What is heavy water? Why is it used in a nuclear power reactor?

Unlike ordinary water, heavy water contains heavy hydrogen, or deuterium, instead of normal hydrogen. Deuterium is an isotope of hydrogen that has a greater atomic mass than normal hydrogen. It is present in water in small amount. Nuclear reactors use heavy water as moderator, i.e. to slow down fast-moving neutrons. The basic concept of a nuclear reactor is simple: assemble a critical mass of fissionable material and bombard it with neutrons in order to break Uranium-235 (U-235) atom. For complicated reasons, the U-235 nuclei seek out slow moving neutrons and capture them. Because U-235 nuclei emit fast moving neutrons when they fission, most nuclear reactors contain some-thing else besides uranium. Along with uranium, they use a material called a moderator that slows the neutrons down, so that U-235 nuclei can grab them and break up afterwards, releasing more neutrons. A fast moving neutron from a fissioning U-235 nucleus enters the moderator, rattles around for about a thousandth of a second, and emerges as a slow moving neutron, one with only thermal energy left. It then induces fission in another U-235 nucleus. To be a good moderator, a material must simply remove energy and momentum from the neutrons without absorbing them. When a fission neutron leaves a good moderator, it has only thermal energy left. The best moderators are nuclei that rarely or never absorb neutrons and don't fall apart during collisions with them. Water, heavy water and graphite are the best moderators for nuclear reactors. They slow neutrons down to thermal speeds without absorbing many of them. Of these moderators, heavy water slows the neutrons quickly yet doesn't absorb them at all. However, heavy water is expensive because only 0.015% of hydrogen atoms are deuterium and separating that deuterium from ordinary hydrogen is difficult. The core of a typical thermal fission reactor consists of small uranium oxide fuel pellets, separated them at all. However, heavy water is expensive because only 0.015% of hydrogen atoms are deuterium and separating that deuterium from ordinary hydrogen is difficult. The core of a typical thermal fission reactor consists of small uranium oxide fuel pellets, separated