The storage of fissile solutions in tall cylinders neutronically isolated from each other by a moderating material, such as water or concrete, permits critically safe arrays that can be much more compact than arrays in air. Calculations show that up to ten times as many cylinders can be placed on a given floor area in such isolated arrays than in air-spaced arrays. However, this advantage must be weighed against the cost of construction and maintenance of moderated arrays. The water or concrete used as the isolating moderator also provide some personnel shielding against penetrating radiation.