A series of experiments was performed at the Batelle-Pacific Northwest Laboratories on water-flooded arrays of Fast Test Reactor fuel elements (PUO2-UO2) intermixed with solid neutron absorbers. The objective of these experiments was to provide reliable experiment data that could be used to benchmark computer codes and calculational schemes commonly used in criticality analysis of such systems. The neutron absorbers used were cadmium and Boral plates and gadolinium cylindrical rods. Critical array sizes were determined for square lattice pitches ranging from 9.7 to 24.9 mm, corresponding to water-to-fuel volume ratios of 3.5 to ∼31 (hydrogen atom to fissile atom ratios of 58 to 473). For both systems, poisoned and unpoisoned, the minimum number of rods required for criticality occurred at a lattice pitch of ∼20.5 mm, which also corresponds to a water-to-fuel volume ratio of 20.5. The smallest critical number of rods for the unpoisoned array was 157. Boral was the most effective absorber, irrespective of degree of moderation or its position in the assembly. However, all three absorbers varied in degree of effectiveness with moderation.