THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

Figure 14.3: Boiling Water Reactor (BWR).
Appendix B: Nuclear Reactor Technology
14.3 Appendix B3: Pressurised Heavy Water Reactor (PHWR or CANDU)
The Pressurised Heavy Water Reactor (PHWR) is a reactor fuelled with natural uranium and
cooled and moderated with heavy water (deuterium, D2O). This reactor design is often called
CANDU (Canada Deuterium Uranium) since it was developed in Canada. There are 44 heavy
water moderated reactors based on the CANDU design in operation worldwide accounting about
10 % of the nuclear reactors installed.
The PHWR/CANDU design is similar to the PWR in that fission reactions inside the reactor core
heat coolant - heavy water in CANDU and normal (light) water in PWR - in the primary loop.
This loop is pressurised to prevent boiling and steam formation. As in a PWR, steam is generated
in a secondary coolant loop at reduced pressure to drive the turbine and generator. CANDU
overall thermal efficiency is typically about 31 %. A major difference is that, whereas the core and
moderator of a PWR are in a single large, thick-walled steel pressure vessel, the CANDU fuel
bundles and coolant are contained in some hundreds of horizontal pressure tubes penetrating a
large tank of heavy water moderator. A schematic drawing of a Pressurised Heavy Water Reactor
(PHWR) or CANDU is shown in Figure 14.4.
Because of the good neutron economy the CANDU reactor design can utilize natural uranium
dioxide (UO2) containing 0.7wt% U-235 as fuel. A CANDU fuel assembly consists of a number of
Zircaloy tubes containing ceramic fuel pellets arranged into a cylinder that fits within the
horizontal fuel channels in the reactor. The assemblies have between 28 and 43 half-meter long
fuel tubes lying end to end in a fuel channel.
One feature of the CANDU design is the online re-fuelling. New fuel assemblies are inserted in
one end of the fuel channel while old assemblies are unloaded from the opposite side.
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