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 short-lived 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