of the earliest nuclear reactors consisted of large piles of graphite blocks riddled with pellets of nuclear fuel. However, heavy water was easier to handle and had superior moderation properties; rapid progress in nuclear fission, given the state of knowledge at that time, required heavy water.
However, heavy water was rare. The only commercial producer of heavy water in the world in the late 1930s was Norsk Hydro, the state-owned Norwegian hydroelectric company. In 1940, the Germans invaded and occupied Norway, seizing the heavy-water production facility at Rjukan-Vemork, Norway. By 1942, U.S. intelligence was aware that the German nuclear research program was using heavy water produced using the electrolysis method at Rjukan-Vemork. In November 1942, British commandos (special forces trained to operate in small numbers behind enemy lines) attempted to land in Norway and destroy essential machinery at Rjukan-Vemork; they were all killed in crashes or captured and executed by the Germans. (Hitler had ordered that all captured commandos were to be shot.) In February 1943, a second commando raid was attempted. This raid succeeded in putting the Rjukan-Vemork heavy-water plant temporarily out of commission. All commandos involved escaped, and the German fission program was delayed by some months. However, the facility was repaired and put back into operation. In November 1943, a force of 460 U.S. bombers was dispatched from England to bomb the Norwegian plant. Not all essential heavy-water machinery at the site was destroyed, but the German government decided to move what was left, including whatever stocks of heavy water had been accumulated, to Germany, where they could be better defended. However, Norwegian resistance personnel succeeded in sinking the ferry that was to carry the precious barrels of heavy water across a lake on its way to Germany, further impeding German nuclear efforts. In the months remaining before the Germans were defeated they could not produce sufficient quantities of heavy water, and their nuclear program (which was mostly devoted to the goal of producing electricity, rather than a nuclear bomb) did not succeed. The extreme scarcity of heavy water in Japan was also a factor in that country's decision not to pursue development of nuclear explosives during World War II.
█ FURTHER READING:
Dahl, Per F. Heavy Water and the Wartime Race for Nuclear Energy. Bath, UK: Institute of Physics Publishing, 1999.
Glasston, Samuel, and Alexander Sesonske. Nuclear Reactor Engineering: Vol. 1, Reactor Design Basics. New York: Chapman & Hall, 1994.