The intra-cell structure of the fast neutron flux has been measured in several TRX lattices with 238U (n, fission) and Al (n, α) detectors. The lattices were light-water-moderated, with cylindrical, 0.387-in.-diam fuel rods of slightly enriched uranium. The fuel rods were arranged in hexagonal arrays, with H2O:U volume ratios of 1.0, 2.35, 4.02, and 8.1. Measured activation shapes and integral fast advantage factors were compared with calculated results obtained with the MOCA2A Monte Carlo program. Agreement was very good. A one-group Monte Carlo calculation and a one-group collision-probability model were found to perform well in comparison with MOCA2A.