A few-group delayed neutron model based on a consistent set of decay constants

As part of an international effort, the Los Alamos National Laboratory has been asked to (1) determine if there is a set of dominant precursors that are common to all fissionable isotopes and all incident neutron energies, (2) expand the existing experimentally-measured few-group models commonly used in the nuclear industry into their 8-group equivalent using a consistent set of decay constants corresponding to these dominant precursors, and (3) formulate new group spectra for the equivalent 8-group model. In response to this request, LANL has calculated the theoretical delayed neutron yield for 14 different isotopes using three different incident neutron spectra (i.e., thermal, fast, and 14.1 MeV) using the current fission-yield and emission probability data found in ENDF-VI. An example of these results is shown in a figure in which the theoretical delayed neutron yields for the 271 precursors produced during thermal fission of {sup 235}U are plotted against the half-lives of the precursors. By comparing the results of all 14 isotopes, a preliminary set of precursors has been identified that are dominant within the various half-life regimes of the delayed neutron precursors. Also plotted on a figure are the group yields of the 8-group equivalent model of Keepin`s 6-group model. And finally, an example of the delayed neutron spectra for group 7 in the 8-group equivalent model is shown. A final report summarizing all results is expected to be released for review by the international steering committee by the summer of 1998