On the Discrepancy Between Measured and Calculated Central Reactivity Coefficients

Systematic errors responsible for the large discrepancy between the measured and calculated central reactivity coefficients were examined. These errors were narrowed to two sources: the normalization integral (or perturbation denominator) and the conversion factor of inhour, or dollars, to Δk/k units. The magnitude of both sources of error is uniquely determined by the ratio of the measured-to-calculated normalization integral when the measurement is carried out using the ²⁵² Cf source-reactivity method. The measured-to-calculated normalization integral ratios for ZPR-6 Assemblies 6A and 7, two typical demo-plant-size Liquid Metal Fast Breeder Reactor criticals, were 1.19 and 1.21, respectively. The magnitude of this discrepancy is essentially the same as that found for the central reactivity coefficient. Analysis of the available fission rate distribution in both assemblies indicates that the calculated normalization integral may be underestimated by 6 to 8% and that the remainder of 10 to 14% must come from the conversion factor. The delayed-neutron data of Krick and Evans, when used with the appropriate average number of neutrons per fission in each assembly, yield conversion factors 9 to 13% higher than the delayed-neutron data of Keepin. This would provide the explanation of the central reactivity discrepancy. Unfortunately, the method of calculating β_eff could also produce errors of this magnitude even if one has an absolutely correct set of delayed-neutron fractions. More definitive measurements of the delayed-neutron fractions of pertinent isotopes, as a function of the incident neutron energy, are needed. In addition, measurements of β_eff in various assemblies by different methods are required.