Close-packed storage of light water reactor (LWR) fuel assemblies is needed to expand the capacity of existing underwater storage pools. This increased capacity is required to store the large volume of spent fuel that arises from prolonged on-site storage. To provide benchmark criticality data in support of this effort, an experimental program sponsored by the U.S. Department of Energy was undertaken. Low-enriched UO2 fuel pins in a water-moderated lattice were used to construct 20 critical assemblies that simulated a variety of close-packed LWR fuel storage configurations. The critical assemblies consisted of nine LWR-type fuel assemblies (clusters) grouped in a radially reflected 3×3 array. Both the spacing and material between the fuel clusters were varied to provide numerous critical configurations. All pertinent data for each critical assembly are documented in sufficient detail to validate calculational methods according to the American National Standards Institute standard N16.9-1975. Criticality calculations using the Monte Carlo code KENO IV were performed for comparison with the experimental data. The comparison shows that the calculational model underestimates keff when separation between fuel clusters is >1 pin pitch (1.64 cm), and that the degree of underestimation increases as the spacing widens.