When 235U is introduced into 238U-Pu-fueled tight-lattice water reactors, it improves the performance of these reactors in two ways. First, 235U tends to improve the reactivity void response of the core by making it less positive or even negative. Second, introducing 235U makes a high conversion reactor breed with respect to plutonium. This impacts positively on the plutonium inventory and the rate of the introduction of the high conversion tight-lattice reactors.