Pressurised heavy-water reactors do have some drawbacks. Heavy water generally costs hundreds of dollars per kilogram, though this is a trade-off against reduced fuel costs. The reduced energy content of natural uranium as compared to enriched uranium necessitates more frequent replacement of fuel;{{citation needed|date=January 2022}} this is normally accomplished by use of an on-power refuelling system. The increased rate of fuel movement through the reactor also results in higher volumes of [[spent nuclear fuel|spent fuel]] than in LWRs employing enriched uranium. Since unenriched uranium fuel accumulates a lower density of [[nuclear fission product|fission products]] than enriched uranium fuel, however, it generates less heat, allowing more compact storage.<ref>{{cite book|url=http://books.nap.edu/openbook.php?record_id=11320&page=50 |title=An International Spent Nuclear Fuel Storage Facility - Exploring a Russian Site as a Prototype: Proceedings of an International Workshop |doi=10.17226/11320 |year=2005 |isbn=978-0-309-09688-1 |author1=National Research Council }}{{page needed|date=August 2019}}</ref> While deuterium has a ''lower'' neutron capture cross section than [[protium (isotope)|Protium]], this value isn't ''zero'' and thus part of the heavy water moderator will inevitably be converted to [[tritiated water]]. While [[tritium]], a radioactive isotope of hydrogen, is also produced as a fission product in minute quantities in other reactors, tritium can more easily escape to the environment if it is also present in the cooling water, which is the case in those PHWRs which use heavy water both as moderator and as coolant. Some CANDU reactors separate out the tritium from their heavy water inventory at regular intervals and sell it at a profit, however.

 

 

==Nuclear proliferation==