page_title: Chernobyl disaster
section_title: Reactor cooling after shutdown
hierarchical_section_title: Chernobyl disaster / Background / Reactor cooling after shutdown
caption_attribution_description: English: Graph of Decay heat as fraction of full power from a SCRAMed operating power reactor. Two models are displayed in the graph, one being Retran, which uses a 11 group exponential decay, the other being Todreas and Kazimi from published work in 1990, which was adapted from previous empirical correlations. The Retran model inputs no assumptions of operating history, and the Todreas model requires the operating time before shutdown, which was assumed to be 2 years for this application.
context_page_description: The Chernobyl disaster was caused by a nuclear accident that occurred on Saturday 26 April 1986, at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR. It is considered the worst nuclear disaster in history and was caused by one of only two nuclear energy accidents rated at seven—the maximum severity—on the International Nuclear Event Scale, the other being the 2011 Fukushima Daiichi nuclear disaster in Japan.
  The accident started during a safety test on an RBMK-type nuclear reactor, which was commonly used throughout the Soviet Union. The test was a simulation of an electrical power outage to aid the development of a safety procedure for maintaining reactor cooling water circulation until the back-up electrical generators could provide power. This gap was about one minute and had been identified as a potential safety problem that could cause the nuclear reactor core to overheat. It was hoped to prove that the residual rotational energy in a turbine generator could provide enough power to cover the gap. Three such tests had been conducted since 1982, but they had failed to provide a solution.
context_section_description: In power-generating operation, most of the heat generated in a nuclear reactor by its fuel rods 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. The RBMK reactors like those at Chernobyl use water as a coolant, circulated by electrically-driven pumps.The coolant flow rate is considerable. Reactor No. 4 had 1661 individual fuel channels, each requiring a coolant flow of 28,000 litres (7,400 US gal) per hour at full reactor power.
  To guard against an electrical power failure to the pumps, each of Chernobyl's reactors had three backup diesel generators, but they took 60–75 seconds to attain full speed and generate the 5.5‑megawatt output required to run one main pump. This delay was considered a significant safety risk. It had been theorized that the rotational momentum of the steam turbines could be used to generate the required electrical power to cover this gap. Analysis indicated that this might be sufficient to provide electrical power to run the coolant pumps for 45 seconds, not quite bridging the gap between an external power failure and the full availability of the emergency generators, but alleviating the situation.
caption: 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 fuel cladding damage, or in the worst case, a full core meltdown.
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