There are extensive gaps in the charge distribution data for fission, and in the chain yields for the more important fission reactions; there are also significant discrepancies between chain-yield measurements. Many fission products of importance in decay-heat calculations are short-lived, and their decay characteristics are either poorly defined or entirely unknown because of the difficulties associated with their direct study. Under such circumstances, sound theoretical extrapolation procedures and modelling techniques have been adopted to generate comprehensive fission-yield data sets, while various methods have been successfully explored to derive beta-strength functions and use these approximations to estimate half-lives and mean beta and gamma energies.

Isomeric states are normally low-lying metastable states (< 1 MeV) that occur when the angular momentum differences between this nuclear level and all lower levels are large. Electromagnetic transition probabilities are significantly reduced under such circumstances, and the lifetimes of the states are long (i. e., metastable). The half­lives of both the ground and metastable states can span many orders of magnitude (~1015 sec) due to variations in the form of P" decay (end-point energies and beta — strength functions). Over 150 of the fission products formed in the thermal-neutron fission of U, U and Pu have known isomeric states with half-lives > 0.1 sec.

These isomeric states play an important role in decay-energy release, since this time — dependent phenomenon depends on the relative populations between the ground and metastable states. Madland and England (1977) have developed a simple model to calculate the independent yield branching ratios between the ground and metastable states, and this approach has been extended by Rudstam et al (IAEA-CRP, 2000).