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[[File:Decay heat illustration2.PNG|thumb|Reactor [[decay heat]] shown as % of thermal power from time of sustained fission shutdown using two different correlations. Due to decay heat, solid fuel power reactors need high flows of coolant after a fission shutdown for a considerable time to prevent [[Behavior of nuclear fuel during a reactor accident|fuel cladding damage]], or in the worst case, a full [[core melt accident|core meltdown]].]]
In power-generating operation, most of the heat generated in a nuclear reactor by its [[fuel rod]]s is derived from [[nuclear fission]], but a significant fraction (over 6%) is derived from the [[radioactive decay]] of the accumulated fission products; a process known as [[decay heat]]. This decay heat continues for some time after the fission [[chain reaction]] has been stopped, such as following a reactor shutdown, either emergency or planned, and continued pumped circulation of coolant is essential to prevent core overheating, or in the worst case, [[core meltdown]].<ref>{{cite web |url=http://www.ewp.rpi.edu/hartford/~ernesto/F2011/EP/MaterialsforStudents/Petty/Ragheb-Ch8-2011.PDF |title=Decay Heat Generation in Fission Reactors |first=M. |last=Ragheb |website=University of Illinois at Urbana-Champaign |date=22 March 2011 |access-date=26 January 2013 |archive-url=https://web.archive.org/web/20130514074247/http://www.ewp.rpi.edu/hartford/~ernesto/F2011/EP/MaterialsforStudents/Petty/Ragheb-Ch8-2011.PDF |archive-date=14 May 2013 |url-status=dead }}</ref> The [[RBMK]] reactors like those at Chernobyl use water as a coolant, circulated by electrically driven pumps.<ref>{{cite web |title=DOE Fundamentals Handbook – Nuclear physics and reactor theory |volume=1 of 2, module 1 |page=61 |publisher=United States Department of Energy |date=January 1996 |url=http://energy.gov/sites/prod/files/2013/06/f2/h1019v1.pdf#page=85.5 |access-date=3 June 2010|url-status=dead |archive-url=https://web.archive.org/web/20140319145623/http://energy.gov/sites/prod/files/2013/06/f2/h1019v1.pdf#page=85.5 |archive-date=19 March 2014 }}</ref><ref>{{cite web |title=Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition (NUREG-0800) |website=United States Nuclear Regulatory Commission |date=May 2010 |url=https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/ |access-date=2 June 2010 |archive-url=https://web.archive.org/web/20100619163526/http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/ |archive-date=19 June 2010 |url-status=live }}</ref> The coolant flow rate is considerable
In case of a total power loss at the station, each of Chernobyl's reactors had three backup [[diesel generator]]s, but they took 60–75 seconds to attain full load<ref name="MedvedevZ">{{Cite book |last=Medvedev |first=Zhores A. |author-link=Zhores A. Medvedev |title=The Legacy of Chernobyl |publisher=W.W. Norton & Company |year=1990 |isbn=978-0-393-30814-3 |edition=First American}}</ref>{{rp|15}} and generate the 5.5{{nbhyph}}[[megawatt]] output required to run one main pump.<ref name="MedvedevZ"/>{{rp|30}} In the interim, special counterweights on each pump would enable them to provide coolant via inertia, thereby bridging the gap to generator startup.<ref>{{cite web |url=http://accidont.ru/rotor.html |title=Turbogenerator Rundown |last=Dmitriev |first=Viktor |date=30 November 2013 |website=Причины Чернобыльской аварии известны |publisher=N/A |access-date=19 September 2021 |quote=На АЭС с реакторами РБМК-1000 используется выбег главных циркуляционных насосов (ГЦН) как самозащита при внезапном исчезновении электропитания собственных нужд (СН). Пока не включится резервное питание, циркуляция может осуществляться за счет выбега. С этой целью для увеличения продолжительности выбега, на валу электродвигателя –привода ГЦН установлен маховик с достаточно большой маховой массой. |archive-date=3 October 2021 |archive-url=https://web.archive.org/web/20211003020646/http://accidont.ru/rotor.html |url-status=live }}</ref><ref>{{cite web |url=http://reactors.narod.ru/rbmk/08_mcp.htm |title=Main Circulating Pumps |author=<!--Not stated--> |date=19 September 2021 |website=Справочник "Функционирование АЭС (на примере РБМК-1000)" |publisher=N/A |access-date=19 September 2021 |quote=Для увеличения времени выбега на валу электродвигателя установлен маховик. |archive-date=20 September 2021 |archive-url=https://web.archive.org/web/20210920212739/http://reactors.narod.ru/rbmk/08_mcp.htm |url-status=live }}</ref> However, a potential safety risk existed in the event that a station blackout occurred simultaneously with the rupture of a {{convert|600|mm|adj=on}} coolant pipe (the so-called Design Basis Accident). In this scenario the [[emergency core cooling system]] (ECCS) needed to pump additional water into the core, replacing coolant lost to evaporation.<ref name=insag7/> It had been theorized that the rotational momentum of the reactor's [[steam turbine]] could be used to generate the required electrical power to operate the ECCS via the feedwater pumps. The turbine's speed would run down as energy was taken from it, but analysis indicated that there might be sufficient energy to provide electrical power to run the coolant pumps for 45 seconds.<ref name="MedvedevZ"/>{{rp|16}} This would not quite bridge the gap between an external power failure and the full availability of the emergency generators, but would alleviate the situation.<ref name="NV Karpan: 312–13">{{Harvard citation no brackets|Karpan|2006|pp=312–313}}</ref>
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