Methods of long-term reactivity control for plutonium-fueled D2O-moderated reactors that favor high conversion ratios are considered. One method uses annular gaps around the fuel elements that can be selectively filled with the D2O moderator. Reactivity compensations ranging from 8 to 15% can be achieved with gaps of 6 to 8-cm thickness and a corresponding reduction in conversion ratio of 3 to 5%. In the second method, depleted uranium sleeves that can be removed as required during long-term operation are utilized around fuel elements in annular regions of the reactor. Sleeves of only 0.2-cm thickness, around fuel elements in the central region of the reactor, provide reactivity compensations of up to 10% and actually increase the conversion ratio in the design studied. Average conversion ratios of about 0.90 are obtained in a large D2O-cooled and -moderated reactor using Zircaloy pressure tubes at fuel burnup of 104 MWd/t. The average conversion ratios would increase to about 0.97 if beryllium-based pressure tubes could be developed.