Nuclear meltdown: Difference between revisions

[[File:3MileIsland.jpg|thumb|right|[[Three Mile Island Nuclear Generating Station]] consisted of two [[pressurized water reactor]]s manufactured by [[Babcock & Wilcox]], each inside its own [[containment building]] and connected [[cooling tower]]s. Unit 2, which suffered a partial core melt, is in the background.]]

A '''nuclear meltdown''' ('''core meltdown''', '''core melt accident''', '''meltdown''' or '''partial core melt'''<ref>{{cite web|url=https://books.google.com/books?id=PqaRAAAAMAAJ&q=core+melt+accident|title=Reactor Safety Study: An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants|first1=U. S. Nuclear Regulatory|last1=Commission|first2=Norman C.|last2=Rasmussen|date=18 June 1975|publisher=W.S. Hein|via=Google Books}}</ref>) is a severe [[Nuclear and radiation accidents and incidents|nuclear reactor accident]] that results in [[nuclear reactor core|core]] damage from overheating. The term ''nuclear meltdown'' is not officially defined by the [[International Atomic Energy Agency]]<ref name="IAEASafetyTerminology">{{cite book|last=International Atomic Energy Agency (IAEA)|title=IAEA Safety Glossary: Terminology Used in Nuclear Safety and Radiation Protection|publisher=International Atomic Energy Agency|location=[[Vienna]], [[Republic of Austria|Austria]]|year=2007|isbn=978-92-0-100707-0|url=http://www-pub.iaea.org/MTCD/publications/PDF/Pub1290_web.pdf|access-date=17 August 2009}}</ref> or by the [[Nuclear Regulatory Commission|United States Nuclear Regulatory Commission]].<ref name="USNRC-MeltdownAndNuclearMeltdownUndefined">{{cite web|url=https://www.nrc.gov/reading-rm/basic-ref/glossary.html|title=Glossary|last=[[United States Nuclear Regulatory Commission]] (NRC)|date=14 September 2009|work=Website|publisher=[[Federal Government of the United States]]|pages=See Entries for Letter M and Entries for Letter N|location=Rockville, Maryland, USA|access-date=3 October 2009}}</ref> It has been defined to mean the accidental melting of the core of a [[nuclear reactor]],<ref>{{cite web|url=https://www.merriam-webster.com/dictionary/meltdown|title=Definition of MELTDOWN|website=merriam-webster.com}}</ref> however, and is in common usage a reference to the core's either complete or partial collapse.

A core meltdown accident occurs when the heat generated by a nuclear reactor exceeds the heat removed by the [[Cooling system (nuclear reactor)|cooling systems]] to the point where at least one nuclear fuel element exceeds its [[melting point]]. This differs from a [[fuel element failure]], which is not caused by high temperatures. A meltdown may be caused by a [[loss-of-coolant accident|loss of coolant]], loss of coolant pressure, or low coolant flow rate or be the result of a [[criticality excursion]] in which the reactor is operated at a power level that exceeds its design limits.

Once the fuel elements of a reactor begin to melt, the fuel cladding has been breached, and the nuclear fuel (such as [[uranium]], [[plutonium]], or [[thorium]]) and [[fission products]] (such as [[caesium-137]], [[krypton-85]], or [[iodine-131]]) within the fuel elements can leach out into the coolant. Subsequent failures can permit these radioisotopes to breach further layers of containment. [[Superheated steam]] and hot metal inside the core can lead to [[fuel-coolant interaction|fuel–coolant interactionsinteraction]]s, [[hydrogen explosion]]s, or [[water hammer|steam hammer]], any of which could destroy parts of the containment. A meltdown is considered very serious because of the potential for [[radioactive materialsmaterial]]s to breach all containment and escape (or be released) into the [[Natural environment|environment]], resulting in [[radioactive contamination]] and [[radioactive fallout|fallout]], and potentially leading to [[radiation poisoning]] of people and animals nearby.