As discussed above, the U. S.-designed P^^ and B^^ reactors require consid­erable enrichment of the uranium in order to overcome the relatively high ab­sorption of neutrons by the light-water coolant. This disadvantage can be overcome by using heavy water as a moderator and either heavy water or boil­ing light water as the coolant. If heavy water itself is used as the coolant, it is possible to operate with natural uranium. This is the principle adopted in the Canadian-designed CANDU (Canadian deuterium-uranium) reactors, which are illustrated in Figure 2.11.

CANDU reactors dispense with the massive thick-walled pressure vessel used in P^WRs and B^WRs; instead, the fuel elements are placed in horizontal pressure tubes constructed from zirconium alloy. These pressure tubes pass through a calandria filled with heavy water at low pressure and temperature. In the CANDU reactor, heavy-water coolant is also passed over the fuel elements at a pressure of approximately 90 bars (1400 psia). It then passes to a steam generator, which is very similar to that used in the P^^ (see Figure 2.7). It should be noted that CANDU reactors have not experienced the same steam generator problems as the P^WRs, possibly because of the lower operating tem­perature on the primary side. The fuel elements consist of bundles of natural

Pnt^uurtsed heavy ^water reactor (CANDU)

End v^v inside pressure tube

— ZireolO>’ end support elate

Figure 2.11: (a) Heavy water-moderated and —cooled CANDU reactor; the fuel is natural uranium oxide; (b) end and side views of a horizontal pressure tube.

U02 pellets clad in zirconium alloy cans; individual bundles are about 50 cm long, and about 12 such bundles are placed in each pressure tube. The average volumetric power density in a CANDU core is approximately one-tenth that in a P^^ (since the moderator volume is taken into account in calculating the av­erage volumetric power density) and nearly four times that in an AGR.

However, the fuel rating is comparable to that in a P^WR Furthermore, the fuel is very much cheaper since natural uranium can be used. Although the CANDU has operated with remarkable success, difficulties have been experienced with hydriding of the zirconium alloy pressure tubes, necessitating their replacement in some cases. Even though it has a lower fuel cost, CANDU needs considerable amounts of expensive heavy water, which makes its capital cost high.