water with a lot of minerals in it

Heavy water is water in which the two hydrogen atoms have an extra neutron. This extra mass makes the water more dense and, thus, useful as a shield against nuclear radiation and to limit the reaction inside a fission reactor. Being water, it can be pumped from the reactor, cooled, and pumped back in again. It has other uses, as well, in the nuclear industry.

A type of water used in the production of nuclear fuels. Here's a link that explains it better than I can. http://www.fas.org/nuke/intro/nuke/heavy.htm

1) "Heavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O. Heavy water may contain as much as 100% D2O, and usually the term refers to water that is highly enriched in deuterium. The isotopic substitution with deuterium alters the bond energy of the hydrogen-oxygen bond in water, altering the physical, chemical, and especially biological properties of the pure or highly-enriched substance to a larger degree than is found in most isotope-substituted chemical compounds. Relatively pure heavy water was produced in 1933, soon after the discovery of deuterium, the stable heavy isotope of hydrogen. With the discovery of nuclear fission in late 1938 and the need for a neutron moderator which captured few neutrons, heavy water soon achieved importance in relation to early nuclear programs during World War II. Due in part to German reliance on scarce heavy water for reactor research in this war, Germany did not succeed in producing a functioning reactor during World War II. Since this war, heavy water has played a part in a number of reactor designs, both in designs for power and for nuclear weapon-making. Reactors which use enriched uranium, however, are able to use normal “light-water” for neutron moderation, and remain the most common type of reactor in use today. Heavy water itself is not radioactive, and has physical properties similar to water save for being about 11% more dense. When the water in eukaryotic organisms is replaced by more than about 25 to 50% heavy water, they experience toxicity due to interference by the deuterium with the mitotic apparatus of these cells. Higher organisms, including mammals, if given only heavy water, soon become ill and die at the point that about half their body water has been replaced. Bacteria, however, are able to grow slowly in pure heavy water. Small concentrations of heavy water are nontoxic. The adult human body naturally contains deuterium equivalent to the amount in about 5 grams of heavy water, and comparable doses of heavy water are still used as safe non-radioactive tracers for metabolic experiments in humans and other animals." Source and further concentration: http://en.wikipedia.org/wiki/Heavy_water 2) "The atoms of a chemical element can exist in different types. These are called isotopes. They have the same number of protons (and electrons), but different numbers of neutrons. Different isotopes of the same element have different masses. Mass is the word for how much substance (or matter) something has. Things with different masses have different weights. Because different isotopes have different numbers of neutrons, they do not all weigh the same or have the same mass. Different isotopes of the same element have the same atomic number. They have the same number of protons. The atomic number is decided by the number of protons. Isotopes have different mass numbers, though, because they have different numbers of neutrons. The word isotope, meaning at the same place, comes from the fact that isotopes are located at the same place on the periodic table. In a neutral atom, the number of electrons equals the number of protons. Isotopes of the same element also have the same number of electrons and the electronic structure. Because how an atom acts is decided by its electronic structure, isotopes are almost the same chemically, but different physically to their original atoms. Heavier isotopes react slower than lighter isotopes of the same element. This "mass effect" is large for protium (1H) and deuterium (2H), because deuterium has twice the mass of protium. For heavier elements, the relative mass difference between isotopes is much less, and the mass effect is usually small. Atomic nuclei are protons and neutrons held together by the strong nuclear force. Because protons have a positive charge, they push against other protons. Neutrons are electrically neutral, and they attract the protons more than the protons push against other protons. This makes the nucleus more stable. So, we need neutrons for more than two protons to be in a nucleus. As there are more protons, more neutrons are needed to have a stable nucleus. But, if there are too many neurtrons, the nucleus becomes unstable. In Nature some elements have just a single isotopes. For example, fluorine 19F is the only isotope of fluorine. Other elements have many isotopes. For example, xenon has 9 isotopes. Also, some isotopes are radioactive. These are called radioactive isotopes. Others are not radioactive. These are called stable isotopes. Hydrogen has three common isotopes. The most common isotope of hydrogen is called protium (1H). A hydrogen atom with an extra neutron (atomic mass of 2) is called deuterium(2H). Hydrogen with and two neutrons (atomic mass of 3) is called tritium(3H). Protium and deuterium are stable isotopes, while tritium is a radioactive isotope." Source and further concentration: http://simple.wikipedia.org/wiki/Isotope