Residual heat : effects of radioactive decay
The residual heat produced by radioactive materials is a good indicator of the change in radioactivity that occurs after a reactor is shut down. In a reactor accident, fresh radioactive products are instantly dispersed by the explosion (at t=0). The most hazardous of these radioelements – iodine-131 – exerts its effects over a period of a few weeks. In contrast, waste stored on-site at a reprocessing plant has been out of the reactor for at least three years. The fission products in the waste have disappeared. Although the activity level of the stored radioactive materials remains very high, it has decreased by a factor of 800 to 2,000 since t=0. The main contributors are elements such as caesium-137.
Source: Clefs CEA