Most problems of nuclear criticality safety correspond to an array of fissile units arranged in a definite pattern in a surrounding medium of specified composition. For an infinite array, mathematical complications are greatly reduced. A suitable model consists of a purely mathematical portion, bulk material parameters, and boundary parameters.The model can be tied to experimental data at one point by modifying a single material parameter. If the tie-in with integral data is made for systems of nearly the same neutronics, strictly geometric differences may be handled with considerable confidence.Several significant reactivities are discussed including that corresponding to an extremum in the spacing of units.