Editing CANDU reactor (section)

=== Purpose of using heavy water ===
{{Further|nuclear reactor physics|nuclear fission|heavy water}}
[[File:Bruce-Nuclear-Szmurlo.jpg|thumb|right|[[Bruce Nuclear Generating Station]], operating eight CANDU reactors]]

Natural uranium is a mix of [[isotope]]s: approximately 99.28% [[uranium-238]] and 0.72% [[uranium-235]] by atom fraction. Nuclear power reactors are usually operated at constant power for long periods of time, which requires a constant rate of fission over time.  In order to keep the fission rate constant, the neutrons released by fission must produce an equal number of fissions in other fuel atoms.  This balance is referred to as "[[critical mass|criticality]]." Neutrons released by nuclear fission are fairly energetic and are not readily absorbed (or "captured") by the surrounding [[fissile material]]. In order to improve the capture rate, the neutron energy must be reduced, or "moderated", to be as low as possible. In practice, the lower energy limit is the energy where the neutrons are in thermal equilibrium with the moderator.  When neutrons approach this lower energy limit, they are referred to as "[[thermal neutron]]s."

During moderation it helps to separate the neutrons and uranium, since <sup>238</sup>U has a large affinity for intermediate-energy neutrons ("resonance" absorption), but is only easily fissioned by the few energetic neutrons above ≈1.5–2&nbsp;[[MeV]]. Since most of the fuel material is usually <sup>238</sup>U, most reactor designs are based on thin fuel rods separated by moderator, allowing the neutrons to travel in the moderator before entering the fuel again. More neutrons are released than the minimum needed to maintain the chain reaction; when uranium-238 absorbs neutrons, plutonium is created, which helps to make up for the depletion of uranium-235. Eventually the build-up of [[fission product]]s that are more neutron-absorbing than <sup>238</sup>U slows the reaction and calls for refuelling.

Light water makes an excellent moderator: the [[hydrogen|light hydrogen]] atoms are very close in mass to a neutron and can absorb a lot of energy in a single collision (like a collision of two billiard balls). However, light hydrogen can absorb neutrons, reducing the number available to react with the small amount of <sup>235</sup>U in natural uranium, preventing criticality. In order to allow criticality, the fuel must be [[enriched uranium|enriched]], increasing the amount of <sup>235</sup>U to a usable level. In [[light-water reactor]]s, the fuel is typically enriched to between 2% and 5% <sup>235</sup>U (the leftover fraction with less <sup>235</sup>U is called [[depleted uranium]]). Enrichment facilities are expensive to build and operate. They may also pose a [[nuclear proliferation|proliferation]] concern, as they can be used to enrich the <sup>235</sup>U much further, up to [[weapons-grade]] material (90% or more <sup>235</sup>U). This can be remedied if the fuel is supplied and reprocessed by an [[International Atomic Energy Agency|internationally approved]] supplier.

The main advantage of [[heavy water]] [[Neutron moderator|moderator]] over light water is the reduced absorption of the neutrons that sustain the chain reaction, allowing a lower concentration of fissile atoms (to the point of using unenriched natural uranium fuel). [[Deuterium]] ("heavy hydrogen") already has the extra neutron that light hydrogen would absorb, reducing the tendency to capture neutrons. Deuterium has twice the mass of a single neutron (vs light hydrogen, which has about the same mass); the mismatch means that more collisions are needed to moderate the neutrons, requiring a larger thickness of moderator between the fuel rods. This increases the size of the reactor core and the leakage of neutrons. It is also the practical reason for the calandria design, otherwise, a very large pressure vessel would be needed.<ref name="Basic CANDU Design">B. Rouben, [http://www.unene.ca/un802-2005/ben/BasicCANDUDesign.pdf "Basic CANDU Design"] {{Webarchive|url=https://web.archive.org/web/20110409011840/http://www.unene.ca/un802-2005/ben/BasicCANDUDesign.pdf |date=9 April 2011 }}, University Network for Excellence in Nuclear Engineering, 2005.</ref> The low <sup>235</sup>U density in natural uranium also implies that less of the fuel will be consumed before the fission rate drops too low to sustain criticality, because the ratio of <sup>235</sup>U to fission products + <sup>238</sup>U is lower. In CANDU most of the moderator is at lower temperatures than in other designs, reducing the spread of speeds and the overall speed of the moderator particles. This means that most of the neutrons will end up at a lower energy and be more likely to cause fission, so CANDU not only "burns" natural uranium, but it does so more effectively as well. Overall, CANDU reactors use 30–40% less mined uranium than light-water reactors per unit of electricity produced. This is a major advantage of the heavy-water design; it not only requires less fuel, but as the fuel does not have to be enriched, it is much less expensive as well.

A further unique feature of heavy-water moderation is the greater stability of the [[chain reaction]]. This is due to the relatively low binding energy of the deuterium nucleus (2.2&nbsp;MeV), leading to some [[n,2n|energetic neutrons]] and especially [[Photodisintegration|gamma rays]] breaking the deuterium nuclei apart to produce extra neutrons. Both gammas produced directly by fission and by the decay of [[Nuclear fission products|fission fragments]] have enough energy, and the half-lives of the fission fragments range from seconds to hours or even years. The slow response of these gamma-generated neutrons delays the [[Nuclear chain reaction#Timescales of nuclear chain reactions|response of the reactor]] and gives the operators extra time in case of an emergency. Since gamma rays travel for meters through water, an increased rate of chain reaction in one part of the reactor will produce a response from the rest of the reactor, allowing various negative feedbacks to stabilize the reaction.

On the other hand, the fission neutrons are thoroughly slowed down before they reach another fuel rod, meaning that it takes neutrons a longer time to get from one part of the reactor to the other. Thus if the chain reaction accelerates in one section of the reactor, the change will propagate itself only slowly to the rest of the core, giving time to respond in an emergency. The independence of the neutrons' energies from the nuclear fuel used is what allows such fuel flexibility in a CANDU reactor, since every fuel bundle will experience the same environment and affect its neighbors in the same way, whether the fissile material is uranium-235, [[uranium-233]] or [[plutonium]].

Canada developed the heavy-water-moderated design in the post–[[World War II]] era to explore nuclear energy while lacking access to enrichment facilities. War-era enrichment systems were extremely expensive to build and operate, whereas the heavy water solution allowed the use of natural uranium in the experimental [[ZEEP]] reactor. A much less expensive enrichment system was developed, but the United States classified work on the [[Zippe-type centrifuge#Centrifuge uranium enrichment|cheaper gas centrifuge]] process. The CANDU was therefore designed to use natural uranium.