Delayed Neutron Data for Fast Reactor Analysis

The delayed neutron precursor decay curves which result from a pulse of fissions of the isotopes ²³³ U, ²³⁵U, ²³⁸U, ²³²Th, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, and²⁴²Pu are consistently fit to a single set of six isotope-independent decay constants. When delayed neutron yields based on a single set of group decay constants are employed in reactor transient analysis, it becomes possible to introduce macroscopic precursor production cross sections for each of the delayed neutron groups. The newly fitted data are tested by the analysis of a typical fast reactor transient; the analysis shows that the utilization of the new data in comparison to the use of original data results in an insignificantly small deviation, while permitting a considerable reduction of calculational effort. The new technique developed for the fitting is shown to preserve important integral kinetics parameters. The fitting technique also allows the recasting of original five group data into the more conventional six group structure.Recent measurements on the energy dependence of delayed neutron yields and emission spectra are briefly discussed. The lack of energy dependence of the total delayed neutron yield in the range of interest for fast reactor analysis leads to a modification of the definition of the effective delayed neutron fraction.