Question

Why can a heavy-water moderated reactor use a lower enrichment uranium fuel than a light water moderated reactor?

Answer 1

Most commercial nuclear reactors use normal water (also called light water) as a neutron moderator. Some reactor designs, such as the CANDU reactor, use heavy water.  

A heavy water molecule contains deuterium (Hydrogen-2), which is an isotope of hydrogen that has a neutron in its nucleus in addition to the proton. The hydrogen in light water molecules contains only the proton (and is therefore called protium or Hydrogen-1).  

Deuterium is much less likely to absorb neutrons than protium. As a result, more of the neutrons in a heavy water reactor are available to be absorbed by uranium than in a light water reactor. The result is that the uranium in a CANDU reactor does not need to be enriched in U-235; natural uranium can be used as fuel. This in turn means that a heavy water reactor can produce more energy per unit of uranium mine.

Thats why heavy-water moderated reactor use a lower enrichment uranium fuel than a light water moderated reactor.