What exactly is heavy water?
Heavy water and its importance to nuclear technology both have to do with isotopes of chemical elements.
Elements - the basic building blocks of chemistry, like hydrogen, oxygen, and uranium - are made up of single atoms. Atoms in turn, are made of a nucleus containing protons and neutrons, and a cloud of electrons loosely orbiting that nucleus.
Each element is characterized by its atomic number - how many protons are contained in the atom's nucleus. All hydrogen atoms contain one proton, all oxygen atoms contain eight protons, and all uranium atoms contain 92 protons.
While the number of protons is the same for all atoms of a particular element, different atoms can have different numbers of neutrons. Different isotopes of elements have different numbers of neutrons.
The most common isotope of hydrogen is called protium, or hydrogen-1. The 1 refers to the total number of protons and neutrons together in a particular isotope. Hydrogen-1 atoms are made up of a just a single proton and no neutrons.
A much rarer isotope of hydrogen is hydrogen-2, or deuterium. Hydrogen-2 atoms are made up of one proton and one neutron.
Water is made up of two hydrogen atoms and one oxygen atom, bonded together. In light water - by far the most abundant type of water in nature - the two hydrogen atoms are both of the hydrogen-1 isotope. In heavy water, the hydrogen atoms are both of the hydrogen-2 isotope.
The reason heavy water is important in some types of nuclear reactors also has to do with different isotopes. Uranium has two main naturally occurring isotopes - uranium-235, with 92 protons and 143 neutrons, and uranium-238, with 92 protons and 146 neutrons.
Those three neutrons make a huge difference. Uranium-238 cannot sustain a nuclear chain reaction, but uranium-235 can.
In naturally occurring uranium, there is vastly more of the uranium-238 isotope than of the uranium-235 isotope. The latter makes up only a fraction of a percent of the overall mass of the ore.
Most nuclear reactors require a much higher percentage of uranium-235 in order to maintain a nuclear reaction. This is where uranium enrichment is important - giant rows of centrifuges slowly increase the amount of uranium-235 in a sample of uranium, allowing it to be used in power plants or weapons.
However, some reactors that use heavy water can use unrefined uranium as a fuel, removing the expensive and time consuming enrichment process. These reactors also tend to produce more plutonium as a waste product that can be used in weapons.
This is why heavy water reactors are a concern, and why we are talking about heavy water this week.