Analysis of Central Reactivity Worths in Fast Critical Assemblies

An integral transport method was developed and applied to the calculation of the central reactivity worths of various samples measured in a 2700-liter uranium-carbide fast core and in a 4000-liter UO₂ fast core. The method was used to obtain corrections to multigroup first-order perturbation calculations, incorporating self-shielding effects inside and outside the sample, as well as scattering, fission, and edge-effect perturbations. Resonance interaction between a ²³⁸U sample and the ²³⁸U in the core as well as resonance self-shielding in the sample itself were explicitly considered. A similar treatment was used for ²³⁵U samples. ENDF/B data were used in all of the analyses. The calculated central worths of ¹⁰B and tantalum were in very good agreement with the measured values, indicating that the calculated value of β_eff used to convert percent Δk/k to period measurements was reasonably accurate. The calculated worths of ²³⁵U were 5 to 10% greater than the measured values. The discrepancy between the calculated and measured values for ²³⁸U was 15 to 20%. The discrepancy for graphite in particular and scatterers in general was rather large, indicating that the distribution of the adjoint function is not adequately calculated.