Experimental data are presented to show the conditions under which a complex fissile system may be conservatively approximated by a simpler system. The complex system consists of an unpoisoned uranium-solution slab in contact, on one face, with a thick region of heavily boronpoisoned uranium solution. The simpler system consists of an unpoisoned uranium-solution slab reflected, on one face, by Plexiglas. A calculated correction to yield a similar result for water is also presented. Use of this approximation will simplify a nuclear-safety engineer's evaluation of complex interacting fissile regions containing heavily poisoned and unpoisoned vessels.Measured critical thicknesses are reported for uranium-solution slabs unreflected, reflected on one face only, and reflected on both faces by Plexiglas. These data and calculations on infinitesolution slabs similarly reflected confirm that the critical height decreases linearly as the percent of the surface area reflected increases.