The Canadian designed Canadian deuterium uranium (CANDU) reactors used natural uranium as a fuel, by employing heavy water as both moderator and coolant. The first CANDU, NDP2-Rolphton of just 23 MW, entered commercial operation in 1962 and a number of 2, 3 and 4 unit plants evolved of commercial power capacity, individual units delivering power in the range 500-800 MW. A schematic of the Darlington PHWR is shown in Figure 1.5.

CANDU reactors consist of horizontal pressure tubes constructed with Zircaloy alloy. They pass through a large vessel (Calandria) filled with heavy water (deuterium oxide) at low pressure and temperature. Uranium oxide pellets are sealed in Zircaloy alloy cans, which are assembled in bundles or fuel assemblies. In a 500 MW plant, each bundle has

Steam

Steam

generator

neavy-water

(in calandria)

about 28 elements. There are about 4860 bundles in total with 12 or 13 such bundles in each pressure tube.

The heat generated is removed by heavy water at about 9 MPa, a sufficiently high pressure to prevent boiling. The water circulates around the fuel elements and passes to a steam generator, a similar principle to the PWR and BWR concepts.

The CANDU reactor is controlled by cadmium absorber rods. When fully inserted, these also provide the shutdown margin. In addition, the reactor can be shut down by voiding the cold heavy water from the reactor core. Vertical steel ‘adjusting’ rods are used to smooth out the sometimes uneven power distribution due to the use of different burn-up fuel segments, even within one fuel channel.

As for the PWR, the CANDU primary system is located in a concrete containment building, of sufficient strength to accommodate a large coolant system break within its design basis. Modern CANDUs are connected by valves to a large vacuum building. In the event of an accident these enable steam to pass from the affected containment to the vacuum building.

The CANDU reactor has a low volumetric power density, about 10 times lower than a PWR, despite fuel ratings that are comparable with PWR. It also has the lowest fuel costs because of utilisation of natural uranium. Against these cost benefits, the CANDU reactor needs considerable quantities of heavy water.

The CANDU reactor does have a number of advantages. It has on-load refuelling and hence has very high load factors. The plant has high availability and high reliability also. Since the design incorporates individual tubes, there is no requirement for a large pressure vessel. From a safety perspective, the reactivity excess is smaller than in reactors employing enriched fuel and hence power excursion transients are less likely.

In terms of disadvantages, the CANDU has a very large core (compared with a PWR or BWR) to achieve a similar power output.