Editing CANDU reactor (section)

===Calandria and fuel design===
[[File:CANDU fuel bundles.jpg|right|thumb|Two CANDU fuel bundles: each is about 50&nbsp;cm in length and 10&nbsp;cm in diameter, and can generate about {{convert|1|GWh|lk=in|abbr=on}} of electricity during its time in a CANDU reactor]]

In conventional [[light-water reactor]] (LWR) designs, the entire fissile core is placed in a large [[pressure vessel]]. The amount of heat that can be removed by a unit of a coolant is a function of the temperature; by pressurizing the core, the water can be heated to much greater temperatures [[boiling point|before boiling]], thereby removing more heat and allowing the core to be smaller and more efficient.

Building a pressure vessel of the required size is a significant challenge, and at the time of the CANDU's design, Canada's heavy industry lacked the requisite experience and capability to cast and machine reactor pressure vessels of the required size. This problem is amplified by natural uranium fuel's lower fissile density, which requires a larger reactor core. This issue was so major that even the relatively small pressure vessel originally intended for use in the [[Nuclear Power Demonstration|NPD]] prior to its mid-construction redesign could not be fabricated domestically and had to be manufactured in Scotland instead. Domestic development of the technology required to produce pressure vessels of the size required for commercial-scale heavy water moderated power reactors was thought to be very unlikely.<ref name="Canada Enters the Nuclear Age">{{cite book|author1=Atomic Energy of Canada Limited|author2=Alastair S. Bain|author3=Frederic C. Boyd|author4=Eugene Critoph|author5=Maurice F. Duret|author6=T. Alexander Eastwood|author7=Charles E. Ells|author8=Ralph E. Green|author9=Geoffrey C. Hanna|author10=Robert G. Hart|author11=Donald G. Hurst|author12=Arthur M. Marko|author13=J.C. Douglas Milton|author14=David K. Myers|author15=Howard K. Rae|author16=J.A.L. (Archie) Robertson|author17=Benard Ullyett|author-link1=Atomic Energy of Canada Limited|title=Canada Enters the Nuclear Age: A Technical History of Atomic Energy of Canada Limited as Seen from Its Research Laboratories|date=1997|publisher=[[McGill-Queen's University Press]]|isbn=0773516018|language=en|jstor=j.ctt9qf2g1}}</ref>

In CANDU the fuel bundles of about 10&nbsp;cm diameter are composed of many smaller metal tubes. The bundles are contained in pressure tubes within a larger vessel containing additional heavy water acting as a moderator. This larger vessel, known as a calandria, is not pressurized and remains at lower temperatures, making it easier to fabricate. In order to prevent the heat from the pressure tubes from leaking into the surrounding moderator, each pressure tube is enclosed in a calandria tube. [[Carbon dioxide]] gas in the gap between the two tubes acts as an insulator. The moderator tank also acts as a large [[heat sink]] that provides an additional [[nuclear safety|safety]] feature.

In a conventional [[pressurized water reactor]], refuelling the system requires to shut down the core and to open the pressure vessel. In CANDU reactors, the tube being refuelled remains pressurized. This allows the CANDU system to be continually refuelled without shutting down, another major design goal. In modern systems, two robotic machines attach to the reactor faces and open the end caps of a pressure tube. One machine pushes in the new fuel, whereby the depleted fuel is pushed out and collected at the other end. A significant operational advantage of online refuelling is that a failed or leaking fuel bundle can be removed from the core once it has been located, thus reducing the radiation levels in the primary cooling loop.

Each fuel bundle is a cylinder assembled from thin tubes filled with ceramic pellets of uranium oxide fuel (fuel elements). In older designs, the bundle had 28 or 37 half-meter-long fuel elements with 12–13 such assemblies lying end-to-end in a pressure tube. The newer [[CANFLEX]] bundle has 43 fuel elements, with two element sizes (so the power rating can be increased without melting the hottest fuel elements). It is about {{convert|10|cm|in}} in diameter, {{convert|0.5|m|in}} long, weighs about {{convert|20|kg|lbs}}, and is intended to eventually replace the 37-element bundle. To allow the [[neutron]]s to flow freely between the bundles, the tubes and bundles are made of neutron-transparent [[zirconium alloy|zircaloy]] ([[zirconium]] + 2.5% wt [[niobium]]).