Evaluated Delayed Neutron Spectra and Their Importance in Reactor Calculations

Delayed neutron emission spectra from thermal-neutron fission of 233U, 235U, 239Pu, and 241Pu, from fast-neutron fission of 232Th, 235U, 238U, and 239Pu and from high-energy neutron (14.7-MeV) fission of 235U and 238U, for six groups of delayed neutrons are evaluated. The evaluation is based on recent measurements of delayed neutron spectra from 20 fission product isotopes. The data are presented in graphic form and are compared to directly measured equilibrium spectra whenever available. Tables with a convenient 54-energy-group structure are provided to facilitate their use in reactor calculations. The results of a limited number of two-dimensional, multigroup, transient calculations for the Clinch River Breeder Reactor core, using the newly evaluated spectra, are compared with calculations using some older spectra. The importance of the inclusion of these data in reactor dynamic calculations is evaluated.