When reactors are shut down, the nuclear chain reaction which releases energy – and thus heat – from the uranium atoms of fuel shuts down as well. But the fuel continues to release smaller amounts of energy from the radioactive decay of fissile fragments.
This “decay heat” amounts to about 6 percent of the total heat generated by the reactor core when it is running, according to a fact sheet on nuclear energy physics from the World Nuclear Association, an industry trade group.
This decay heat falls off quite rapidly. After one hour, it will equal 1.5 percent of the previous core power, and so on. But the amount of energy generated by the core can be so enormous to begin with that even this residual heat can be dangerous. After one year offline, used fuel still emits about 10 kilowatts of decay heat energy per ton. After 10 years, it emits 1 kW of heat per ton.
“The decay heat produced is significant ... and systems must be provided to keep the reactor cool even after shutdown,” says a US Department of Energy handbook on nuclear physics.
Source: csmonitor.com