Gentilly-2 facilities

Generating Station Characteristics

CANDU reactors

Every nuclear generating station in Canada uses CANDU (CANada Deuterium Uranium) reactors, which are based on technology designed and developed in Canada. Gentilly-2 is equipped with a CANDU-PHW reactor that uses pressurized heavy water as a reactor coolant and a moderator.

CANDU reactors offer many advantages. From a safety perspective, they have multiple containment barriers. Fuel supply is readily available, and the reactors can be refueled while online. Gentilly-2 and stations like it are designed, built, maintained and operated to guarantee maximum safety. The probability of a failure, defect or accident is extremely low.


CANDU reactors are fueled with natural uranium. The fuel used at Gentilly-2 is Canadian-mined and consists primarily of uranium-238 (99.3%) with a small amount of uranium-235 (0.7%). Only uranium-235 nuclei are fissionable.
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  • The concentration of uranium-235 is too low to create the chain reaction needed to operate a nuclear reactor. Therefore, the technology used at Gentilly-2 combines natural uranium with a heavy water moderator, which increases the efficiency of the nuclear fission.

Heavy water

Chemical composition

The chemical composition of heavy water is almost identical to that of ordinary water (H2O). However, its hydrogen atoms are heavy isotopes (deuterium) whose nuclei contain a neutron in addition to the proton found in every hydrogen atom.



  • In CANDU reactors like Gentilly-2, heavy water acts as a neutron moderator – it slows the results of nuclear fission to increase the probability of a sustained nuclear fission chain reaction.


  • Pressurized heavy water acts as a coolant – it absorbs the heat produced by fission and then heats the ordinary water that drives the steam generators.

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