Received August 4, 1981 Accepted for Publication March 18, 1982 The uranium ore requirement under a given world energy demand scenario is studied for the light water reactor, high temperature reactor (HTR), and fast breeder reactor (FBR), respectively. Both HTRs, when operated in the thorium fuel cycle, and FBRs achieve favorable conservation of uranium, but in different chronological periods: the HTR during the introductory phase of the nuclear power for ∼100 yr; the FBR during the later phase of saturation for many centuries. The advantages can be combined by introducing both systems in parallel, which brings a definite further improvement. Beyond that, the world uranium requirement can be limited to a total of 5 million Mg if accelerator breeders or fusion-fission hybrids are introduced for breeding 233U out of thorium starting 40 yr from now.