A series of experiments performed at the High Temperature Test Facility of the Bettis Atomic Power Laboratory indicated that placing lead in the water-reflector region of a water-moderated thermal reactor causes the reactivity of the core to increase. Two-dimensional diffusion theory calculations of the above-mentioned experiments also predicted this, but undercalculated the Δk/k effect of 6 in. of lead by 25%. In addition, two-dimensional diffusion theory and Monte Carlo calculations were used to analyze the reactivity effect of a lead shipping container surrounding a Seed 2 cluster from Shippingport Core 2. All the calculations revealed that the presence of lead in the reflector region of a water-moderated core causes the reactivity of the core to be significantly higher than a core with a pure-water reflector.