Pressurized Heavy Water Reactor (PHWR or CANDU)
The Pressurized Heavy Water Reactor is referred
to as the CANDU in Canada where it is used.
Figure 3 (Courtesy of the World Nuclear Association)
This design uses natural uranium oxide for fuel (only 0.7% uranium-235, as opposed to the enriched uranium oxide at 3% uranium-235) and D2O as a moderator ("heavy water" that is
made up of deuterium, or "heavy hydrogen", and oxygen instead of regular hydrogen. The economic trade-off here is that they don't have to enrich the uranium oxide, but they do have to enrich the water.
The CANDU is similar to the PWR design in that it has two water loops. The primary, radioactive water loop pumps the heavy water moderator through a large tank called a
calandria, instead of the more standard pressure vessel. The calandria is penetrated by several hundred horizontal pressure tubes that form channels where the fuel is contained, and control
rods are inserted vertically into the calandria. The calandria is cooled by the flow of the heavy water moderator, although this circuit cannot be seen in the diagram.
As in the PWR design, the water is pressurized to keep it from boiling as it reaches 290°C. The secondary, non-radioactive water loop carries steam from the steam generator to drive the turbine.
The advantage of the pressure tube design is that the reactor can be refueled by isolating individual pressure tubes, without having to shut the reactor down each time.
A secondary shutdown system involves adding gadolinium to the moderator.