|
|
| Fig. 1: Fig. 1: A CANDU reactor design - 1- Fuel Bundle, 2 Calandria, 3 Adjuster Rods, 4 Heavy Water pressure reservoir, 5 Steam generator, 6 Light Water Pump, 7 Heavy Water Pump, 8 Fueling Machines, 9 Heavy Water Moderator, 10 Pressure Tube, 11 Steam going to steam turbine, 12 Cold Water returning from turbine, 13 Containment Building. (Source: Wikimedia Commons) |
Heavy water reactors differ from more common nuclear power sources by their neutron moderator: heavy water. [1] They are less often discussed because light water reactors dominate. Heavy water offers better neutron economy because it has a more favorable neutron absorption cross section than light water, preserving neutrons needed to sustain fission. Chemically, heavy water replaces hydrogen with the isotope deuterium, making it slightly heavier and giving it distinct physical properties. [1]
A common reactor type that uses heavy water as a neutron moderator is the CANDU reactor (Fig. 1). CANDU stands for Canadian Deuterium Uranium. [2] This technology was developed by scientists who arrived in Canada in 1942 during the Second World War. [2] The CANDU reactor is a pressurized heavy water reactor (PHWR) design. A key significance of PHWRs is that they can be fueled with natural (unenriched) uranium. Natural uranium mined from the ground is about 0.7% U-235 and 99.3% U-238. In light-water reactors, this fissile-to-nonfissile ratio is insufficient to sustain a chain reaction, so the uranium must be enriched. This is an expensive process that increases the concentration of fissile isotope U-235. Because this enrichment barrier can be avoided, heavy water reactors can be attractive in some contexts.[1] Moreover, as of December of 2023, there are 46 PHWRs in operation generating 24.2 GW(e) around the world. [3]
In conclusion, heavy water reactors are an important alternative to the more widely used light water designs because their heavy water moderator enables efficient neutron economy and allows operation on natural (unenriched) uranium. Even so, adoption depends on more than physics and chemistry alone, since national infrastructure, economics, and regulatory or policy priorities strongly influence which reactor types are built and operated.
© Hayden Hamilton. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Yu, "CANDU Reactor Design," Physics 241, Stanford University, Winter 2017.
[2] C. Liekhus-Schmaltz, "The History and Current State of Canada's CANDU Nuclear Reactor," Physics 241, Stanford University, Winter, 2013.
[3] "Nuclear Power Reactors in the World," International Atomic Energy Agency, IAEA-RDS-2/44, July 2024.
