Dominant Delayed Neutron Precursors to Model Reactivity Predictions for Multiple Fissioning Nuclides

The product of cumulative yield and probability of neutron emission is used to assess the relative importance of known delayed neutron precursors. Thirteen precursors are consistently dominant. Nonlinear fits to experimental delayed neutron decay data distinguish the decay constants of the three longest-lived dominant precursors: ⁸⁷Br, ¹³⁷I, and ⁸⁸Br. Sensitivity calculations based on a six- to seven- group transformation lead to a proposed seven-group formulation in which the group decay constants are those of dominant precursors: ⁸⁷Br, ¹³⁷I, ⁸⁸Br, ⁹³Rb, ¹³⁹I, ⁹¹Br, and ⁹⁶Rb. An alternative six-group formulation is obtained by using the mean of the ¹³⁷I and ⁸⁸Br decay constants for group 2. The use of the suggested dominant precursor decay constants improves the goodness of fit to experimental data compared to that obtained from nonlinear least squares in which both group yields and decay constants are determined empirically. Reactivity worth and transient analyses confirm that the positive reactivity scale is preserved in the transformation. A known bias in the negative reactivity scale is eliminated by forcing the half-life of the longest-lived group to be the 55.9-s half-life of ⁸⁷Br. The proposed use of dominant precursor decay constants offers significant simplifications in data analysis and the analysis of fast, epithermal, and thermal reactors with multiple fissioning nuclides.