Aggregate decay behavior of fission products in nuclear reactors

There exist a lot of important quantities which have their origin in aggregate behavior of unstable fission products (FP) in nuclear reactors. Decay heat is a typical example of importance. These quantities are calculated by summing up all the contributions coming from each unstable nucleus accumulated during reactor operation by using of FP decay data libraries. In their infancy, however, they were suffered from so-called the pandemonium problem. For decades, the difficulty caused by pandemonium problem has been circumvented by introduction of a β-decay theory. In recent years, the total absorption gamma-ray spectroscopy (TAGS) is saving the situation remarkably even though leaving some open problems.