Critical experiments are carried out in order to validate, improve, and benchmark the extensive data calculations available. A series of such experiments was performed at the Los Alamos Criticality Experiments Facility. These experiments were performed to provide criticality safety data for waste matrix materials. These critical experiments were fueled with highly enriched uranium, moderated and reflected with polyethylene, and mixed with silicon dioxide (SiO2), aluminum (Al), magnesium oxide (MgO), and gadolinium (Gd). The uncertainties affecting the experiment were divided into three broad categories: mass measurement, geometry, and material composition. Each category is considered in turn, and then the total experimental uncertainty is derived. All four experiments had a measured keff of 1.001. The sensitivity analyses of these critical experiments yielded uncertainties in the measured keff of ±0.0026 for SiO2, ±0.0026 for Al, ±0.0021 for MgO, and ±0.0029 for Gd. These experiments were judged to be of benchmark quality.