Decay heat
Shutting a reactor down stops the chain reaction in about two seconds, but not the heat. Fission products in the fuel keep decaying, releasing roughly 6% of full thermal power immediately after shutdown, about 1% an hour later, and a few tenths of a percent a week later. On a reactor the size of Taishan nuclear power plant 1-1 that last figure is still around 12 MW of heat with the reactor switched off, which is why cooling has to continue for years and why losing it is the mechanism behind most of the accident record.
Where the heat comes from
Fission does not just split a nucleus into two pieces and stop. The fragments are themselves unstable, wildly neutron-rich isotopes that decay through a chain of further transformations, each one releasing energy. A reactor that has been running for a year holds an inventory of these fission products built up over that whole period, and the chain reaction has nothing to do with them. Dropping every control rod removes the fissions; it leaves the inventory exactly where it was.
What that means in megawatts
Percentages hide the scale. Taishan nuclear power plant 1-1 is the largest unit on this site by thermal output, at 4,590 MWt. Apply the standard decay curve to it:
| Time after shutdown | Share of full power | Heat still produced |
|---|---|---|
| 1 hour | 1.3% | 60MWt |
| 1 day | 0.55% | 25MWt |
| 1 week | 0.27% | 12MWt |
Percentages are indicative values from the standard decay curve for a core at the end of a normal irradiation cycle; the exact figure depends on how long the unit had been running and at what power. The megawatt column is that percentage applied to Taishan nuclear power plant 1-1's stated thermal output.
Why this is the accident mechanism
A reactor that loses its cooling does not need anything to go wrong with the chain reaction. The chain reaction can be entirely, correctly, permanently stopped and the fuel will still heat up, because nothing is removing the decay heat. Given enough time without cooling the cladding oxidises, releasing hydrogen, and the fuel melts.
Three of the accidents on this site followed exactly that path. In none of them was the reactor critical when the fuel was damaged.
- Three Mile Island Unit 2 1979-03-28, INES 5, accident with wider consequences.
- Saint-Laurent A2 fuel melt 1980-03-13, INES 4, accident with local consequences.
- Fukushima Daiichi 2011-03-11, INES 7, major accident.
It does not stop when the fuel leaves the reactor
Spent fuel carries its decay heat out of the core with it, which is why it goes into a pool rather than a warehouse, and why it stays there for years before it is cool enough for dry cask storage. Every step of the back end of the fuel chain, from interim storage onward, is shaped by the need to keep removing heat from material that nobody is using any more.
It is also the reason a unit reading 0% on the daily status feed is not an idle machine. It is a machine still being actively cooled.
Sources
- Definition: US NRC glossary, Decay heat
- ANSI/ANS-5.1, Decay Heat Power in Light Water Reactors · the source of the percentage curve used here; the standard itself is paywalled, so the link is to its listing
- Thermal output: Global Nuclear Power Tracker, Global Energy Monitor, August 2026 release , CC BY 4.0
How these sources are used, including the corrections applied on top of them and what is deliberately left blank · download the underlying data