A '''pressurized heavy-water reactor''' ('''PHWR''') is a [[nuclear reactor]] that uses [[heavy water]] ([[deuterium]] oxide D₂O) as its [[Nuclear reactor coolant|coolant]] and [[neutron moderator]].<ref>{{Cite web|date=2015|title=Pocket Guide Reactors|url=http://www.world-nuclear.org/uploadedFiles/org/WNA/Publications/Nuclear_Information/Pocket%20Guide%20Reactors.pdf|access-date=2021-12-24|website=World-Nuclear.org}}</ref> PHWRs frequently use [[natural uranium]] as fuel, but sometimes also use [[Enriched uranium#Low -enriched uranium (LEU)|very low enriched uranium]]. The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for a [[pressurized water reactor]] (PWR). While [[heavy water]] is very expensive to isolate from ordinary water (often referred to as ''light water'' in contrast to ''heavy water''), its low absorption of neutrons greatly increases the [[neutron economy]] of the reactor, avoiding the need for [[Isotope separation|enriched fuel]]. The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or [[nuclear fuel cycle|alternative fuel cycles]]. As of the beginning of 2001, 31 PHWRs were in operation, having a total capacity of 16.5 GW(e), representing roughly 7.76% by number and 4.7% by generating capacity of all current operating reactors.

[[File:U235 Fission cross section.png|thumb|right|450px|{{chem|235|U}} fission [[neutron cross section|cross section]] - while a [[nonlinear]]{{disambiguation needed}} relationship is apparent, it is clear that in most cases lower [[neutron temperature]] will increase the likelihood of fission, thus explaining the need for a [[neutron moderator]] and the desirability of keeping its temperature as low as feasible.]]

In addition, the use of heavy water as a moderator results in the production of small amounts of [[tritium]] when the [[deuterium]] nuclei in the heavy water absorb neutrons, a very inefficient reaction. Tritium is essential for the production of [[boosted fission weapon]]s, which in turn enable the easier production of [[thermonuclear weapon]]s, including [[neutron bomb]]s. This process is currently expected to provide (at least partially) tritium for [[ITER]].<ref>{{Cite journal|date=2018-11-01|title=Tritium supply and use: a key issue for the development of nuclear fusion energy|journal=Fusion Engineering and Design|language=en|volume=136|pages=1140–1148|doi=10.1016/j.fusengdes.2018.04.090|issn=0920-3796|last1=Pearson |first1=Richard J. |last2=Antoniazzi |first2=Armando B. |last3=Nuttall |first3=William J. |s2cid=53560490 |doi-access=free|bibcode=2018FusED.136.1140P }}</ref>