page_title: Spent nuclear fuel
section_title: Spent fuel decay heat
hierarchical_section_title: Spent nuclear fuel / Spent fuel decay heat
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: Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and depending on its point along the nuclear fuel cycle, it may have considerably different isotopic constituents.
context_section_description: When a nuclear reactor has been shut down and the nuclear fission chain reaction has ceased, a significant amount of heat will still be produced in the fuel due to the beta decay of fission products. For this reason, at the moment of reactor shutdown, decay heat will be about 7% of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, and after a week it will be 0.2%. The decay heat production rate will continue to slowly decrease over time.
  Spent fuel that has been removed from a reactor is ordinarily stored in a water-filled spent fuel pool for a year or more (in some sites 10 to 20 years) in order to cool it and provide shielding from its radioactivity. Practical spent fuel pool designs generally do not rely on passive cooling but rather require that the water be actively pumped through heat exchangers.
caption: Decay heat as fraction of full power for a reactor SCRAMed from full power at time 0, using two different correlations
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