Few-group analysis of current delayed-neutron data

Recent improvements in the knowledge of fission product yields and decay data have made it possible to study the yield of delayed neutrons per fission based on the summation of individual precursor properties. Earlier analyses of delayed-neutron data were based on 20, 105, and later 110 individual precursor nuclides. A current set of delayed-neutron data has been compiled and evaluated for 272 fission product nuclides that have been identified as delayed-neutron precursors based on energetics and/or measured emission probabilities. All 272 precursors are used in the current analysis, although in most cases, ∼90% of the total delayed-neutron yield can be accounted for by 30 to 40 of the more important precursors. In the present analysis, data for the 272 precursors are used to calculate delayed-neutron activity following a prompt irradiation of 235U (fast fission). A least-squares analysis is performed on this exponentially decaying curve to produce few-group constants much in the manner in which Keepin et al. analyzed their experimental data in 1956. The analysis is performed for different few-group sets: 3, 6, 9, and 12 delayed-neutron groups and a comparison of the results is given