The trick to achieving [[critical mass|criticality]] using only natural or low enriched uranium, for which there is no "bare" ''critical mass'', is to slow down the emitted neutrons (without absorbing them) to the point where enough of them may cause further nuclear fission in the small amount of ²³⁵U which is available. (²³⁸U which is the bulk of natural uranium is also fissionable with fast neutrons.) This requires the use of a [[neutron moderator]], which absorbs virtually all of the neutrons' [[kinetic energy]], slowing them down to the point that they reach thermal equilibrium with surrounding material. It has been found beneficial to the [[neutron economy]] to physically separate the neutron energy moderation process from the uranium fuel itself, as ²³⁸U has a high probability of absorbing neutrons with intermediate kinetic energy levels, a reaction known as "resonance" absorption. This is a fundamental reason for designing reactors with separate solid fuel segments, surrounded by the moderator, rather than any geometry that would give a homogeneous mix of fuel and moderator.

 

 

One complication of this approach is the need for uranium enrichment facilities, which are generally expensive to build and operate. They also present a [[nuclear proliferation]] concern; the [[dual-use technology|same systems]] used to enrich the ²³⁵U can also be used to produce much more "pure" [[weapons-grade]] material (90% or more ²³⁵U), suitable for producing a [[nuclear weapon]]. This is not a trivial exercise by any means, but feasible enough that enrichment facilities present a significant nuclear proliferation risk.

== Advantages and disadvantages==

[[File:U235 Fission cross section.png|thumb|right|450px|{{chem|235|U}} fission [[neutron cross section|cross section]] - while a [[nonlinear]] 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.]]

===Advantages===

The use of heavy water as the moderator is the key to the PHWR (pressurized heavy water reactor) system, enabling the use of natural uranium as the fuel (in the form of ceramic UO₂), which means that it can be operated without expensive uranium enrichment facilities. The mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons have lower energies ([[neutron temperature]] after successive passes through a moderator roughly equals the temperature of the moderator) than in traditional designs, where the moderator normally is much hotter. The [[neutron cross section]] for fission is higher in {{chem|235|U}} the lower the neutron temperature is, and thus lower temperatures in the moderator make successful interaction between neutrons and fissile material more likely. These features mean that a PHWR can use natural uranium and other fuels, and does so more efficiently than [[light water reactor]]s (LWRs). CANDU type PHWRs are claimed to be able to handle fuels including [[reprocessed uranium]] or even [[spent nuclear fuel]] from "conventional" [[light water reactor]]s as well as [[MOX fuel]] and there is ongoing research into the ability of CANDU type reactors to operate exclusively on such fuels in a commercial setting. (More on that in the article on the [[CANDU]] reactor itself)

===Disadvantages===

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.