Examination of the 2×2 SiO₂, Al, and Fe Experiments Mixed with Highly Enriched Uranium on the Thermal Energy Region

The basic characteristics of waste materials such as silicon dioxide, aluminum, and iron fueled with highly enriched uranium (HEU) and moderated and reflected by polyethylene were investigated. These critical experiments were performed at the Los Alamos Criticality Experiments Facility. The primary intention of these experiments is to provide supplementary data that can be used to validate and improve criticality data for the Yucca Mountain and the Hanford Storage Waste Tanks Projects. The secondary intention of the 2×2 experiments is to reduce the H/U ratio and increase the waste material/U ratio from previously published experiments. These experiments were designed to supply data for interlaced waste material/fuel/moderator systems on the thermal region. The experiments contained silicon dioxide (SiO₂), aluminum (Al), and iron (Fe) mixed with 93.23% enriched uranium and moderated and reflected by polyethylene. This analysis systematically examines uncertainties associated with the critical experiments as they affect the calculated multiplication factor. The systematic analysis is separated into uncertainties due to mass measurements, uncertainties due to fabrication, and uncertainties due to composition. Each type of uncertainty is analyzed individually, and a total combined uncertainty is derived. The SiO₂-HEU experiment had a measured k_eff of 0.993, the Al-HEU experiment had a measured k_eff of 0.990, and the Fe-HEU experiment had a measured k_eff of 1.000. The calculated k_eff values tend to agree well with the experimental values. The sensitivity analysis of these critical experiments yielded a total combined uncertainty on the measured k_eff of ±0.0044 for SiO₂, of ±0.0048 for Al, and of ±0.0046 for Fe.