Editing Nuclear meltdown (section)

== Causes ==
Nuclear power plants generate electricity by [[Cooling system (nuclear reactor)|heating fluid]] via a nuclear reaction to run a [[Electrical generator|generator]]. If the heat from that reaction is not removed adequately, the fuel assemblies in a reactor core can melt. A core damage incident can occur even after a reactor is shut down because the fuel continues to produce [[decay heat]].

A core damage accident is caused by the loss of sufficient cooling for the nuclear fuel within the reactor core. The reason may be one of several factors, including a [[loss-of-pressure-control accident]], a loss-of-coolant accident (LOCA), an uncontrolled power excursion. Failures in control systems may cause a series of events resulting in loss of cooling. Contemporary safety principles of [[Defense in depth (nuclear engineering)|defense in depth]] ensure that multiple layers of safety systems are always present to make such accidents unlikely.

The containment building is the last of several safeguards that prevent the release of radioactivity to the environment. Many commercial reactors are contained within a {{convert|1.2|to|2.4|m|ft|adj=on}} thick pre-stressed, steel-reinforced, air-tight concrete structure that can withstand [[hurricane]]-force winds and severe [[earthquake]]s.
* In a loss-of-coolant accident, either the physical loss of coolant (which is typically deionized water, an inert gas, [[NaK]], or [[liquid sodium]]) or the loss of a method to ensure a sufficient flow rate of the coolant occurs. A loss-of-coolant accident and a loss-of-pressure-control accident are closely related in some reactors. In a pressurized water reactor, a LOCA can also cause a "steam bubble" to form in the core due to excessive heating of stalled coolant or by the subsequent loss-of-pressure-control accident caused by a rapid loss of coolant. In a loss-of-forced-circulation accident, a gas cooled reactor's circulators (generally motor or steam driven turbines) fail to circulate the gas coolant within the core, and heat transfer is impeded by this loss of forced circulation, though natural circulation through convection will keep the fuel cool as long as the reactor is not depressurized.<ref name="H&C-DPZF">{{cite book|title=Introduction to nuclear power|year=2000|publisher=Taylor & Francis|isbn=978-1-56032-454-6|chapter-url=https://books.google.com/books?id=YvLum7UFjK8C&q=depressurization%20fault&pg=PA133|author=Hewitt, Geoffrey Frederick|author2=Collier, John Gordon |access-date=5 June 2010|location=London, UK|page=133|chapter=4.6.1 Design Basis Accident for the AGR: Depressurization Fault}}</ref>
* In a loss-of-pressure-control accident, the pressure of the confined coolant falls below specification without the means to restore it. In some cases, this may reduce the [[heat transfer]] efficiency (when using an [[inert gas]] as a coolant), and in others may form an insulating "bubble" of steam surrounding the fuel assemblies (for pressurized water reactors). In the latter case, due to localized heating of the "steam bubble" due to decay heat, the pressure required to collapse the "steam bubble" may exceed reactor design specifications until the reactor has had time to cool down. (This event is less likely to occur in [[boiling water reactor]]s, where the core may be deliberately depressurized so that the [[emergency core cooling system]] may be turned on). In a depressurization fault, a gas-cooled reactor loses gas pressure within the core, reducing heat transfer efficiency and posing a challenge to the cooling of fuel; as long as at least one gas circulator is available, however, the fuel will be kept cool.<ref name="H&C-DPZF" />