Reactor noise techniques have been used to determine how the quantity Beff/I changes when water regions of several sizes and shapes are introduced into a light water lattice of plutonium-enriched rods. The measured value of Beff/l for the unperturbed uniform lattice was 89 ± 2 sec−1. This value was reduced significantly as successively larger water regions were introduced into this lattice. Calculated values of Beff/l obtained with a typical reactor design calculational method, agree with measured results to within ±5% for these experiments.