The moderator, the substance that slows down neutrons, so they can be captured by uranium atoms, is usually graphite, hydrogen or some other light element. An isotope of hydrogen called deuterium is a particularly good moderator. A hydrogen atom has a single proton in its nucleus and a single electron. Deuterium has a nucleus of one proton and one neutron. Water H2O that is made from 2 deuteriums instead of hydrogens is called heavy water. The extra neutrons make heavy water weigh 10% more than regular water.
NRU is cooled and moderated with heavy water. It occurs naturally but only in tiny quantities. Separating heavy water from light water is a difficult process that makes heavy water an expensive material.
Heavy water has two excellent properties. Firstly, it does not absorb many neutrons. Because neutrons are what NRU is trying to make, the use of heavy water maximizes the number of useful neutrons in the reactor. Secondly, because heavy water still behaves like water, it can be circulated around with a pump and used to cool the reactor at the same time it moderates the neutrons.