Results and Analysis of the Spherical ²³⁷Np Critical Experiment Surrounded by Highly Enriched Uranium Hemispherical Shells

An experiment to investigate the critical mass of ²³⁷Np was performed at the Los Alamos Critical Experiments Facility. The critical configuration consisted of a 6.07-kg neptunium sphere surrounded by 62.555 kg of highly enriched uranium hemispherical shells. The experiment was performed in order to decrease the large uncertainty in the critical mass of ²³⁷Np for criticality safety and nonproliferation issues. The critical configuration had an experimental k_eff of 1.003. Comparison of the experimental results with computational methods used to predict the k_eff of the system led to identification of a large discrepancy in the ²³⁷Np cross-section data from ENDF/B-VI used by the analysis performed with the MCNP code. In an effort to bound the uncertainty on the experimental k_eff, a sensitivity analysis was performed. This analysis systematically examines uncertainties associated with the critical experiment as they affect the calculated multiplication factor. The systematic analysis is separated into uncertainties due to mass measurements, uncertainties due to geometry of materials, and uncertainties due to impurities. Each type of uncertainty is analyzed individually, and a total combined uncertainty is derived. The sensitivity analysis on this experiment yielded a total combined uncertainty on the measured k_eff of ±0.0032.