A double cascade...
The products found in the spent fuel from reactors are no longer the fresh fragments from fission. In this example, a heavy fragment of xenon-143 through a series of intermediate nuclei, whose period is a fraction of a second to 14 days. After one minute, the fragment was transformed into lanthanum-143. After a few months, all kernels have reached the neodymium-143, a stable core of the chain terminus. The light fragment of bromine-90 is changing rapidly at first, but the waterfall is still pending with the fourth element, strontium-90 whose period is around 30 years. It will take about 10 times that long for the radioactivity of the fragment has completely disappeared. Strontium-90 is one of the main fission products found in the average life reactors.
IN2P3