AbstractCalculations are made of the infinite multiplication factor in natural uranium dioxide lattices employing mixtures of light and heavy water as a moderator. The three parameters varied in the study are (a) the metal-to-water volume ratio, (b) pin diameter, and (c) light-to-heavy water atomic ratio. Maxima in plots of K∞vs hydrogen-to-deuterium ratio indicate that such H2O + D2O mixtures would prove advantageous in certain cases. However, in these cases, slight fuel enrichment might be required.