Heat generated by the decay of radioactive fission products in a nuclear reactor after shutdown – termination of the chain reaction. The residual heat in the first seconds after shutdown amounts to about 5% of the power prior to shutdown. The residual heat in the fuel elements is equal to approx. 2 kW per tonne nuclear fuel after three years of decay time, i.e. about 1 kW per fuel element of a pressurized water reactor.