13.52. The use of heavy water as a moderator instead of light water permits natural uranium to be used as the fuel. This freedom from the need for enriched uranium has made the concept attractive in several countries, especially in Canada where heavy-water moderated reactors have been developed and operated successfully in commercial power plants. Although other coolants have been suggested and used to some extent, the present discussion will be devoted to reactors of the CANDU (Canadian- Deuterium-Uranium) type in which heavy water is the coolant as well as the moderator.

13.53. A special feature of the CANDU (pressure-tube) reactors is that the heavy-water coolant and the heavy-water moderator are separated from each other forming two completely independent circuits. This is possible because heavy water is a poor neutron absorber; consequently, the moderator — to-fuel ratio can be higher than is acceptable in light-water reactors.* Hence, the fuel rods in a core moderated by heavy water are less tightly packed. Space is thus available for confining the heavy water coolant under pressure (to prevent boiling) in tubes containing the fuel; the moderator, which does not need to be pressurized, surrounds the pressure tubes.

A similar situation arises in graphite-moderated reactors.

STEAM

SEPARATORS

RPV

STABILIZER

LOW

PRESSURE FLOODER & SHUTDOWN COOLING SPARGER

TOP GUIDE

CONTROL ROD GUIDE TUBE

CORE

DIFFERENTIAL

PRESSURE

LINES

THERMAL

INSULATION

REACTOR

INTERNAL

PUMP

CONTROL ROD DRIVE HOUSING

FINE MOTION CONTROL ROD DRIVE