The heat generated by decaying fission products and actinides once fission has stopped. It amounts to about 6 percent of full power at the instant of shutdown, falls to roughly 1 percent after an hour and about half a percent after a day, then declines slowly over months. For a large plant even 1 percent is tens of megawatts, which is why loss of all cooling after shutdown, not the chain reaction, causes core damage.
Core's been down 40 hours, decay heat's about 12 megawatts, so one RHR pump is plenty.
A reactor shut down a minute ago still makes enough heat to boil off its coolant; scramming solves the fission problem, not the heat problem.
Also heard as
- residual heat
- afterheat
Related terms
ECCS
Emergency core cooling system: the set of safety systems that inject water to keep the core covered after a loss of coolant.
SBO
Station blackout: complete loss of offsite and onsite AC power at a nuclear plant.
Spent fuel pool
A deep, water-filled pool at a reactor where used fuel is stored to cool and to shield its radiation.
Severe accident
An accident involving significant damage to the reactor core, with possible fuel melting and large releases.