Heavy water reactors use water, or deuterium oxide as a moderator. Deuterium oxide has a lower neutron absorption ratio than regular water, allowing the use un-enriched, natural uranium as fuel. Although most heavy water reactors are cooled with heavy water, the Swiss Lucens reactor was gas-cooled.
1. The Pressurized Heavy Water Reactor (PHWR)
Most examples of the PHWR are of the CANDU (CANada Deuterium-Uranium) design which originated in Canada. The PHWR is a radically different design from the light water reactor family because the fuel bundles of the reactor are contained within their own pressurized tubes rather than the reactor containing one large pressure as is seen in the BWR. This is because Canada lacked the large steel forging presses required to manufacture large pressure vessels and found it easier to make a series of smaller tubes, one for each fuel rod bundle. Surrounding pressure the tubes is the reactor core, and the control rods are inserted at a perpendicular angle. Each tube contains pressurized heavy water which flows around the fuel bundles within, serving as a moderator during the primary coolant loop. Heated primary coolant flows into a heat-exchanger within the steam generator where it causes the temperature of the surrounding fluid in the secondary coolant loop to heat up and turn into steam which turns the turbine. After the steam has condensed into water again, the secondary coolant is pumped into the bottom of the steam generator to repeat the cycle. The returning primary coolant from the heat exchanger is pumped into the pressure tubes to be heated up again by the fuel bundles.
Moderator Type: Heavy Water
Technology: Generation II
Existing Examples: Twenty-nine in several countries.
-It can use naturally occurring uranium without requiring further enrichment.
-It is more efficient in its fuel utilization than light-water moderated designs.
-It can be refueled without having to shut down the reactor.
-The resulting spent fuel can be stored in a more compact manner because it generates less heat when coming out of the reactor as it is less reactive.
-Heavy water is relatively expensive, even though this cost is partially offset by the reduced cost of fuel processing.
-Because the fuel for PWHR reactors is not enriched, fuel is used up at a faster rate.
-It generates higher volumes of spent fuel because of the greater frequency of refueling.
Variants: Advanced CANDU, CANDU 6
2. Heavy Water Gas Cooled Reactors (HWGCRs)
These reactor designs used heavy water as a moderator but used carbon dioxide as a coolant. Although there were experimental reactors of this type, such as the KS 150 and Lucens reactor, both of these experimental reactors proved to be fraught with problems. Since then, there have been no existing reactors of this type.