Heavy water reactors use natural uranium as fuel and heavy water as moderator. The coolant can be "heavy water" or ordinary boiling or pressurized water, gas, or organic liquid. In the most common variants the coolant circulates in pressure-holding tubes holding pressure, within which the fuel is placed. These force tubes cross the vessel containing heavy water. The fuel is uranium oxide [CFDT 80].

In France, the reactors Zoé or EL1 (Fontenay), EL2 and EL3 (Saclay) and EL4 or Brennilis (Finistère) , which are no longer in service, belong to this line. The difficulties encountered with EL2 "even well after its going critical in 1952, contributed to turning the programs [of the CEA] towards electricity generating reactors with graphite" [EnNu xii.65].