Measured fast fission activations of depleted U238 foils in thin clean critical slabs are compared with theoretical predictions made using the P-1 and P-3 approximations. Various methods of analyzing fast activation experiments are considered. Finally, the effect of the P-3 approximation in the fast groups on the thermal flux is studied by comparing Mn wire activations out to 50 cm in the reflector of a thin clean critical slab with several theoretical calculations. It is found, as would be expected, that the deviations of the calculated P-1 activations from experiment increase as the observer moves farther out into the reflector. The use of the P-3 approximation gives marked improvement. As regards eigenvalues, experience in the analysis of thin clean critical slabs with highly enriched fuel and metal-to-water volume ratios from 1 to 1.7 indicates that use of the P-3 approximation in the first few fast groups results in an increase in eigenvalue of more than a per cent.