[en] The accurate determination of minimum material thicknesses required for isolating multiplying media from neutron interaction is a problem of considerable interest in the design of shipping containers and storage vaults. The net amount of storage space required for nuclear materials can be effectively reduced and criticality controls, improved through the proper use of neutron-absorbing and -moderating materials near the fuel boundary. A series of critical approach experiments was conducted at the Battelle-Northwest Critical Mass Laboratory utilizing a split-table machine in order to measure effective isolation thickness values for several materials. Analytical methods were developed for investigating the effect of fuel geometry and material compositions upon the experimental results. As a result of the data analysis, a simplified experimental technique was developed for determining isolation thickness values from the reflector savings data. Also, it was determined that many of the currently acceptable isolation thickness values are low by at least a factor of two. It was found that the minimum isolation thickness values vary significantly with the cross-sectional area of the fuel system. Diffusion theory calculations indicated that the thickness of material necessary for isolating two critical fuel slabs of essentially infinite cross-sectional dimensions was approximately a factor of four greater than that required for the experimental system. Also, it was found that the minimum isolation thickness value of concrete varies significantly with the hydrogen concentration. 12 figures, 4 tables