Editing CANDU reactor (section)

==Safety features==
The CANDU includes several active and passive safety features in its design. Some of these are a side effect of the physical layout of the system.

CANDU designs have a positive [[void coefficient]], as well as a small power coefficient, normally considered bad in reactor design. This implies that steam generated in the coolant will increase the reaction rate, which in turn would generate more steam. This is one of the many reasons for the cooler mass of moderator in the calandria, as even a serious steam incident in the core would not have a major impact on the overall moderation cycle. Only if the moderator itself starts to boil would there be any significant effect, and the large thermal mass ensures that this will occur slowly. The deliberately "sluggish" response of the fission process in CANDU allows controllers more time to diagnose and deal with problems.<ref name=safe>{{cite web|url = http://www.nuclearfaq.ca/cnf_sectionD.htm | title = Canadian Nuclear FAQ, Section D | work = The Canadian Nuclear FAQ by Dr. Jeremy Whitlock | access-date=  5 March 2005 }}</ref>

The fuel channels can only maintain criticality if they are mechanically sound. If the temperature of the fuel bundles increases to the point where they are mechanically unstable, their horizontal layout means that they will bend under gravity, shifting the layout of the bundles and reducing the efficiency of the reactions. Because the original fuel arrangement is optimal for a chain reaction, and the natural uranium fuel has little excess reactivity, any significant deformation will stop the inter-fuel pellet fission reaction. This will not stop heat production from fission product decay, which would continue to supply a considerable heat output. If this process further weakens the fuel bundles, the pressure tube they are in will eventually bend far enough to touch the calandria tube, allowing heat to be transferred into the moderator tank. The moderator vessel has a considerable thermal capability on its own and is normally kept relatively cool.<ref name=safe/>

Heat generated by fission products would initially be at about 7% of full reactor power, which requires significant cooling. The CANDU designs have several emergency cooling systems, as well as having limited self-pumping capability through thermal means (the steam generator is well above the reactor). Even in the event of a catastrophic accident and core [[Nuclear meltdown|meltdown]], the fuel is not critical in light water.<ref name=safe/> This means that cooling the core with water from nearby sources will not add to the reactivity of the fuel mass.

Normally the rate of fission is controlled by light-water compartments called liquid zone controllers, which absorb excess neutrons, and by adjuster rods, which can be raised or lowered in the core to control the neutron flux. These are used for normal operation, allowing the controllers to adjust reactivity across the fuel mass, as different portions would normally burn at different rates depending on their position. The adjuster rods can also be used to slow or stop criticality. Because these rods are inserted into the low-pressure calandria, not the high-pressure fuel tubes, they would not be "ejected" by steam, a design issue for many pressurized-water reactors.

There are two independent, fast-acting safety shutdown systems as well. Shutoff rods are held above the reactor by electromagnets and drop under gravity into the core to quickly end criticality. This system works even in the event of a complete power failure, as the electromagnets only hold the rods out of the reactor when power is available. A secondary system injects a high-pressure [[gadolinium nitrate]] neutron absorber solution into the calandria.<ref>{{cite web | url = http://www.nuclearfaq.ca/cnf_sectionA.htm#candu_control | title = Canadian Nuclear FAQ, Section A | work = The Canadian Nuclear FAQ by Dr. Jeremy Whitlock | access-date = 5 March 2005 | archive-url = https://web.archive.org/web/20131101054647/http://nuclearfaq.ca/cnf_sectionA.htm#candu_control | archive-date = 1 November 2013 | url-status = dead }}</ref>