AbstractA simple technique is described for computing the interaction in groupings of fissionable units. The units may be slabs, cylinders, or spheres and may be surrounded by a reflector. The validity of the approximations employed is discussed. The technique is illustrated by applying it to the specific example of calculating the interaction in a reflected cubic array of eight spheres of uranium (93.5% U235). Comparisons are made with experiment and with other methods of calculation.